3.2 directional drilling...directional drilling and spontaneous deviation before describing in full...

VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

3.2.1 Introduction

In the oil industry directional drilling refers to thedrilling of wells where, apart from vertical sections,sections with a curved axis can also be found. Ingeneral, the latter have a radius with a constantcurvature where the inclination with regard to thevertical varies evenly until it reaches angles between30° and 60°, but also greater angles (even 90°), ifnecessary; the curved sections are followed by straightsections whose inclination is kept constant. Directionaldrilling is, therefore, a technique that makes it possibleto reach deep mining targets even at a considerablehorizontal distance from the location of the surface rig.

There are numerous types of directional wells eachhaving configurations that can be very different andcomplex (Fig. 1): from the standard slant and S-shapedholes, to slanted holes that are drilled starting fromfacilities which are also slanted, to the different typesof horizontal, single and multiple, wells. Besides theinclination, even the direction in which wells can bedrilled may vary; consequently, a directional well canhave several shapes. Each of these shapes meetsspecific operational demands. Moreover, during thedrilling process, on the basis of the information as andwhen it is relayed to the drilling engineer, both theinclination and the direction can still be modified;therefore it is possible, and indeed quite accurate, to

337

3.2

Directional drilling

slant hole S-shaped hole deep inclined hole horizontal hole

objective’sdisplacement




Fig. 1. Main configurations of a directional or horizontal well.

talk about navigational drilling. In other words, withthe state-of-the-art technologies available nowadays,the hole may take highly complex, even three-dimensional shapes, and it is possible to ‘navigate’through the subsurface in the way deemed mostappropriate.

Applications of directional drillingDirectional drilling is used in a number of

operational situations, the most recurrent of which arelisted below.

Multi-well drilling from a single position. Thissolution is applied both offshore and onshore. In theformer case, where it has been systematically used forvery many years, the main reason for applying such asolution is the need to minimize a field’s drilling andexploitation costs. This is due to the fact that, from asingle platform (Fig. 2 A), it is possible to drill a greatnumber of wells (even more than thirty), whereas, ifthe vertical well option had been chosen, it would benecessary to drill them one by one with dedicated rigsand then connect each of them to the same productionplatform.

In the case of onshore wells, besides the need toreduce the drilling and production costs of the field,the main reason for using a single cluster is the need toreduce the environmental impact of the operations,especially when working in areas such as parks,natural reserves and habitats of certain protectedanimal and plant species. Onshore directional drillinghas been adopted more recently than offshoredirectional drilling. In particular it spread after theintroduction of horizontal drilling due to the

undeniable advantages that it also brings in terms ofincreased well productivity.

Inaccessible places. Resorting to the drilling ofdeviated and horizontal wells is unavoidable when thetarget is located in mountains regions or below orrivers (Fig. 2 C), or in any circumstance where it isdifficult or costly to set up the drilling rig directly overthe vertical of the target.

Drilling in salt domes or in particular geologicalformations. In order to reach a target located belowformations difficult to drill (e.g., salt domes, extensivefaults), it is sometimes preferable to start drilling thewell laterally with respect to the structure and thendeviate around the ‘cap’ of the salt dome (Fig. 2 E), orperpendicularly to the fault plane to minimize thenatural tendency of the hole to deflect (Fig. 2 B) whichcould cause great problems in maintaining theinclination and the direction.

Sidetracking. When a section of hole becomesunworkable due to pipe sticking or failure, or when thetarget is changed once the drilling has already started,a new hole is drilled starting from the maximum depthwhere the well proves to be free (Fig. 2 D) to later takeup drilling again in a direction which is compatiblewith the newly planned well path.

Relief wells. These directional wells are drilled tointercept a formation from which a well is blowing outwith the main purpose to ‘kill’ it (Fig. 3).

Horizontal wellsAs mentioned earlier, the drilling of directional

wells is a technological option that has been employedfor a long time by the oil industry and which makes it

338 ENCYCLOPAEDIA OF HYDROCARBONS

DRILLING AND COMPLETION OF WELLS

multiwell drillingfrom an offshore platform

fault crossinginaccessible

locationswell

sidetrackingsaline dome

crossing

Fig. 2. Various types of directional wells.

A B C D E

possible to reach all targets. In contrast, horizontalwells have been introduced more recently and havepermitted an increase in the productivity of singlewells and a decrease, at the same time, in both theoperating costs of the development of a particular fieldand its environmental impact. A horizontal well, infact, may cost from 20% to 2-3 times as much as avertical or deviated well but permits a productivity 2to 10 times greater, and therefore a smaller number ofwells is necessary to develop a field, with positiveeffects both on the global costs and on theenvironment. This does not mean that a directionalwell, or even more so a horizontal one, is the bestsolution in all circumstances (the positive aspects arealways to be compared to the disadvantages, especiallyin terms of wellbore stability, penetration rate, limitsof productivity, etc.) but, undoubtedly, thetechnologies available nowadays tend to impose thechoice of directional and horizontal wells over verticalones as a priority, particularly when drillingdevelopment wells in a field. Vertical wells still remainthe most common in drilling exploratory wells.

The first efforts at drilling horizontal wells dateback to the beginning of the Twentieth century.However, it was necessary to wait until the 1980s forthe drilling of horizontal wells to spread. After a fewisolated attempts carried out in North America and inEurope (the former Soviet Union, France, the NorthSea), the drilling of horizontal wells became socommon in subsequent years as to be considered a‘routine’ technology and to be applied whenever thecircumstances allow it.

A horizontal well, apart from greater drillingcosts, has the following advantages compared to botha vertical and a directional well: a) wells may have

problems related to gas coning; or water coning; along section of horizontal well increases the exposureof the producing zone, thus making it possible toobtain greater productivity and a smaller differencebetween bottomhole and wellhead pressures duringproduction without a high risk of attracting gas orwater back into the well as it can navigate far enoughfrom both the gas cap and the water table; b)fractured reservoirs; there is a greater chance tointersect the fractures from which the well produces;c) reservoirs difficult to access; it is possible to reachreservoirs located in particular areas, such as belowinhabited areas, lakes, rivers, on inaccessible land inmore severe conditions and at greater distancescompared to those that may be resolved by drilling awell which is simply deviated; d) application ofenhanced oil recovery techniques; horizontal wellsensure a better injection and effectiveness of oildisplacement from the mineralized areas; e) moreeffective reservoir exploitation; in the case of layerswhich have already been drained and which hadpreviously gone into production by the use of verticalwells, the drilling of a subsequent horizontal wellmay make it possible to drain even the remotest areasof the field, which would not otherwise be exploited.

3.2.2 Characteristics, configurationsand planning of a directionalor horizontal well

Directional drilling and spontaneous deviationBefore describing in full detail the engineering

aspects connected to directional and horizontaldrilling, it is necessary to consider some of the

339VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

DIRECTIONAL DRILLING

Fig. 3. Drilling of a relief well(right) to ‘kill’ a blowout well visible on the left (courtesy of D. Giacca).

problems which may arise and that must be resolvedwhen making a hole follow a pre-established path,even if such a hole is several thousands of metres long.In fact, it is especially when drilling a directional orhorizontal well and when monitoring the inclinationand the direction, that it becomes clear how difficult itis, from a practical viewpoint, to constrain the drill bitto go along a path that accurately follows the designtrend. This is due to the fact that the drill bit in thewell is subject to a number of mechanical interactionswith the formations to be drilled. These interactionsare difficult to predict and quantify and their maineffect is to deflect the hole away from the design path.The above is also true of vertical wells: there exists nosuch thing as a truly vertical well but, on the contrary,there will always be a deviation from the vertical. Suchdeviations may be more or less pronounced and haveto be kept within pre-established limits in order toreach the target at the planned depth. The design of awell, be it vertical or directional, therefore establishesthe ideal trajectory of reference which must beconstantly followed by applying all the solutions thatmodern technology puts at our disposal.

Before relating in detail the characteristics andproblems of directional and horizontal drilling, it isnecessary to discuss, on the one hand, the causes of thespontaneous deviation of the hole and, on the otherhand, the effects brought about by the drill string onthe hole trajectory as these elements are also veryimportant in planning a directional well.

The conceptual difference between directionaldrilling and spontaneous deviation is that, in the formercase, the deviation is a desired effect and the course ofthe hole is calculated, whereas, in the latter case, thetrajectory the well follows is random and is determinedby changing lithological and geostructural conditionsthat arise during the drilling phase as well as by theinteractions between the drill string and the rockformations. The trajectory that drilling wouldspontaneously follow thus differs from the one designed.In operating practice, consequently, perfectly vertical ordeviated holes are never identical to the programmedones as the deviation forces in action cannot entirely becompensated for. It is, however, necessary to limit thedeviation from the pre-established well path as it cancause some serious operating problems if it becomesexcessive. For instance, these problems may consist of adrill pipe failure due to fatigue, formation of ‘keys’(recesses created in the wall of the hole due to rotation ofthe drill string), variations in inclination and directionthat are too abrupt (dog legs) where the drill string canbecome stuck, problems in the stability of the rocks thatmay even result in collapse and, in the most seriouscases, in the need for well sidetracking, a difficulty inlowering the casings and the tools into the well to record

the electric logs, the failure to reach the target and, inshort, a huge waste of economic resources.

It is not possible to control certain factors whichdetermine the spontaneous tendency of the well todeflect. Such factors are, for example, the lithologicalfeatures of the formations, as well as their pressuregradients, sequence, thickness and density. In contrast,other factors such as the features of the drill string(diameter, number and position of the drill collars andthe stabilizers, etc.), the drilling parameters applied (inparticular, the weight on the bit), and the annulusbetween the hole and the string, can be chosen at thedesign and execution stages of the well.

Configurations of a directional wellDirectional and horizontal wells are drilled on the

basis of a design that follows precise technical criteriain order to obtain a regular and ‘practicable’ hole bothat the drilling stage and during all of its subsequentproductive life.

Broadly speaking, the drilling of a directional orhorizontal well has the following operating stages:

Beginning of the drilling. The first step is thedrilling of the vertical section until the point where thedeviation begins (the Kick-Off Point, KOP) is reachedwith a drill string that is normally stabilized (Fig. 4).The vertical section may be more or less longdepending on the final depth of the well, the numberand location of the mining targets, the configuration –more or less complex – that the well will have to take(wells having a shallow or deep KOP).



total horizontal displacement

vert

ical

dep

th

slant-hole section

vertical section

surface location

drop-off section

build-up section

kick-off point

Fig. 4. Directional well with an S-shaped hole.

In addition, there exist a few particular cases ofrigs having a tower whose inclination may vary fromthe vertical to an angle of 35° (tilted or slant rig). Forthis reason the hole already appears to be deviatedfrom the very beginning and keeps a constant angleuntil the target is reached. This solution is appliedwhen the targets to be reached are located at verysuperficial levels and there is not enough space to putthe deviation in place. Sometimes the terminal sectionof the hole may be horizontal in order to reachsuperficial targets that are located at a considerabledistance away from the drilling rig.

During this first stage, the control of the wellcourse is generally carried out by means of deviationmeasurements that are not very frequent except in thecase of wells starting from the same cluster where,instead, the control of the inclination and of thedirection must be as precise and accurate as possible toavoid the risk of ‘collision’ among the various wells.Such an event could have dramatic consequences.

Deviation of the well. After reaching the planneddepth where the deviation away from the vertical mustbegin, the next step is to implement the deviation bydirecting the well in the planned direction andgradually increasing the angle with the vertical untilthe angle of maximum inclination is reached (seeagain Fig. 4). The angular increase by unit of length(Build-Up Rate, BUR) may vary, depending on thecircumstances, from 1° to 3°-4° every 30 m and canalso be zero along certain sections, thus producing asequence of curvilinear and straight sections. Ingeneral terms, the angles of maximum inclinationrange between 30° and 60° (values smaller than thiscause problems in keeping the inclination, whereasgreater values present difficulties which graduallybecome similar to those typical of horizontal wells),although the angle may be significantly increased,even up to 90°. The slower the BUR, the smoother andmore gradual – though longer – the passage from thevertical to the deviated section and the smaller thenumber of operational problems that will have to befaced. However, the conditions of the well do notalways allow this criterion to be applied.

The deviation is achieved by means of the oldwhipstock technique. The whipstock is a tool having awedge-like shape that is lowered down a well, forinstance above a cement plug, and whose function is toforce the bit to drill in a specific direction (see below)either using a jet bit (this is a bit where two of the threenozzles are plugged so that the mud flows at high speedout of the only nozzle that is open and is thus steered inthe desired direction) or, more commonly, by a suitablyequipped bottomhole motor (turbine or volumetricmotor). The section in which the deviation is establishedis, in general, only a few metres long, but has a variable

extension depending on the nature of the subsurface andis lengthened up until a certain inclination angle isreached (usually around 3°-4°). A flexible Bottom HoleAssembly (BHA) is then lowered; this string has onestabilizer only, mounted near the bit to increase theinclination up to the maximum value of the project,which can even be higher than 90°. This situation mayarise when, navigating through a thin layer, the wellcomes dangerously near the water table, which makes itnecessary to steer it in an upward direction. During thisvery delicate stage, the deviation measurements must bevery frequent. As long as the deviation angle is low, thehole path is rather unstable because the bit tends tospontaneously deviate, and one is obliged tocontinuously vary the drilling parameters, the BHAcomposition or orientation in order to carry out thecorrections required by the hole path.

Constant inclination. Once the angle of maximuminclination is reached, the well is drilled by keepingthe inclination constant until the target is reached or, inthe case of wells where an S-shaped hole is planned,until the depth at which the drop-off section starts (seeagain Fig. 4). To keep the angle at the prefixed values,a suitably stabilized BHA is used.

The distance between the deviation measurementsis generally greater, unless there are some difficultiesin keeping the inclination and/or the direction, orunless it is a horizontal well. In the latter case, themeasurements can be markedly stepped up and, insome cases, even a continuous and real-time dataacquisition system can be installed. Respecting theplanned direction and inclination is achieved byvarying the drilling parameters or the stringcomposition and, in the case that the hole tends todecidedly deviate and runs the risk of going out of theallowed ‘tolerance cylinder’ (the space within whichthe well path is considered acceptable), suitableequipment is lowered down the well so that it forces thestring to follow a well-defined path.

S-shaped hole. If the design envisages an S-shapedhole to reach the target, or a decrease in the inclinationangle to a pre-determined value, at the established depththe inclination of the hole path begins to be decreased bya prefixed gradient (Drop-Off Rate, DOR). An S-shaped hole is usually carried out when severalmineralized levels located at different depths areconcerned, or when it is necessary to avoid certainformations that may cause drilling problems, when thereare unstable or fractured rocks or zones having a lowfracture gradient, etc. In this case, a pendulum BHA isused that has a single stabilizer mounted 1 or 2 drillcollars above the bit. This permits a decrease – more orless rapid – in the inclination. Otherwise, it is possible touse special equipment that has recently been introducedinto standard operating practice, such as the steerable



systems. Also during this stage the deviationmeasurements and the trajectory corrections are morefrequent.

Configurations of a horizontal wellThere are also several types of horizontal wells that

can be drilled: horizontal wells can schematically besubdivided into three main categories according to theangular gradient with which the horizontal section isreached, i.e. long, medium and short-radius wells.

Long-radius wells use standard technology to drilldirectional wells. The BUR may vary between 3° and 8°every 30 m and requires 2 or 3 sections. In the firstsection, the angle that is reached measures about 40°-50°and is then kept constant throughout the second sectionwhich may even be of considerable length beforestarting the third section. In the third section, theinclination is further increased up to 90°. At this stage,the horizontal section is accomplished and it can even bevery long (over 1,500-2,000 m and up to 6,000-7,000m). There are several examples of wells which have beendrilled horizontally with displacements between 8,000and 10,000 m, for instance the Wytch Farm M-16Z wellin the United Kingdom has a displacement measuring10,727 metres. Many experts of the field go as far as toclaim that, in the years to come, displacements up toabout 20 miles, i.e. more than 30 km, will be possible.

In medium-radius wells standard equipment is stillbeing used, although suitably modified to face theproblems arising during horizontal drilling. The BURincreases significantly compared to the preceding case(between 8° and 20° every 30 m), although in theory itis possible to have increases measuring about 50°every 30 m. The length of the horizontal section mayeven measure over 1,000 m and the hole diameters canhave the same values as those of conventional wells orlong-radius wells.

Finally, the drilling technique of short-radius wellsmakes it possible to obtain build-up rates rangingbetween 30° and 60° every m and therefore has thepossibility to arrive to the horizontal section in lessthan 3 m. Lateral holes up to 300 m long are typical ofthis well configuration, and it is possible to navigatethrough layers whose thickness measures only a fewmetres. However, in this case, it is necessary to resortto special equipment which combines the features ofrotary drilling with those that have been devised forthe specific purpose of drilling short-radius wells. Forexample, it is possible to use bottomhole motorshaving short, thin sections that may easily passthrough the curved portions of the well or, in buildingthem, it is possible to use materials having a lowmodule of elasticity such as aluminium alloys. Finally,another option is to resort to pipes having knucklejoints. More than one horizontal hole (called a ‘drain’)

is very often drilled from the same vertical part of thewell so as to involve a greater reservoir surface.

There are several applications of horizontaldrilling. They depend on the location of the surface rigwith respect to the prefixed targets, the reservoirfeatures, the nature and properties of the formationfluids or, also, on the general development plan for thefield. In general terms, long-radius wells arepreferable when it is necessary to obtain a longdisplacement between the surface and the target at thewell bottom (extended-reach wells). Medium-radiuswells are planned when the depth and thickness of themineralized layers require an accurate control of thetrajectory. Finally, short-radius wells are drilledespecially when the plan is to enter formations havinga low permeability but which are fractured and,therefore, drilling a long horizontal section is notcrucial, or when the space available to establish thedeviation and arrive to the horizontal section from thevertical one is extremely reduced.

Multilateral wells are a variation on theseconfigurations and consist of several horizontal wellsstarting from the same vertical well. The shapes ofthese wells depend on the particular situation and theresults to be achieved (Fig. 5). There are lateral wellswhich are drilled in the opposite direction one withrespect to the other (Fig. 5 A) and which turn out to beparticularly suitable to bring deep wells intoproduction. The aim is also to reduce their costs asthe information gathered during the lateral drilling ofthe first well may be used to optimize the drilling ofthe second well as the formations have the samefeatures. Moreover, it is possible to drill wells goingin the same direction but which are located atdifferent depths (Fig. 5 B). Such a solution ispreferable when the plan is to bring into productiondifferent mineralized levels as it regularly happens,for example, in Canada in developing heavy oilfields. Horizontal wells have also been drilled thatare arranged ‘fanwise’ or like a ‘fishbone’ (forexample, as in the Orinoco basin in Venezuela) wherelateral wells also have, in turn, other lateral wellswith a smaller diameter and which are shorter (Fig. 5 C).Multilateral wells can be drilled both at low depth(250 m) and at great depth (�5,000 m) and in fieldsof all kinds, be they light oil, heavy oil or gas fields.It is possible to have up to 4 horizontal wells startingfrom the same well with the horizontal section thatmay measure over 1,300-1,500 m and at a depthranging between 5,300-5,500 m.

Planning of a directional well:general features

In addition to the data necessary to plan wells of anykind (see Chapter 3.6), the data needed beforehand to



plan a directional or horizontal well are: a) thegeographic coordinates of well on the surface; b) thenumber of targets, their respective depths andgeographic coordinates of the well bottom; c) thelithology, the physico-mechanical and geometric features(thicknesses, slope and direction of the layers) of theformations to be drilled through; d) the general data,concerning deviations, obtained from correlated wells(directional and non directional) which may havepreviously been drilled in the area in question.

The sequence of steps to follow to plan adirectional well is summarized as follows. First of all,it is necessary to establish the location of the drillingsite fixing its geographic coordinates as precisely aspossible. It is then necessary to determine the targetarea with a well-defined margin. The following stepinvolves the determination of the well trajectory in thehorizontal plane by calculating the deviation betweenthe starting and the final coordinates, the choice of thewell configuration (it is necessary to decide whetherthe well will have to have a slant hole or a S-shapedhole or whether it will be preferable to drill ahorizontal well with long, medium or short-radius withone or more lateral branches), the definition of thedeviation’s geometric parameters (angle of maximuminclination, depth of the KOP, BUR, direction andlength of the horizontal section, etc.). The trajectory ofthe vertical section is then established. The final stepconsists of choosing the casing depths of the variouscolumns depending on the anticipated pressuregradients, the formations to drill through, theconfiguration taken by the well and the choice of thedrilling fluids that are most suitable to face theexpected problems.

The main aspects of this operating stage are brieflydescribed in the following section.

Location of the drilling rig. The choice of the bestlocation for the drilling rig must be made taking intoconsideration the position of the targets and the stateof the layers. When more than one directional well isto be drilled starting from a single location, thedrilling rig must be located so as to minimize thehorizontal displacement to the targets. A platform or acluster must be located at the barycentric position withrespect to the targets of the single wells.

A further criterion is to choose the rig location soas to take advantage of the natural tendency of theformations to be drilled through in order to favour thedeviation. In some cases, a well that deviates naturallymay turn out, in fact, to be easier to drill as it may beeasier to keep the hole inside the cylinder of prefixedtolerance, provided the formations are known and it ispossible to anticipate a preferential direction ofdeviation.

Once the surface rig position has been established,it is necessary to exactly determine its geographiccoordinates.

Limits of the target area and definition of theradius of tolerance. The target may be defined as thearea that must be reached by the hole at a certain pre-established depth. The horizontal extension of thetarget may be imposed by the structure’s dimensions,the distance between the wells, the limits of theconcession, etc.

During the planning stage one builds a cylinderwith generatrices that are parallel to the theoreticalaxis of the well and with a radius equal to the so-called radius of tolerance. The real trajectory of the



shallow, depletedor heavy-oil reservoirs

laminated or layeredreservoirs

low-permeabilityor naturally fractured

reservoirs dual-opposed laterals

verticallystacked laterals horizontally

fanned lateralsmain wellbore

junctions

Fig. 5. Various types of multilateral horizontal wells (Fraija et al., 2002).

A

B

C

well must be kept within this radius of tolerance untilthe target is reached. Thus, the target area turns out tobe bounded by a circle whose radius is equal to theradius of tolerance. It is important for the target areato be as wide as possible given that a small areacauses an increase in the rig-time as the deviationmeasurements must be carried out more frequentlyand because of the corrections to be made whiledrilling the well. In general, the radius of tolerance isgiven values ranging between 30 and 50 m but,sometimes, this value may be extremely reduced, inparticular when thin layers are being navigated and itis imperative to keep away both from the top and thebed of the layer. In addition, in some cases, becauseof particular geological structures, the target areatakes an elliptic shape, therefore the radius oftolerance has different values in directions which aremutually perpendicular. During the drilling stage, ifthe hole tends to come out of the cylinder oftolerance, the trajectory is suitably corrected byvarying the drilling parameters and by acting on theBHA composition.

In areas where directional wells have already beendrilled and, thus, the effects of the formations areknown, the limit represented by the radius of tolerancemay be interpreted in a more flexible way. For example,if in these areas it is observed that the hole trajectorycomes out of the cylinder of tolerance in a section stillfar from the objective, instead of drastically correctingthe well path, a gradual re-entry is preferred. On thecontrary, if the area is unknown, such an operation isnot acceptable as the behaviour of the formations whenthe drilling is carried out will be unpredictable. On theother hand, close to the target, respect for the radius oftolerance is binding under all circumstances as theremay be a risk of missing the target.

A number of innovative technologies have beendevised permitting the automatic and continuouscorrection of the deviation without having to interruptdrilling. A number of systems have, in fact, beendeveloped which make it possible to modify theorientation and inclination of the string in real andcontinuous time by processing suitable signalstransmitted from the surface to the bottom of the well.These techniques, requiring greater economicinvestments, must be chosen for each single caseaccording to the prefixed targets.

Particular care and frequent controls are requiredwhen planning and drilling the initial and mostsuperficial section of a directional well; this isespecially true when several wells have to be drilledstarting from the same cluster. In this case, thedistance between the wellheads at the surface isextremely short; in fact, it goes from about 5 m foronshore wells – where the space is slightly bigger – to

about 3 m for those offshore. Consequently, there is ahigh risk of collision among wells, with consequencesthat are easy to imagine.

Determination of the horizontal and verticalprojections and choice of the well configuration

In order to determine the horizontal and verticalprojections of a directional or horizontal well, it isnecessary to know the position of the surface rigwhere the well will start from and the plannedposition of the target, or targets. These positions areusually expressed as geographic coordinates, i.e. interms of latitude and longitude, as well as theirplanned vertical depths.

The horizontal projection (frontal view) isdefined by two parameters, i.e. the direction and thehorizontal deviation of the well bottom with respectto the starting point, which is represented by thecentre of the slot. The latter is a tube, alreadyembedded into the ground, starting from which thatgiven well will be drilled. These two dimensions canbe represented in different ways, among which themost common are:

Grid coordinates. Starting from the latitude andlongitude values of the starting and arrival pointsof the well, the angular differences betweenlatitudes and longitudes are calculated. Thesedifferences are later turned into metric lengths bymeans of suitable equations or charts. On the basisof such a method, taking the surface location as theorigin of a system of Cartesian axes orientedtowards the East and the North, the derived pair ofmetric coordinates NT and ET identifies the positionof the horizontal projection of the arrival pointcompared to the starting point.

Field coordinates. The field coordinates, thosemost commonly used, express the position of thearrival point compared to the origin by means of thedistance r of this point from the origin, and the angleb formed by the segment joining the origin and thearrival point to the North-South axis. This angle iscalculated by knowing the metric coordinates, ET andNT , determined in the preceding point.

Polar coordinates. The polar coordinates of thetarget P, called the pole, with respect to the origin O,are the polar radius r, which is equal to that expressedby the coordinates above, and the azimuth v which isthe difference between 360° and the angle b, asdetermined in the preceding point.

Once the horizontal projection has been identified,the next step is the vertical projection. In order todefine it, various designs are generally formulated, allof which, at least theoretically, can reach the target,and each will be characterized by differentcombinations of values concerning the length of the



initial vertical section, the BUR and, thus, the radius ofcurvature, the maximum attainable angle ofinclination, the possible DOP and the depth at which itis best to implement the return to vertical. The choiceof these parameters is not only a geometrical problem,but, more importantly, must be dictated by technicaland economic considerations.

The optimal values for the angle of maximuminclination (30°), for the BUR (1.5°-3.5°every 30 m)and DOP (1.5°-2.5°every 30 m), for the casing settingdepths, etc., must be respected, whenever possible,during the planning stage. This does not alwayshappen in reality, particularly when constrained to acertain solution by the deviation. In particular, thechoice of the build-up and drop-off rates (if return tovertical is planned) must satisfy a number ofrequirements. First, there must be geometriccompatibility with the angle of the planned maximuminclination, with the depth of the kick-off point, withthe vertical depth and with the planned horizontaldisplacement of the well. Second, it is necessary tolimit the drilling problems, such as the risks for thestring to stick, the wear of the string, the formation ofkeys and dog legs. Finally, all of the strings and othertools must be able to pass smoothly through the curvedsections. As far as the geometric aspect is concerned,if the horizontal displacement and the vertical depth ofthe target are known, and once the hole configuration(slant hole, S-shaped hole and a hole having ahorizontal section), the angle of maximum inclinationand the position of the KOP have been decided, thebuild-up and drop-off rates turn out to be implicitlydefined. As an alternative, once the build-up and drop-off rates have been established, the angle of maximum

inclination can be inferred. Indeed, in most cases, it isthe latter approach that is preferable as it allows for awider choice. For example, if the project involves thedrilling of a directional well in rock where the stringmay become stuck because of differential pressure andthe occurence of keys, if the rock is hard and abrasiveor if it is necessary to run in hole casing strings with alarge diameter and which are thus rather rigid, it ispreferable to work with low angular gradients makingthe curved sections more gradual and softer.

Calculating the angle of maximum inclination isdone analytically by means of dedicated software andapplying one of the various approaches available. If thevalue taken for the KOP and the angle of maximuminclination, which is calculated starting from theestablished angular gradients, are deemed acceptableand in accordance with the good engineering practice,the next step is to precisely define all the necessaryparameters to describe the hole trajectory. If, forwhatever reason, the values of the inclination angle andof the KOP were considered unacceptable or at leastdifficult to achieve, the calculation can be repeatedaccording to different combinations of parameters untilthe most practicable solution is found. Such a solutionmay involve a directional hole, whether it be an S-shaped or slant hole, superficial or deep, or a singleor multiple horizontal well.

At the end of this process, in which the experienceof the design engineer plays a crucial role, thehorizontal and vertical projections are put at thedisposal of the drilling engineer. These projectionsindicate the ‘ideal’ trajectory that the well will have totake and the geometric parameters that will have to beapplied throughout the various stages of the welldrilling, whether it be directional or horizontal. Athree-dimensional representation of the well trajectoryis very useful when several wells are to be drilledstarting from the same cluster and is fundamentalespecially in the high section of the wells when thedistances between one well and the other come downto a few metres and the risks of collision are high. The‘theoretical’ well trajectory will then be compared tothe real one, which is derived from the interpretationof the deviation measurements. The latter introducefurther approximations that have to be taken into dueaccount by means of the construction ellipse ofuncertainty (Fig. 6).

The casing designIn most cases, the casing depths of the columns in

directional wells are determined by the same criteriaused for vertical wells, i.e. the trend assumed by thepressure gradients, the presence of fractured orunstable formations, the location of mineralized levelsand the expected drilling problems. However, when



0

2,000

4,000

6,000

8,000

10,000

12,000

�1,000�3,000

�5,000�7,000 �5,000

�1,000

�3,000

Fig. 6. Three-dimensional representation (in feet)of the trajectory of a directional well indicating the ellipse of uncertainty (Economides et al., 1998).

choosing the casing depths, especially in the case ofdeep deviated and horizontal setting wells, it isparticularly important to evaluate the risks of thestring becoming stuck. This may take place either bymeans of jamming (the string may be pushed insidethe keys mentioned above with the risk that the string,getting stuck, may no longer be pulled out to thesurface) or by sticking (the string in directional wellstends to lean on the lower part of the hole and, if theformations are very permeable and the mud isunsuitable, the string may ‘stick’ to the hole wall withno possibility of being recovered). If these risks arehigh, it is good practice to cover the build-up anddrop-off curves with a column as quickly as possible.

In the case of wells drilled from a single cluster, thesuperficial columns are generally run in hole, not onlyapart from each other in the horizontal plane, but also atslightly staggered depths. The aims of this measure aretwofold: firstly, to reduce the interferences that the steelof the casings may cause on the correct survey of thedeviation parameters; and secondly, to minimize thecollision risks among the various wells.

The choice of the mudThe choice of the mud for a directional well must

take into account a few main necessities, in particularthe need to minimize the risk of sticking of the string.To this end it is necessary, on the one hand, to keep theweight of the mud as low as possible, while remainingcompatible with the pressures at play and, on the otherhand, to adequately formulate the composition. Theuse of specific additives and the optimization of therheological and the chemical and physicalcharacteristics of the mud, in addition, allow theminimization of the friction between the hole and thedrill string (in this case by the addition of lubricants)and of the risk of pressure differentials caused byfiltration processes due to the creation of a thin, elasticand impermeable ‘mud cake’ (the mud cake is thelayer formed by the deposition of solid particles in themud onto the walls of the well during the filtrationprocess) which prevents pushing of the pipes against,and thus their ‘sticking’ to, the walls of the hole. Bothproblems can be markedly reduced by using invertedoil-emulsion or oil-based mud. In this case, it isadvisable to consider whether it is convenient,economically speaking, to use mud that is soexpensive and to take into account the environmentalproblems that the these types of mud can posedepending on the ecological features of the area takeninto consideration.

3.2.3 Methods for assessing and surveying the deviation

Establishing the deviationThere are several systems to establish and carry out

the deviation and some of them have been highlyimproved in recent times, particularly since the carryingout of extended-reach horizontal wells or those having aparticular shape have become more common. In fact,the drilling industry has gone from using the whipstockand jetting to the systematic use of bottomhole motors,steerable systems and the geosteering.

Establishing the deviation by means of the whipstock.The most commonly used tool to put a deviation in placewas, for many decades, the whipstock (Fig. 7). Basically,this is a tool having a wedge-like shape about 6 metreslong which has, on top, a collar inside which a drill bit isinserted. This drill bit has a smaller diameter than that ofthe hole in which the deviation will be carried out, forexample a 6� bit will be used in a 8 e 1/2� hole. Abovethe drill bit, a stabilizer and then a pipe are screwed. Atfirst, the drill bit-string system is joined to the whipstockby means of a shearable pin to permit its descent into thewell. Once the system has arrived to the bottom of thewell, i.e. the depth where the deviation is to begin, the



Fig. 7. Establishing a deviation by means of a whipstock(Bosworth et al., 1998).

tool is oriented in the direction where the well will haveto proceed. Once the correct direction of the whipstockface has been established, the whipstock is inserted intothe ground by means of a suitable weight, which alsocauses the stop pin to be sheared. In this way, the drillpipes are free to rotate while the whipstock stays still atthe bottom.

Due to the rotation given to the string, the drill bitslides along the wedge going in the planned directionand producing a hole that has a smaller diameter than theoriginal diameter of the well. After drilling in this way ahole about 3-4 m long, the drill pipes-stabilizer-whipstock-drill bit system is taken back to the surfaceand an hole expander is lowered down the well, whichbrings the hole diameter back to its planned size.Originally, the orientation of the whipstock to the Northwas done through a telescope: on the surface, the toolface was oriented towards a fixed point of reference, andmarkers were placed on each pipe, in the course of itsrunning in hole, and thus, by sighting through thetelescope the prefixed point of reference, the orientationof the whipstock face could be followed. Once the stringhad reached the bottom, it was rotated until the tool facewas oriented in the desired direction and only at thisstage was the tool forced to penetrate the soil. This

method, which is rather complicated and approximate,has later been surpassed by introducing into the drillingstring above the whipstock a short piece of slightly bentequipment, and for this reason called a bent sub, havinga guide set in a sleeve that can rotate and thus be fixed inthe desired direction and whose position with respect tothe North is known, just as the position of the whipstockface must be known with respect to the guide. When thestring has reached the bottom of the well, a tool, forexample a single shot, is lowered inside the pipes inorder to observe the inclination and direction of the hole.This tool has a small shoe guide that will insert itselfinto a suitable slot present inside the bent sub. Thedevelopment of the picture of the compass of the singleshot will highlight the point of reference of the guidewith respect to the North and, thus, with respect to theposition of the whipstock face. At this stage, the stringwill be rotated by the angle which is necessary to line upthe tool face with the planned direction of the well. Thisorientation system of the string is used for wells that arenot particularly difficult to drill or when their cost is tobe minimized.

Establishing the deviation by means of jetting. Theorientation method by means of jetting (Fig. 8) waswidely used up until about the end of the 1980s, when it



Fig. 8. Establishing a deviation by means of the jet bit: A, initial stage of the ‘jetting’ with an increase in the deviation angle; B, penetration of the bit in rotary mode; C, further increase in the deviation angle by jetting.

A B C

was substituted by new technologies. However, it is stillused in wells that are not particularly difficult because,like the preceding method, it allows an appreciablereduction of the costs. Jetting is based on the excavatingaction on the soil by a jet of mud at high pressureissuing from the nozzles of the drill bit and is suitableespecially for shallow, very soft and easily erodible soil.The drilling string used for this purpose usually has aconventional drill bit where two of the nozzles are blindand the mud flows out through a third one having alarge diameter (3/4�). Over the bit there are a near bitstabilizer, the bent sub, two non-magnetic collars (calledMonel rods), a stabilizer and, finally, the succession ofdrill collars and normal pipes. On the surface, beforethe string is lowered into the well, the guide of thesleeve which has been inserted into the bent sub isaligned with the large diameter nozzle and, then, oncethe system has arrived to the bottom, its orientation isobserved by means of a single shot as in the case wherethe whipstock is used. The string is then rotated by theangle necessary to position the open nozzle of the drillbit in the direction planned. With the string kept still, allof the available weight is unloaded onto the drill bit soas to have the near bit stabilizer work as a fulcrum,forcing the drill bit to increase the inclination angle. Thepumps are set in motion so as to create, at the outlet ofthe nozzle, a high pressure, high speed jet able topenetrate the rock for a few metres. The build-up curveis produced in this way, alternating the action of thejetting with the traditional rotary mode.

Establishing the deviation by means of a turbine.Before the systematic spread of the bottomhole motorsof the volumetric type, the turbine was usuallyemployed in putting the deviation in place. At present,this method is used only in particular cases. The bentsub is installed above the turbine; this sub has thefunction of creating an angle between the axis of the

drilling string and the axis of the turbine-drill bitsystem: subs with angles of 1°, 1.5° and 2° areavailable. Establishing the deviation is done in a waysimilar to what has been previously described aboutjetting. Once the turbine has reached the bottom, theposition of the reduction-turbine axis is determinedusing a single shot, and then, when the system hasbeen oriented in the desired direction keeping thestring still, the pumps are set in motion and the drillingbegins. When using a turbine, only the final part of thedrilling system rotates, not the entire string: therotating mode drilling is substituted by the slidingmode. The use of turbines requires a good knowledgeof the area of work as such areas may induce a strongtorsion reaction which, if not duly taken intoconsideration, may push the well towards a totallydifferent direction with respect to that planned. If thedeviation is being put in place by means of a turbineand a system providing the deviation data in real timeis not available, the real path of the hole will be knownonly after drilling a certain number of metres, whenthe device for surveying the direction is taken back tothe surface. Quite often, it is then necessary to put acement plug in place and re-start the deviation.

A remarkable step forward in drilling directionaland horizontal wells came about when systems forsurveying the deviation in real time were coupled tothe turbine. The first systems envisaged theinstallation above the turbine and the bent sub of anumber of accelerometers and magnetometers, whichwere placed in a suitable slot near the bent sub andlinked to the surface by an electric cable. Thanks tothis technique it was possible to control, in real time,direction in which the well was going and, thus,balance the effects of the turbine. Such a system,however, had the big disadvantage of having to usepipes equipped with an electric cable, which made the



ADNazimuthal density,

neutron tool

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resistivity toolgeosteering tool, motor,

inclination,resistivity and gamma ray

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Fig. 9. Typical composition of a system used in geosteering operations (Allen et al., 1997).

operations rather difficult and expensive. The systemwas improved when the cable was substituted by atransmission system like the one included in the MWD(Measurement While Drilling) and the LWD (LoggingWhile Drilling) equipment, and which permits the datato be sent to the surface in the form of pressure pulsestransmitted by the circulating mud.

Establishing the deviation by means of a downholemotor. Over the last few years, Positive DisplacementMotors (PDMs) have been preferred instead of theturbines. They have turned out to be more suitable thanturbines for deviation operations, given their lowernumber of revolutions, the high value of torque andthe simplicity of their manufacture, with aconsiderable reduction in the operating costs and aneasier execution of the deviation. The motors used areof the rotating type (see Chapter 3.1).

Steerable systems and geosteering. The lastgeneration of equipment, which goes under the name ofsteerable systems, is thus made up of a PDM, or of aturbine, on the lower part of which, just above the drillbit, the bent sub is mounted. Above the PDM, either theMWD equipment, which provides in real time the dataof interest to the driller (such as inclination, direction,pressure, temperature, real weight on the drill bit, torquestress, etc.) or the LWD equipment is installed. Thelatter makes it possible to send to the surface, not onlythe information mentioned above, but also geologicaldata (the gamma ray log, the resistivity, density andsonic logs, etc.). The coupling of sensors providinginformation on the course of the well trajectory, in realtime and in a continuous way, with logs characterizingthe formations from a geological viewpoint, goes underthe name of geosteering (Fig. 9). This technique makesit possible to navigate, in the true sense of the word, inthe subsurface following the most suitable route toreach the prefixed targets.

Thanks to such string configurations, it is possible toestablish and carry out all curves with an increasinginclination, to drill sections with a constant inclinationand, possibly, to implement an S-shaped curve withoutever having to take the drill bit out (as long as it is notnecessary to change it because of excessive wear). Thedrilling is accomplished in a sliding mode and,consequently, while the final part of the string is keptrotating, the part above the PDM is kept still and inducedslide along the hole trajectory as the drill bit movesforward, increasing the deviation angle through the bentsub between the PDM and the drill bit. During thisphase, every time the operator requires, the MWD or theLWD relays the deviation and geological data to thesurface, thus permitting exact knowledge of thedeviation parameters of the well and of the formationcharacteristics; this allows the operator to intervene indue time to make the corrections deemed most

appropriate. Once the desired inclination angle has beenreached, the drilling is carried on by rotating the drillingstring. If, during this phase, in which the well inclinationis kept constant, the need arises to make correctionsbecause the hole is going in another direction or ischanging its inclination, the string rotation is interrupted,the PDM is re-oriented and the drilling is done with thestring motionless until the programmed inclination andthe direction values are re-established. A return tovertical, if necessary, will be carried out by means of thetechnique described above. This technology is alsowidely used in drilling horizontal wells thanks to thegreat flexibility of navigation that the steerable systemspermit, particularly when thin layers are drilled andwhen it is imperative to keep away from both the gas capand the water table.

Other systems. Last generation systems are evenmore sophisticated as the bent sub may be substitutedby more complex equipment from a construction pointof view, but much more efficient in controlling theinclination and direction of the well. For example, oneuses stabilizers whose blades can be adjusted, makingthem protrude more or less from their slots, by meansof commands sent from the surface. In this way, theblades exert the necessary force against the formationto steer the drill bit in the desired direction and thusallow its trajectory to be changed at will. These toolsmake it possible to drill, in rotating mode and bymeans of a PDM, also very complex wells. Even ifsignificant and frequent changes of the well trajectoryturn out to be required, it is not necessary to pull outthe string to modify its composition. In addition, theazimuth and the inclination angle can be changed evenduring the build-up and drop-off stages, withundeniable advantages both technically andeconomically speaking.

Other even more recent solutions envisage thepossibility of drilling in rotating mode, always bymeans of steering systems, also horizontal sections athigher penetration rates compared to drilling in slidingmode, using bottomhole equipment made up of, forexample, a rather long transmission mast mountedright above the drill bit and which can bend due to theaction of two counter-rotating cams. In this way, it ispossible to steer the drill bit in the desired direction bysuitably placing the cams. Through a series of pressurepulses of the mud, it is possible to send commands tothe well bottom to modify the position of the camsand, on the surface, signals are received confirmingthat the new inclination and direction have beenestablished. Such systems can be used also in the caseswhere it is impossible to use stabilizers with adjustableblades, for instance, in the cases of strongly ‘caved’holes where the stabilizer blades cannot come intocontact with the wellbore walls.



Methods of surveying the inclination and directionof a directional well

Once the deviated or horizontal well is in thedrilling phase, it is essential that its inclination anddirection be assessed as quickly as possible. This is allthe more important the more complex the wellconfiguration, the smaller the radius of tolerance, theharder the formations to drill and the stricter the safetyrequirements.

There are several types of equipment for surveyingdeviations. Besides the order in which they weredeveloped, they are different with respect to theprecision of their measurements, their difficulty ofuse, the context in which they are used and, ultimately,their cost. The types of equipment most widely used inthe oil industry are: the single shot, the multi shot,accelerometers and magnetometers, gyroscopes.

The single shot. The single shot was the firstinstrument to be regularly used to observe theinclination and the direction. The most common model(Fig. 10) is made up of a timer, which is set on thesurface, by a pendulum that measures the holeinclination, and by a compass giving the direction. Acamera takes a picture of the compass-pendulumsystem at the time set by the operator using the timer.Between the compass and the pendulum there is aglass disc on which several concentric rings areengraved. These rings serve to measure the hole

inclination. To avoid interferences with the compass,all of this equipment is placed inside a non-magneticcontainer, which is made heavier by a bar. Thecontainer may be launched or run in hole by means ofa cable inside a pipe that is also non-magnetic.

The operations for surveying the deviationparameters are the following: the timer sets themoment the photo will be acquired, taking intoaccount the well depth, its inclination and the muddensity and, thus, the speed at which the equipmentwill move inside the pipes and how long it will take toreach the measurement position at the bottom. Thedeeper the well, the steeper it is and the higher themud density, and thus the longer the container willtake to reach the measurement location at the bottom.When the equipment seems to have arrived to thedestination, the drill string stops, which meanwhilehad been kept rotating slightly in order to avoid itbecoming stuck. At this point, one waits for the pictureto be taken. On the picture there remain the image ofthe compass needle (from which the well direction isdeduced) and the projection of the pendulum shadow,thanks to which the inclination is inferred. Also, thetool face is indicated on the picture, i.e. the position ofthe system in the well with respect to the North. Thecontainer is then taken to the surface either when thedrill bit is pulled out of hole, in the case it had beenlaunched, or when it is recovered by means of afishing tool lowered inside the well with a cable. Oncethe inclination and direction values have been obtained(as the direction refers to the magnetic North, it has tobe corrected by calculating the magnetic declination ofthe region) if the course followed by the well differsfrom that planned, all of the elements to intervene in arelatively short time are available.

The multi shot. Another model of the single shot isthe multi shot. The latter uses the same equipment asthe single shot with the difference that it is equippedwith a camera with an 8 mm film that can take a largenumber of pictures. The camera is switched on by atimer that unrolls the film and also turns on thebattery powered lamp to light up the measurementdevice. The operating sequence, switched on by thetimer, schedules a series of images to be takendepending on the number of planned measurementsand the time required to take the device from onedepth to the other. In this way, in a single run, it ispossible to acquire all of the elements necessary toreconstruct the trajectory, position and deviation ofthe hole as a function of depth. Such a system ofsurveying of the inclination and direction is notentirely compatible with putting in place and carryingout a directional well. The multi shot, in fact, can beused either to increase the number of measurementspreviously carried out using the single shot or, at the



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inclinationand direction

film disc

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Fig. 10. Working scheme of a single shot device.

end of a certain drilling phase, even of a vertical well,in order to know more precisely the location of thewell bottom for safety reasons.

Inclinometers, accelerometers, magnetometers. Astep forward was made with instruments in which thecompass is substituted by a group of threemagnetometers to measure the direction, and thependulum by three inclinometers or by threeaccelerometers to determine the inclination. The threeinclinometers are arranged each in an orthogonalposition with respect to the other and they are alsocompensated for the movements of the probe inside thewell. They consist of an electrolytic solution which fillshalf of the space between the two conductors installed atthe opposite ends of a disc-like, glass container. If thecontainer is inclined, the electrolytic solution shiftscausing a variation in the electric resistance between thetwo conductors, which is in proportion to the angle ofinclination. The accelerometers, which measure theearth’s gravitational field, are also three in number,generally of the piezoelectric type and placed at 90° withrespect to each other. The accelerometers, arranged in acontainer, register the position and the shift of the massof a pendulum which, because of the inclination, movesas the position taken by the container itself changes. Themass is taken back into its initial position by aservomotor and the energy absorbed to carry out thismanoeuvre is proportional to the extent of inclination.

The direction is measured by means of threemagnetometers that are arranged in an orthogonalposition one with respect to the other and which

register the intensity and the direction of the earth’smagnetic field. In order to measure the direction, theseinstruments, as mentioned above, measure the earth’smagnetic field and, consequently, the declination ofthe local magnetic field must be taken intoconsideration. Although magnetometers may be usedin most wells, they have some limitations, particularlywhen it is necessary to make measurements inside thecasings, or when there are irregular local magneticfields. All these remarks have lead to development ofnon-magnetic type systems for surveying thedirection.

Gyroscopes. Gyroscopes belong to the categoryof non-magnetic surveying systems. Other thanensuring a very high precision, gyroscopes are notsensitive to the effects of the magnetic field on thematerial of the pipes and casings. They are usuallywidely used both during the drilling of directionaland horizontal wells and the drilling of infillingwells, i.e. those wells that will have to be drilled intoa network of pre-existing wells for which the ‘fielddata’, in particular the deviation data, are absent orpartial. In order to better exploit the old fields it isoften necessary, in fact, to drill new wells that are,indeed, called infilling wells, which may interfereand collide with pre-existing wells. In this case,monitoring surveys on the old wells are carried out inorder to precisely locate them and, thus, decide thetrajectories the new wells may follow.

The basic principle of the gyroscope is rathersimple. It is made up of a rotor that quickly rotatesaround a rod – up to 40,000 revs/min. In turn, the rodis mounted onto a frame that is more or less complex,by means of two gimbals, one on the outside and theother on the inside. The rotor can move freely withrespect to the frame, thus taking any orientation andhas the characteristic of keeping the initial orientationthanks to inertia of its mass and the high angularspeed. The main components of a gyroscope (Fig. 11)are, thus, a rotor, a gimballing system, the outeranchorage of the suspensions (the frame), a system tomeasure the angular shift between the outeranchorage and the gimballing system and, finally, atorquemeter to compensate certain kinds of errors andcontrol the precession phenomenon. A factor to takeinto consideration is the ‘apparent drift’ of the rotorthat is induced by terrestrial rotation. This drift varieswith the latitude going from about 15°/h at the polesto zero at the equator.

First-generation gyroscopes usually employed aconventional system made up of two gimbals, one onthe outside and the other on the inside, with twopossibilities of movement from which the welldirection was derived. The inclination was, instead,provided by a plumb line placed inside the device



gimballing system

pick-off

gyro case

torquer

spin rotor

gimbal

resolver

Fig. 11. Working scheme of a gyroscope to survey inclination and direction in deviated and horizontal wells.

used for the angular measure. A small camera, set bya timer, took pictures of the plumb line placed overthe dial used to measure the direction. In order toobtain good results, the instrument had to bearranged following a known direction beforelowering it into a well.

Second-generation gyroscopes made it possible toread the data directly on the surface by eliminatingboth the camera and the timer. The probe waspowered by a cable and was linked up to a computerlocated on the surface. The computer would check theperformances of the probe by printing the data as theywere gathered: accelerometers were used to measurethe inclination, whereas the direction was provided bythe conventional system, i.e. the gyroscope havingtwo possibilities of movement.

Third-generation gyroscopes are more highlyperforming and sophisticated. In some systems it isnot necessary to orient the instrument on the surfacebefore lowering it down the well. Other systemsconsist of a platform that has four gimbals and thatlets the outer case move in every direction withoutaffecting the rotor position. Finally, other systemsmake it possible to automatically keep that directionand the horizontal position after orientating the axis ofthe rotor toward the North.

Optical gyroscopes, which in practice have nomoving parts, are even more recent. So are systemsbased on sending a light beam from a source located,for example, at the end of a string of pipes towards atarget mounted onto their tip. The inflection the stringis subject to causes the deflection of the light beamand this is converted into a variation of the inclinationand of the direction.

Data communication. Gyroscopes and theensemble of last generation accelerometers-magnetometers are parts of the most advancedsystems for controlling well deviation, for examplesteerable systems. As has already been described,through these systems it is possible to influence thedirectional characteristics of a well without pullingthe drill string out of the hole and thanks to a number

of systems communicating data to the surface theyalso make it possible to know, in real time, theprecise trajectory that the hole is following. The datacommunication systems most widely used nowadaysare the MWD and the LWD, which need no cables tolink the surface to the well bottom. These instrumentscreate pressure pulses during the mud circulationobtained by the opening and closure of special valvesinstalled inside non-magnetic pipes. Informationregarding the status of accelerometers,magnetometers, gyroscopes which measure theinclination, the direction and the tool face, and thedata of the equipment that register resistivity, gammaray, neutron, density, temperature logs and mudpressure, etc, are transmitted to the surface throughthe mud in the annulus following a binary logic, andare picked up on the surface by a pulse transducer.The latter registers even the smallest variations inpressure determined by the opening or closure of thevalves at the bottom. The pressure pulses are thendecoded by a computer. Systems that communicatedata by means of low frequency electromagneticwaves instead of through mud are also available.These waves propagate through the soil until theyreach the surface, where they are picked up anddecoded. The advantages of the electromagneticMDW are the following. It is possible to relay andreceive the signals in two ways, from the bottom ofthe well to the surface and vice versa, even whenthere is no mud circulating. This is an aspect that canbe very important when the drilling is carried out insituations of total loss of circulation. Moreover, thecommunication speed is very high, consequently it ispossible to receive the signals every 2-3 minutes anda few seconds are enough to communicate everyparameter and, on the whole, the system is simpler.However, these positive aspects are counterbalancedby the fact that the system is strongly affected by thedepth, particularly when the signals must go throughvery resistive formations, with the risk that theinformation communicated will not get to its finaldestination.



Fig. 12. Jonah Field area, Wyoming, USA, taken by the Landsat satellite in two consecutive moments of the field life: the way the area looked on August 27, 1986 before the field development (left), the area on October 18, 2002 after drilling about 400 wells (right). The drilling of horizontal wells may havesignificantly reduced the environmental impact the area was exposed to (SkyTruth).

3.2.4 Conclusions

On the basis of what has been stated above, it isclear that the drilling of even very complexdirectional and horizontal wells is nowadayssupported by a wide range of technologies, someof which are still being changed and improved.This makes it possible to use these technologies inall situations, however difficult or demanding. Inaddition, it is evident that drilling, in particular,extended-reach horizontal wells having adisplacement measuring several kilometres, has remarkable consequences not only on theproduction potentials of each well, which canbe markedly increased compared to a vertical orconventionally deviated well and, thus, on thedevelopment costs of a field, but also from anenvironmental viewpoint. Some regulations,especially in the USA, require priority to be givento the drilling of directional or horizontal wells inthe development of an oilfield, precisely with theaim of reducing environmental impact of theoperations (Fig. 12). A smaller number of wells,located even at a great distance from sensitive andprotected areas, implies a significant reduction inthe infrastructures necessary to develop and keep a field in production, such as drilling sites, serviceroads, parking areas, and means of transport. Allof this implies less pressure on the area with greatadvantages for the environment.

Bibliography

Deidda R. (2001) Rilevazione dei pozzi direzionati. Progettazione,sistemi di calcolo della traiettoria, attrezzature di deviazione,Eni-Divisione E&P.

Downton G. et al. (2000) New direction in rotary steerabledrilling, «Oilfield Review», 12,18-29.

Giacca D., Mazzei S. (1985) Tecniche e tecnologie diperforazione. Manuale di perforazione direzionata, Agip.

Killeen P.G. et al. (1995) Surveying the path of boreholes. Areview of orientation methods and experience, in: Proceedingsof the 6th international Mineral and Geotechnical LoggingSociety symposium on borehole geophysics for minerals,geotechnical and groundwater applications, Santa Fe (NM),22-25 October.

Molvar E.M. (2003) Drilling smarter. Using directionaldrilling to reduce oil and gas impacts in the intermountainWest, Laramie (WY), Biodiversity Conservation Alliance.

References

Allen F. et al. (1997) Extended-reach drilling. Breaking the10-km barrier, «Oilfield Review», 9, 32-47.

Bosworth S. et al. (1998) Key issues in multilateral technology,«Oilfield Review», 10, 14-28.

Economides M.J. et al. (edited by) (1998) Petroleum wellconstruction, Chichester-New York, John Wiley.

Fraija J. et al. (2002) New aspects of multilateral wellconstruction, «Oilfield Review», 14, 52-69.

Diego GiaccaEni - Divisione E&P

San Donato Milanese, Milano, Italia



3.2 directional drilling...directional drilling and spontaneous deviation before describing in full...

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