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Copyright 2000, IADC/SPE Drilling Conference
This paper was prepared for presentation at the 2000 IADC/SPE Drilling Conference held inNew Orleans, Louisiana, 2325 February 2000.
This paper was selected for presentation by an IADC/SPE Program Committee followingreview of information contained in an abstract submitted by the author(s). Contents of the
paper, as presented, have not been reviewed by the International Association of DrillingContractors or the Society of Petroleum Engineers and are subject to correction by theauthor(s). The material, as presented, does not necessarily reflect any position of the IADC or
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AbstractA new concept and process for real-time monitoring andcontrol of wellbore stability establishes the drilling parameters
required to optimize the drilling process and thereby reduce
the potential for wellbore instability and subsequent
unscheduled events or lost rig time. Surface and downholemeasurements, recorded while drilling, are used to make
regular updates to a model of the wellbore and to revise thedrilling plan accordingly.
The first step in the process is the generation of a
mechanical earth model (MEM) using information obtained in
offset wells and field and regional data. The proposed well
trajectory for a new well is projected into the MEM and a set
of stability parameters is generated for a given initial drilling
plan. The product identifies potential danger zones within a
well plan.
During drilling, real-time data, including logging-while-
drilling (LWD), measurements-while-drilling (MWD), surface
mechanical measurements, and fluids and solids monitoringinformation, are used to diagnose the state of the wellbore.
Any significant hole instability is detected and a warning is
given to the driller. The state of the wellbore is compared to
the model, and any revision required to align the predicted
with the actual state is made. This real-time update of the
mechanical model is then used to predict the future state of the
wellbore, in front of and behind the bit, for the given drilling
plan. If the drilling plan can be improved, a revision will be
recommended; for instance, reduction in the rate of
penetration, increase in mud weight and circulation, and
change in hole direction. The drillers can independentlyevaluate their own recommendations for changes to the
drilling plan and then decide on the best course of action. The
process also provides a record of wellbore stability
information that can be input to the field description for use infuture wells and continuous improvement of the drilling
process.Use of this concept was validated on the Valhall field in
the Norwegian sector of the North Sea. Extended-reach
drilling (ERD) to downflank targets has been problematic in
recent years; there is a high risk that wells will be suspended
or abandoned because of problems associated with wellbore
instability in this very weak overburden.The Real-Time Wellbore Stability Control (RTWBSC)
project team produced an MEM for the Valhall field, working
closely with the drilling engineers to develop a well plan for a
proposed ERD well. Implementation involved providing
wellsite support to coordinate monitoring and detection ofwellbore instability from real-time data, and on-line support inthe drilling office to interpret data, update the MEM and revise
the well plan. Through this process the team proposed and
implemented a strategy of drilling the well in controlled states
of failurenot a conventional drilling approach. The well
successfully reached its target ahead of schedule and a plannedstring of intermediate casing was not required, mud losses (a
previous problem contributing to instability and cost) were
minimal and the well was cased to below the unstable
overburden intervals.
Introduction
Wellbore instability is a major problem during the drilling ofmany oil and gas wells. Often quoted as costing the industry
between 0.6 and 1 billion dollars per year,1
it currently leads to
major difficulties in such diverse areas as the North Sea,
Argentina, Nigeria and the Tarim basin.2,3
A recent, well-
documented spectacular example of the cost savings availablefrom improved handling of wellbore instability is available for
the Cusiana field operated by BP Amoco and partners in
Colombia. Wellbore instability was very severe there, leading
to costs per well of tens of millions of dollars. An integrated
approach to the problem led to large reductions in these
IADC/SPE 59121
When Rock Mechanics Met Drilling: Effective Implementation of Real-Time WellboreStability ControlI.D.R. Bradford, SPE, Schlumberger Cambridge Research, W.A. Aldred, Schlumberger, J.M. Cook, SPE, SchlumbergerCambridge Research, E.F.M. Elewaut, Netherlands Institute of Applied Geoscience TNO, J.A. Fuller, SchlumbergerHolditch Reservoir Technologies, T.G. Kristiansen, SPE, BP Amoco Norge and T.R. Walsgrove, Consultant
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2 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
costs.4,5
A fundamental aspect of this approach was to accept
that wellbore instability was inevitable and to manage it ratherthan to eliminate it.
The drilling industry historically addresses wellbore
instability issues in two ways. The first approach treats the
problem on an ad hoc basis; for specific problem formations,
data and cores are collected and the drilling history is
analyzed, allowing the formulation of a set of empirical rules.In the Valhall field of the North Sea, for example, wells drilled
through the Middle Eocene formation at inclinations
exceeding 65 are at high risk. These rules do reduce
nonproductive time. However, they do not identify the
underlying instability mechanism and do not appropriatelyrelate it to drilling operations so that the full benefit of this
knowledge is realized. Furthermore, many of these empirical
rules apply to a well, and all need to be taken into account to
determine the drilling parameters (e.g., mud flow rate, rate of
penetration, pump pressure, trajectory). Techniques exist to
solve this type of problem, but these are often not applied andcan lead to an inadequate set of drilling parameters that can
trigger wellbore instability. The second approach is based onlog interpretation methods that estimate the safe mud weight
window using rock strength and in-situ stress state predictions
based primarily on sonic logging. The calculations are made,
however, within the framework of classical rock mechanicswhere it is assumed that the maximum and minimum mud
weights are governed by the onset of breakouts and fractures,
respectively. Several common modes of wellbore instability
(e.g., fractured shales, fault reactivation) are not amenable to
this classical approach. The description of wellbore stability is,
therefore, generally incomplete.
Both approaches can be applied before or after, but not
during, drilling. Any lessons learned from data or experience
gathered on a well can therefore only be applied on subsequentwells in the same field. As a result, several wells can be drilled
before the minimum cost construction technique is found. This
significantly increases both the capital required for field
development and the cycle time. Managing borehole
instability in real time would potentially allow learning to be
implemented on the current well so that the optimal
construction technique is achieved over the minimum number
of wells. Such an approach has not, however, been possibleuntil recently because of technical constraints. The following
developments now make it feasible:
1. There is increasing availability of MWD data.6
2. Wellbore deformation and failure mechanisms, and theirrelation to stress state, are better understood.7
3. There is improved understanding of how drilling practices(e.g., frequency of wiper trips, swab and surge pressures)
influence instability and of how, in turn, instabilities of
different kinds influence drilling.
The RTWBSC concept uses real-time measurements and
interpretation to manage wellbore instability (real-time here
means essentially during drilling of the well; some real-time
data arrive immediately as a formation is being drilled, but
other data can be delayed by up to a few hours). Although
wellbore instability can be classified as either mechanical
(e.g., failure of the rock around the hole because of highstresses, low rock strength, or inappropriate drilling practice)
or chemical (damaging interactions between the rock,
generally shale, and the drilling fluid), the integration of
understanding of chemical and mechanical damage remains
problematic despite intensive efforts throughout the oil
industry. Accordingly, the RTWBSC process (a) determineswhether a particular drilling problem is mechanical or
chemical in origin, (b) deals with the mechanical aspects and
makes recommendations, based on known rules of thumb, if
the problem is chemical in origin.
The four main components of this process are described inthe next section. The first component is a wellbore model
consisting of the trajectory, in-situ stress state, rock
constitutive parameters and all types of instability
mechanisms, together with a description of the drilling
practices. It is constructed through the two approaches by
which wellbore instability is currently addressed, and it usesoffset well data, drilling experience and in some cases a
seismic survey to define the geological structures. Theaccuracy of the model depends on the information available,
but it always provides a framework against which real-time
observations and interpretations are judged. The second
component is the data acquisition program, which defines thetypes of data and sampling rate necessary to provide a reliable
diagnosis of the instability mechanisms, their severity and the
conditions under which they occur. The third component is a
software tool that accepts data from a wide range of sources
and manages the data flow, diagnoses the instability
mechanisms, and quantifies both their severity and corrective
drilling practices. A key part of this third component is the
refinement of the subsurface model. The fourth component is a
communication tool, such as an intranet Web site, that acts asa data repository and enables rapid dissemination of
information and recommendations.
The RTWBSC process was validated on an ERD well in
the Valhall field of the North Sea (see Valhall field test
section). This field, operated by BP Amoco Norge, is located
in offshore blocks 2/8 and 2/11 in the Central Graben area of
the southern part of the Norwegian North Sea. It was
discovered in 1975, when the exploration well 2/8-6encountered over 100 m of hydrocarbon-bearing section in
Late Cretaceous chalk formations. Production began in 1982
from the highly porous Tor and Hod chalk formations.8
Valhall was originally developed to recover reserves of
250 million barrels. There are ongoing projects to increase
recoverable reserves to 1000 million barrels.9
One projectinvolves accessing downflank reserves in the far northern and
southern parts of the field through ERD wells. Although this is
economically attractive, because of the potential for significant
gains in recoverable reserves over a relatively small cycle
time, wellbore instability is a major problem: There is a high
risk that wells will be abandoned or suspended before reaching
their target. This factor, together with the availability of a
comprehensive data set, meant that Valhall was suited to
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 3
demonstrate the value and viability of real-time detection and
control of wellbore instability.
Real-time wellbore stability control processThe process uses four main components: the MEM, a data
acquisition program, data management software and a
communication system. The implementation of the process
and its components, for drilling optimization, is shownschematically in Fig. 1 and has three phases:
1. In the design phase, relevant data are gathered and theMEM is constructed. The wellbore stability and drilling
plans are then formulated: these are taken into account
during the design of the data acquisition program.2. In the execution phase, the drilling process is monitored
and data are aquired to detect instability.
3. In the evaluation phase, which also occurs during drilling,real-time data are interpreted, the MEM is updated as
necessary and recommendations relating to drilling
practices are made to the rig crew. Interpretation of real-time data should be made within the context provided by
the MEM and the wellbore stability predictions:assessments of the validities of the interpretation and/or
the MEM will be more reliable.
The four components are discussed further in the following
paragraphs. The implementation of the design-execute-evaluate cycle is discussed in the Valhall field test section
and is illustrated using events that occurred during the drilling
of the well.
Planning. Before drilling, the optimal, or least damaging, well
construction techniques are identified through prognoses of the
geology and instability mechanisms likely to be encountered
and estimates of the conditions, including the stress state, that
trigger the mechanisms.In areas where drilling has occurred, the geology can be
characterized using offset well data such as logs and
geological reports, perhaps combined with a seismic survey. In
areas where no exploration has occurred (the case in Cusiana),
it is necessary to rely on a geological prognosis only, albeit
one now aided by geological modeling software tools.10,11
The process of analyzing the likely instability mechanisms
and estimating their trigger conditions is described in thefollowing paragraphs.
Review of offset well construction. This review should
include the drilling phase, with trips and casing runs. Attention
is typically focused on (a) mud losses, cavings rates and
morphology, geological reports and any (partial or full) stuck
pipe incidents and (b) relating instability issues to theoperation (tripping, backreaming) and comparing the mud
density and/or equivalent circulating density (ECD) to the
predicted stable mud weight window.
The product of this review includes the instability
mechanisms and their severity, indexed to true vertical depth
(TVD) or, more generally, incorporated within an earth model.
Any key factors influencing the instability, such as well or
bedding inclination, should also be noted.12
The instability
mechanism at a given depth is categorized as either breakouts,
sloughing, natural fractures, weak planes, drilling-inducedfractures, faulting, undergauge hole, interbedded sequence,
overpressured formation, unconsolidated formation, mobile
formation, permeable formation or chemical activity. This list
is not exhaustive; further categories can be envisaged. The
severity of the instability is categorized as low, medium or
high.1. A low severity problem is one for which symptoms exist,
but no remedial action is required.
2. An instability of medium severity has noticeablesymptoms; minor action is required either to inhibit the
problem or to deal with its consequences. An example isminor breakouts manifested by an increased cavings rate,
or perhaps even a partially stuck pipe. The hole cleaning
could be emphasized (to deal with breakout debris without
stopping breakouts) or the mud weight could be increased
by a small amount, thus inhibiting the problem.
3. A problem of high severity is a potential well-stopper.Without major remedial action (running casing), a total
loss of borehole integrity is highly likely and will result ina sidetrack or abandonment.
Density, sonic and gamma ray logs. Data can be
constructed using logs from several offset wells. The sonic log
should ideally consist of compressional and shear slownesses.In many cases, however, only compressional slowness is
available: an empirical correlation is then needed to derive the
shear wave speed. These data form the primary input for the
MEM, which consists of the in-situ stress state, the formation
constitutive parameters and the failure mechanisms. The
accuracy of the MEM can be enhanced by correlating (a) the
log-derived results to point data, such as information from
cores or leakoff tests, and (b) quantities such as sonic
velocities to constitutive parameters such as formationstrength.
13The MEM and proposed well trajectory may then
be used to predict the safe mud weight window.14
The instability evaluation must be combined with other
factors considered during well planning, such as mud
hydraulics, hole cleaning, torque and drag calculations, and
casing programs. A discussion of how the relevant factors are
integrated exceeds the remit of this paper. It is evident,
however, that many iterations are required before the finaltrajectory and drilling practices are decided.
Planning in Valhall. The geological structure of Valhall is
dominated by a central uplift, elongated about a North-
Northwest axis.8 Otherwise, the stratigraphy is relatively
uniform, with formations varying a little in thickness anddipping away from the center of the field at an angle of
approximately 5o
to the horizontal. Figure 2 shows a generic
stratigraphic column.
Owing to the relatively uniform geology, 1D mechanical
earth models (where the properties are only a function of
TVD) are adequate for wellbore stability purposes in this field.
The structure of Valhall is not, however, entirely
axisymmetric, so it was necessary to construct MEMs that are
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4 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
locally valid for the northern and southern parts of the field.
Since the RTWBSC concept was validated in an ER welldrilled in a northwesterly direction (Figs. 3 and 4), attention is
restricted in the remainder of this paper to the MEM
constructed for northern Valhall. The MEM derived prior to
drilling is shown in Figs. 5 and 6.15
The associated mud
window, derived using an undrained linear elastic-brittle
model, is shown in Fig. 7.16 It is important to note, however,that in cases where the geological structure and/or rock
behavior is more complicated (e.g., a salt diapir), fully
numerical techniques, such as finite element analyses, are
necessary to model the in-situ stress state and derive the mud
weight window.The classical rock mechanics approach just described
determines the risk of breakouts and mud losses. It is,
however, increasingly recognized that many wellbores,
especially those drilled at higher inclinations, fail because of
instability mechanisms that are not amenable to this approach.
Examples of such mechanisms include fractured zones, mobileformations and faulting. Practical quantitative or
semiquantitative modeling of these instabilities requiresdevelopment. Currently, issues pertaining to them are handled
in a soft manner: drilling histories are analyzed to identify
the location and severity of nonclassical failure. The
dominant instability mechanisms for the discussed well areshown in Fig. 8. Medium and high severity instabilities are
denoted by the thick vertical dotted and solid lines on the right
side of the figure, respectively. Experience indicates that the
naturally fractured zone lying between 2000 and 2200 m TVD
[4160 and 4570 m measured depth (MD)] poses the most
severe risk, particularly if the well inclination through this
zone exceeds 65o. The region from 1510 to 1850 m TVD
(2370 to 3680 m MD) contains rock with weak bedding
planes; it becomes more unstable with time.Drilling strategy. The combination of the mud window
(Fig. 7) and analyses of other hazards (Fig. 8) indicated it was
impossible to drill the well without continuous rock failure
because simultaneous remedies to all the instabilities did not
exist:
1. The mud weight needed to be high to avoid bothbreakouts and underbalanced drilling.
2. The mud weight needed to be less than the minimum in-situ horizontal stress to prevent fluid loss, particularly into
the fractured zone between 2000 and 2200 m TVD.
To formulate a strategy for drilling the well, it was necessary
to assess the risk posed by each instability:
1. Breakouts are a controllable failure. This type of failure iseither self-stabilizing (breakouts tend to stop growingafter reaching a certain size) or can be controlled by
remedial actions (increasing mud weight prevents
breakout development), or both.
2. Destabilized fractured zones are an uncontrollable failure.This type of failure, once initiated, cannot be stopped
easily and is expected to become ever more severe.
Thus, the strategy for the well was to prevent destabilization of
the fractured zone between 2000 and 2200 m TVD. This
approach is contrary to conventional drilling practices, which
emphasize breakout control. This strategy involved thefollowing:
1. A relatively low mud weight. It was accepted that thiswould induce breakouts. The resulting cavings were dealt
with using hole-cleaning procedures and rate of
penetration (ROP) control. The mud weight was increased
in steps of 0.1 lbm/gal only if the rate of cavings influxinto the annulus overwhelmed hole-cleaning capabilities.
2. Specific attention, within the monitoring program(discussed below), to cavings and mud losses to provide a
warning of a destabilized fracture zone.
Recommendations for drilling parameters, such as ROP, couldonly be quantified as drilling progressed and trends for
parameters such as ECD became established.
Data acquisition. A reliable diagnosis of the instability
mechanisms, their severity and their trigger conditions
requires a combination of MWD and LWD measurements,mud analysis, geological/micropalaeontological analysis and
other surface information such as hookload and mud flow rate.The variety of data is notable and necessary because (a)
wellbore instability and the influence of operations, together
with the relationship between them, are very complex, and (b)
the process cannot rely on any single source of information.Thus, sensible interpretations require integration of all
available information. It is also important that the sampling
rates are such that interpretations can be provided on an
appropriate timescale.
Clearly, data acquisition programs are designed on an
individual well basis, taking into account the nature and risk
posed by the anticipated hazards, together with other factors
such as budget constraints, formation evaluation requirements
and contingency plans. The benefits provided by acquiringspecific types of data and desirable sampling rates are
summarized below: use and flow of the data are discussed in
the following paragraphs.
LWD measurements can include annular pressure, caliper,
gamma ray, resistivity (phase and attenuation; i.e., shallow and
deep, respectively) and compressional slowness:
1. Annular pressure is an important measurement. It can beused to (a) determine the risk of mud losses or shearfailure, (b) assess hole-cleaning effectiveness, and (c)
evaluate annular cuttings/gas loading.
2. Resistivity measurements can be used to evaluate mudinvasion into fractured or permeable zones and faults.
3. Compressional slowness can be used to determineformation strength or flag overpressured domains.
The evolution of time-dependent instabilities can be assessed
using the appropriate time-lapse data.
MWD and surface measurements must include deviation,
inclination, ROP, pump pressure, rotation rate in revolutions
per minute, downhole torque, downhole weight on bit, surface
torque and hookload, possibly combined with turbine
revolutions per minute. The data are principally used to
determine the risk of stuck pipe and hole-cleaning
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 5
effectiveness. The combination of ROP and ECD data enables
annular cuttings and/or gas loading to be managed.Mud logging data should, for safety reasons and loss
control, consist of mud flow rate in and out, total active tank
volume, change in the total active tank volume, average
background gas and maximum background gas. Periodic mud
measurementssuch as rheological parameters and fluid loss,
and the percentages of oil, water and solidsare alsodesirable, not least to aid interpretation of annular pressure
data.
A cavings analysis greatly reduces the ambiguity in
instability diagnoses; rate (i.e., volume), size range, average
size, morphology, lithology, and source depth are desirablemeasurements. It should also be noted if the cavings are old
(in a cuttings bed for several days) or are new (just become
detached from the wellbore wall). This is discussed in
Appendix A.
LWD, MWD and surface information should be monitored
continuously during drilling and also while tripping, providedthe driller is pumping out of hole at a sufficiently high flow
rate. It is advisable to conduct cavings analyses at 30-minintervals, with periodic mud logging data gathered every few
hours. All data should be indexed to date, time, hole depth and
bit depth to identify the effect of specific operations. Last et al.
correlated greatly increased cavings volumes with trips andback-reaming.
4
An appropriate selection of these measurements forms the
basis of any data acquisition program that is part of a real-time
wellbore stability control process. It is not an exhaustive list;
other key data may be required depending on the nature of the
instability. For example, if swelling shales are a severe
problem, further mud analysis may be required. It is also not a
must have list; the approach to real-time detection and
control must be flexible so that no measurement is critical.The data acquisition program for the Valhall field test
consisted of surface measurements, mud and cavings analyses,
and extensive MWD and LWD measurements. The benefits
provided by this program are discussed in the Valhall field
test section.
Decision support software. The process summarized in Fig. 1
is embodied, to a significant extent, in the decision supportsoftware shown in Fig. 9 and is designed for use on a Pentium
laptop computer. This package contains data manipulation,
evaluation and visualization algorithms that help the user
make efficient, effective real-time decisions. It is not intended
to be an automated drilling optimization tool.
The package supports the user in five main areas:predicting instability mechanisms and their trigger conditions,
diagnosing the wellbore state using real-time data, updating
the earth model to ensure consistency between the predicted
and the diagnosed states, providing recommendations to the
driller, and visualization.
Predicting the instability mechanisms and their trigger
conditions has been discussed. Algorithms enable users to
build trajectories and MEMs; safe mud weight windows are
calculated with an undrained elastic-brittle theory.
Diagnosing the wellbore state using real-time data involvesthe integration of a number of disciplines; namely, geological
analysis, drilling mechanics, formation evaluation, wellbore
stability and mud logging (mud analysis and palaeontology).
This is a complex process requiring human judgment,
particularly to distinguish wellbore instability and poor hole
cleaning. Diagnoses made within the context provided by theMEM and the planning analysis are more reliable than those
made using only the real-time data.
After the diagnosis is completed, the current wellbore state
is compared to the model; human judgment determines if the
two are consistent. If inconsistencies exist, it is necessary toupdate the MEM.
When the predicted and diagnosed wellbore states agree
adequately, recommendations either to suppress the
instabilities or minimize their consequences can be made to
the driller. For example, increasing mud weight will reduce the
amount of breakouts, whereas decreasing the ROP will reducethe rate at which breakouts are exposed, resulting in less debris
in the annulus given constant flow and rotation rates. Therecommendations should apply over the entire open-hole
interval or a specified subsection of it. The aim is to optimize
the condition of the complete open-hole section and not to
focus on remedial actions required just at the bit.Visualization is a key component of the support tool; the
quality of the real-time decisions depends strongly on the
ready and unambiguous assimilation of the output of the
RTWBSC process. For example, Fig. 10 shows the predicted
damage zone around a borehole resulting from shear failure. It
is immediately evident that the failure is extensive enough to
warrant increasing the mud weight to suppress the failure;
hole-cleaning procedures would not be able to cope with the
debris that would fall into the annulus.
Communications. Decisions on well construction are made at
the wellsite and in the office. The influence exerted by each
location varies according to the operator, the level of
actual/anticipated risk and the maturity of the field
development program.
The distribution of wellsite data and the procedures for
implementing decisions resulting from the RTWBSC analysismust be compatible with working practices; there should be
particular attention on communication.
During the Valhall field test, the RTWBSC process was
managed in the office by wellbore stability specialists working
with an existing team of drilling engineers. A Schlumberger
engineer trained in drilling risk management was at thewellsite to ensure (a) the necessary measurements were taken
correctly and (b) the data flowed efficiently to the relevant
people at the wellsite and in the office. This engineer was also
responsible for communicating recommendations for wellbore
stability at the wellsite and for conducting the cavings
analysis. Although these recommendations are usually made
by office-based personnel, a suitably trained engineer can
make recommendations independently in some situations.
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6 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
The acquired data are generally analyzed by personnel of
differing disciplines (geologists, drilling engineers, mudloggers, formation evaluation and wellbore stability
specialists), both at the site and in the office. This joint
evaluation requires a reliable link with sufficient bandwidth
between rig and office and a readily accessible repository for
the information. The experience gained from Valhall and from
the BP Amoco ETAP field has shown that a Web site canfulfill this requirement.
Training classes on wellbore stability in general and
cavings monitoring in particular were given to all drilling and
mud logging crews going offshore on Valhall. The crews
responded positively to these classes, which focused onavoiding, rather than reacting to, instability problems.
Valhall field testThe field test began with the drilling out of the 13 3/8-in.
casing shoe at 1610 m (Fig. 8) and continued until the
reservoir was penetrated at 5602 m (Point C). The sectionbetween the casing shoe and Point A was drilled using a rotary
steerable assembly with a 12.25-in. bit and a 14-in. three-armstabilized reamer. Sections AB and BC were drilled with
conventional steerable assemblies having 12.25-in. bits.
Drilling from casing shoe to Point A (1610-3832 m MD).The 13 3/8-in. casing shoe was drilled out using a mud weight
of 14.2 lbm/gal; a leakoff indicated that fluid loss occurred at
pressures exceeding 15 lbm/gal. During drilling, ECD data
indicated that a safe lower bound to the minimum horizontal
stress was 15 lbm/gal over the interval 1610 to 2040 m MD.
The mud weight had to be raised to 14.6 lbm/gal by 2200
m MD to reduce background gas levels from 20% (gas peaks
of 35% were observed). These high gas levels were consistent
with the drilling hazards prognosis (Fig. 8) and resulted frommatrix gas being released into the annulus as rock was crushed
beneath the bit. The necessity for further mud weight
increases, which would have led to the destabilization of the
critical fractured zone between 4160 and 4570 m MD, was
eliminated by slowing the ROP to below 30 m/h (Fig. 11).
This action reduced the rate at which gas was released into the
annulus and, combined with the mud weight increase of 0.4
lbm/gal, eventually led to background gas levels decreasing toless than 5%.
Wellbore stability in this section was controlled following
the strategy outlined previously. A mud weight of 14.2 lbm/gal
prevented significant breakouts after the shoe was drilled out
(Fig. 7). Subsequent mud weight increases resulted solely
from the overpressure problems described, as hole cleaningcoped with the levels of debris in the annulus caused by
breakouts.
Cavings analysis indicated no failure had occurred as a
result of weak bedding planes while drilling this section (Fig.
8), although the instability mode became active during one trip
(discussed below). The cavings rate is shown in Fig. 12.
During the drilling of this section (0 to 100 hr approximately)
the cavings rate remained reasonably steady, although there
was a reduction at around 3650 m MD caused by a packoff.
The steady cavings rate resulted from the use of a rotarysteerable tool and the absence of severe wellbore instabilities.
The ECD was constrained by ensuring the ROP did not
exceed 30 m/h: this rate controlled the cuttings loading and
gas levels in the annulus. The ROP limit was deduced by
correlating annular pressure while drilling and ROP data.
Figure 13 shows a typical case. During the period 33 to 36 hr,the ROP exceeded 30 m/h and the ECD increased gradually as
the cuttings loading in the annulus increased. Partial packoffs
then occurred, causing the ECD to become highly erratic.
Subsequently, the ROP was reduced to below 30 m/h and the
hole was cleaned more effectively by increasing both therevolutions per minute and flow rate. The ECD became more
stable and decreased gradually to 15.1 lbm/gal, indicating the
ECD effects were a result of inadequate hole cleaning rather
than continued wellbore instability.
As drilling proceeded, mud weight rose to 14.6 lbm/gal
and the ECD increased above the estimated minimumhorizontal stress (Figs. 6 and 7) to between 15 and 15.2
lbm/gal, without mud losses. The minimum horizontal stresswas therefore assumed to be 15.2 lbm/gal in the section 1610
to 3832 m. Although this value is a lower bound ofh , it is
more accurate than the previoush estimate. Figure 14 shows
the refined model of the in-situ stress state.
A severe problem occurred at 3649 m, where a fault was
encountered. This fault was diagnosed using resistivity,
gamma ray and mud loss data, as shown on Fig. 15. It can also
be inferred from this data that a packoff occurred below the
LWD resistivity tool where the ECD sensor is housed. The
surface pump pressure increased significantly while the ECD
remained constant. The reason for the packoff is uncertain, but
it is due to either fault movement or rubbilized rock, whichcan occur around faults, blocking the annulus. This incident
caused seal failure on the rotary steerable system, leading to
lubricant loss. The assembly had to be pulled out of hole after
drilling to 3832 m MD (Point A on Fig. 8). Specific
procedures for wellbore stability control were developed forthese trips and are discussed separately.
The other key problem encountered in this zone was the
presence of limestone stringers at 2943, 3258, 3290, 3305,
3330, 3350, 3546, 3508, 3550, 3596, 3645, 3650, 3668 and
3795 m MD. When the bottomhole assembly (BHA) was
pulled back through these stringers, there was a tendency topack off. It is thought that while the limestone stringers
remained in gauge, hole enlargements either side of themresulted in lower mud velocities, which led to the formation of
cuttings beds. Accordingly, during circulation periods the
BHA was positioned away from these stringers. At the same
time, to limit damage in the weakest formations, the MEMwas used to select the strongest zones for rotation of the BHA
(Fig. 5).
Drilling from Point A to the reservoir (3832-5602 m MD).
This section was drilled in two stages (AB and BC on Fig. 8)with conventional steerable assemblies having 12.25-in. bits.
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 7
Wellbore stability control in this section consisted of ensuring
the ECD did not exceed 15.2 lbm/gal (this initial constraintwas later relaxed to 15.35 lbm/gal) to avoid destabilizing the
naturally fractured zone. This was difficult given the large
amount of sliding that occurred, and there was a strong
emphasis on hole cleaning and ROP control procedures. The
stability of caving beds was also a source of concern. These
beds tend to avalanche down the well at inclinations around60, causing pipe and BHAs to stick.
In Section AB, it was found, unfortunately, that holding
angle was difficult. Drilling was therefore halted at 4306 m
MD (Point B) for the following reasons.
1. If drilling had continued, there was a risk the well wouldhave penetrated a partially drained section of the
reservoir, which is to the left of the fault shown on Fig. 8.
2. The wellsite engineer observed a caving produced throughdestabilization of the naturally fractured zone.
The proximity of the planned trajectory to the fault (Fig. 8),
made it necessary to trip out of hole to change out the BHA.The cavings analysis dictated the trip should occur without
further drilling so as to limit damage to the key fractured zone.During the trip back into the hole, 12 bbl of mud were lost
when the ECD exceeded 15.35 lbm/gal at 4120 m MD. The
minimum horizontal stress in the MEM was therefore revised
to 15.35 lbm/gal from 1610 to 4306 m MD. The refined modelof the in-situ stress state is shown in Fig. 16. Figure 17 shows
the strength profile of the overburden (to Point B) updated
using LWD compressional slowness data. This data verified
the rock strength profile constructed using offset well data
(Fig. 5) and therefore no significant changes were made in the
drilling strategy. The updated mud window is shown in Fig.
18.
In Section BC, the necessity to maintain the ECD at 15.35
lbm/gal or less meant that breakouts were a severe problem(Fig. 18). The difficulty of cleaning hole with such severe
breakouts can be seen in Fig. 12. The cavings rate varied
greatly and, in particular, there were sudden bursts of solids
over the shaker. The reservoir was, however, penetrated (Point
C) ahead of schedule.
Limestone stringers were again encountered at 4000, 4075,
4150, 4700, 4740, 4780, 4830, 4930, 4985, 5024, 5160, 5170,
and 5310 m MD.
Tripping procedures. To prevent problems associated with
swabbing as the downhole assembly was pulled out of hole,
the mud weight was increased from 14.6 to 14.8 lbm/gal
during the first two trips out from Points A and B (Fig. 8). The
procedure required the heavier mud to be circulated into thewell after pulling 10 stands. The increase in mud weight was
deduced from an analysis of pressure while drilling data from
offset wells. Conversely, during the trips in to Points A and B,
the mud weight was reduced from 14.8 to 14.6 lbm/gal to
minimize problems associated with surging. The procedure
required mud gels to be broken, after tripping 10 stands into
the hole, by increasing the revolutions per minute. Lighter
mud was then circulated into the well. It was also found that
increases in ECD during trips into hole were reduced by
shearing the mud on the surface prior to circulating itdownhole.
The trip out of hole following the reservoir penetration
(Point C) required the mud weight to be increased from 14.6 to
14.8 lbm/gal at the start of the 12.25-in. hole (Point A). This
ensured the mud had sufficient carrying capability in the 14-in.
hole section while keeping the effective mud weight in theentire open-hole section to a minimum. This was an important
consideration for the casing operation; as the casing is run,
large surge pressures destabilize the naturally fractured zones.
During the trip out of the hole after the reservoir
penetration, the hole was accidentally swabbed, causing thewell to collapse at 3500 m MD. The wiper trip to clean this
damage unfortunately initiated a sidetrack from around 3600
m MD. This sidetrack also penetrated the reservoir using the
same wellbore stability strategy described previously in the
Planning section, with further emphasis on hole cleaning.
The casing string then re-entered the original track, which hadbeen open through the fractured zone for several weeks, and
was landed below the fractured zone. It could not, however,quite reach the bottom of the hole. The well as a whole cannot,
therefore, be called a success. However, since the reservoir
was penetrated ahead of schedule and the casing could still be
installed in the troublesome fracture zone after several weeksof open-hole exposure to drilling fluid, the original wellbore
stability strategy and the real-time approach can still be
considered a success.
ConclusionsReal-time monitoring and control of wellbore stability
systematically reduce the drilling risks associated with
wellbore instability and other geological hazards. This real-
time process treats such instabilities and hazards as conditionsthat impose constraints on the drilling parameters (mud
weight, ROP, revolutions per minute, etc.) and then provides
recommendations on the drilling practices most likely to
ensure the entire hole section is maintained in the best, or least
damaged, state.
The concept and process discussed here have been
validated on an ER well drilled in the Valhall field of the
North Sea. The well reached its target ahead of plan and withmuch lower mud loss to the formation than usual (around 10%
of the typical value for a well such as this on Valhall) and
negligible activation of the fracture zones. The well was cased
to below the unstable overburden intervals.
NomenclatureUCS = Rock uniaxial compressive strength
= Rock friction angle
h = Total in-situ minimum horizontal stress
H = Total in-situ maximum horizontal stress
V = Total in-situ overburden stress
PP = Pore pressure
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8 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
AcknowledgmentsThe Real-Time Wellbore Stability project was partly funded
by the European Commission, under the THERMIE initiative
(contract number OG-0199-95).
During the Valhall field test, Schlumberger personnel
located offshore (Paul Benoit, Ruth Bertelsen, Gael Boche,
Andy Foster, Caroline Hatch, Vidar Haugen and Al Pattillo)
were responsible for the data acquisition program. Theycontributed greatly to the success of the field test through their
initiative and dedication. The assistance provided by Charles
Jenkins of Schlumberger Cambridge Research is also
gratefully acknowledged.
Appendix A - Cavings monitoringAn analysis of cavings can provide a signal that the borehole is
failing and indicates both the nature of the instability and the
troublesome formations. Cavings dimensions range from a few
millimeters to 10 cm or more, with larger examples rising to
the surface while lodged in the BHA.There are four main types of caving: tabular, angular,
splintered and those that cannot be characterized. Examples ofthe first three types are shown in Figs. 19 to 21. Tabular
cavings, shown in Fig. 19, are the result of natural fractures or
weak planes. In the case of natural fractures, the fluid pressure
in the annulus exceeds the minimum horizontal stress,resulting in mud invasion of fracture networks surrounding the
wellbore. This can result in severe destabilization of the near-
wellbore region (resulting from movement of blocks of rock),
leading rapidly to high cavings rates, lost returns, stuck pipe
and tools lost in hole. The blocks of rock are bounded by
natural fracture planes and therefore have flat, parallel faces
(Fig. 19). The other characteristic is that bedding, if any, will
not be parallel to the faces of the caving. In the case of weak
planes, the combination of low mud weight and a boreholeaxis that is within approximately 15
oof the bedding direction
can induce massive failure along the planes of weakness,
leading to the symptoms described above.12
Cavings resulting
from weak planes are characterized by having flat, parallel
faces. The bedding direction is also parallel to the faces.
Figure 20 shows angular cavings, which are a consequence of
breakouts. These cavings are characterized by curved faces
with a rough surface structure. The surfaces intersect at acuteangles (much less than 90
o). Splintered cavings are shown in
Fig. 21. These cavings have two nearly parallel faces with
plume structures. This type of caving is due to tensile failure
occurring parallel to the borehole wall and commonly occurs
in overpressured zones drilled with a small overbalance.
The cavings rate can indicate the severity of failure,coupled with the efficiency of hole cleaning. It is measured
every 30 min by the time required to fill a bucket placed
underneath the shakers. This method may seem crude, but it is
versatile (in terms of the number of different models of rig that
it can be applied to) and reliable; more sophisticated solids
measuring devices have been tried on a number of rigs, but
very few have been satisfactory.
Micropalaeontological analyses determine the geological
age of cavings. During the field test, an analysis of tabularcavings indicated that they originated from the upper section
of the open hole, where the exposure time was longest, rather
than from the dangerous naturally fractured zone.
References
1. Santarelli, F.J.: Rock mechanics characterization of deepformations: a technico-economical overview, paper SPE 28021presented at the 1994 Eurock Rock Mechanics in Petroleum
Engineering Conference, Delft, August 29-31.
2. Charlez, P.A., Bathellier, E., Tan, C. and Francois, O.:Understanding the present in-situ state of stress in the Cusianafield Columbia, paper SPE/ISRM 47208 presented at the
1998 Eurock Rock Mechanics in Petroleum EngineeringConference, Trondheim, July 8-10.
3. Charlez, P.A. and Onaisi, A.: Three history cases of cases rockmechanics related stuck pipes while drilling extended reach
wells in North Sea, paper SPE/ISRM 47287 presented at the1998 Eurock Rock Mechanics in Petroleum EngineeringConference, Trondheim, July 8-10.
4. Last, N., Plumb, R.A, Harkness, R., Charlez, P., Alsen, J. andMcLean, M.: An Integrated Approach To Evaluating andManaging Wellbore Instability in the Cusiana Field, Colombia,South America, paper SPE 30464 presented at the 1995 AnnualSPE Techical Conference and Exhibition, Dallas, Oct 22-25.
5. Last, N., Plumb, R.A and Harkness, R.: From theory to practice:evaluation of the stress distribution for wellbore stability in anoverthrust region by computational modelling and fieldcalibration, paper SPE/ISRM 47209 presented at the 1998
Eurock Rock Mechanics in Petroleum Engineering Conference,
Trondheim, July 8-10.6. Rosthal, R.A., Best, D.L. and Clark, B.: Borehole caliper while
drilling from a 2-MHz propagation tool and borehole effectscorrection, paper SPE 22707 presented at the 1991 Annual SPE
Techical Conference and Exhibition, Dallas, Oct 6-9.
7. Bradford, I.D.R. and Cook, J.M.: A semi-analytical elastoplasticmodel for wellbore stability with application to sanding, paperSPE 28070 presented at the 1994 Eurock Rock Mechanics in
Petroleum Engineering Conference, Delft, Aug 29-31.8. Munns, J.W.: The Valhall field: a geological overview, Marine
and Petroleum Geology (1985), February, p. 23-43.9. Kristiansen, T.G., Mandzuich, K., Heavey, P, and Kol, H.:
Minimizing drilling risk in extended-reach wells at Valhallusing geomechanics, geoscience and 3D visualizationtechnology, paper SPE 52863 presented at the 1999 SPE/IADC
Drilling Conference, Amsterdam, March 9-11.
10. Bryant, I.: Cybergeologist: 3D reservoir modelling using digitalgeological analogs, GasTIPS, Spring 1998, GRI-98/0144-001.
11. Bryant, I., Kaufman, P.S., McCormick, D.S. and Tilke, P.G.:
Knowledge capture and reuse in geological modelling, paperpresented at Gulf Coast Section of Society of EconomicMineralogists and Paleontologists Annual Meeting, December1999.
12. Okland, D. and Cook, J.M.: Bedding-related instability in high-
angle wells, paper SPE/ISRM 47285 presented at the 1998
Eurock Rock Mechanics in Petroleum Engineering Conference,Trondheim, July 8-10.
13. Plumb, R.A.: Influence of composition and texture on the failure
properties of clastic rocks, paper SPE 28022 at the 1994Eurock Rock Mechanics in Petroleum Engineering Conference,
Delft, August 29-31.
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 9
14. Bradford, I.D.R., Fuller, J., Thompson, P.J. and Walsgrove, T.R.:
Benefits of assessing the risk of solids production in a North
Sea reservoir using elastoplastic modelling, paper SPE/ISRM47360 presented at the 1998 Eurock Rock Mechanics inPetroleum Engineering Conference, Trondheim, July 8-10.
15. Kristiansen, T.G.: Geomechanical characterization of the
overburden above the compacting chalk reservoir at Valhall,paper SPE/ISRM 47348 presented at the 1998 Eurock RockMechanics in Petroleum Engineering Conference, Trondheim,July 8-10.
16. Fjaer, E., Holt, R.M., Horsrud, P., Raaen, A.M., and Risnes, R.:
Petroleum related rock mechanics, Elsevier, Amsterdam(1992).
SI Metric Conversion Factorsbbl x 1.589873 E-01 = m
3
ft x 3.048* E-01 = m
gal (U.S. liq) x 3.785412 E-03 = m3
in. x 2.54* E+00 = cm
lbm/gal x 1.198264 E+02 = kg/m3
psi x 6.894757 E-03 = MPa
* Conversion factor is exact.
Fig. 1. The design-execute-evaluate cycle for real-time wellborestability control. The starting point is at the top, with initial datagathering and construction of the first MEM in the planning phase.The remainder of the cycle occurs as the well is being drilled.
Fig. 2. Generic stratigraphic column for the Central Graben.(Extracted from Kristiansen et al.
9)
Fig. 3. A plan view of the Valhall field and ER well. (Extracted fromMunns
8)
Fig. 4. Trajectory of the ER well.
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10 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
Fig. 5. Uniaxial compressive strength and friction angle in theValhall overburden, estimated before drilling the ER well.
Fig. 6. The in-situ stress state in the Valhall overburden, estimatedprior to drilling the ER well.
Fig. 7. Mud weight window, estimated prior to drilling the ER well.
Fig. 8. Anticipated instability mechanisms and their severities.The thick vertical dotted and solid lines on the right of this figuredenote medium and severe instabilities, respectively.
Fig. 9. The flow of information and decisions through theprototype system. Ellipses represent data input, diamonds aredecision or comparison points, and rectangles are processes. Thestarting points are the two upper ellispes, and the finish point isthe lower left corner.
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL11
Fig. 10. A schematic of shear-induced borehole failure.
Fig. 11. The influence of mud weight and ROP on gas levels(shown as squares).
Fig. 12. Cavings data. The cavings rate is 1440 x the reciprocal ofthe time, in seconds, taken to fill a 4.5-L bucket placed under theshakers.
Fig. 13. Time-based data acquired during drilling of the intervalbetween the 13 3/8-in. casing shoe and Point A.
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12 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121
Fig. 14. The in-situ stress state, refined following the drilling of theinterval between the 13 3/8-in. casing shoe and Point A.
Fig. 15. Identifying a large fault at 3649 m MD. PUMP and TVCAdenote surface pump pressure and volume change in active mudtanks, respectively. TVCA takes account of the increase in holevolume during drilling.
Fig. 16. The in-situ stress state, refined following the trip into holeto Point C.
Fig. 17. Strength parameters calculated using LWD compressionalslowness data.
Fig. 18. Revised mud window calculation.
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IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL13
Fig. 19. Tabular caving.
Fig. 20. Angular caving.
Fig. 21. Splintered caving.