Well Logging Course (1st Ed.)

1. Rudimentary definitions

2. hydrocarbons presence determination

3. hydrocarbons quantity and recoverability determination

4. The Borehole Environment

1. Reading A Log

2. Examples of Curve Behavior And Log Display

3. Electrical Properties Of Rocks And Brines

Standard log presentation formats

Reading a log with ease requires familiarity with some of the standard log formats.

The formats for traditional logs and most field logs are shown in Figure.It contains three tracks.

A narrow column containing the depth is found between track 1 and tracks 2 and 3.

The latter are contiguous

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Different scale types

In the normal linear presentation, the grid lines in all three tracks

having linear scales each with ten divisions

In the logarithmic scaleWe have logarithmic presentation for tracks 2 and 3 Four decades are drawn to accommodate the electrical

measurements, which can have large dynamic rangesscale begins and ends on a multiple of two rather than unity

In a hybrid scale We have a logarithmic grid on track 2 and a linear in track 3Electrical measurements that

may spill over from track 2 into track 3 will still be logarithmic even though the indicated scale is linear

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SP and GR log headings used forclean formation determinationFigure shows the typical

log-heading presentation for several of the basic logs.

The upper two presentations show two variations for SP, which is always in track 1. the SP decreases to the left

The bottom presentation shows the caliper,

a one-axis measurement of the borehole diameter,

the gamma ray, which are also generally

presented in track 1.

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clean sections determination

The rule given for finding clean sections was thatthe SP becomes less negative for increasing shale,

so that deflections of the SP trace to the right will correspond to increasing shale content

The GR curve, as it is scaled in increasing activity

(in American Petroleum Institute (API) units) to the right,

will also produce curve deflections to the right for increasing shale content.

Thus the two shale indicators can be expected to follow one another as the shale content varies.

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The induction log heading and schematic of the formationAlthough modern tools

have a larger selection of curves with different depths of investigation, the displays are similar

A traditional resistivity log heading along with a schematic indication of the zones of investigation is shown in the figure three zones corresponding

approximately to the simultaneous electrical measurements of different depths of investigation

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dual induction-SFL

The particular tool associated with this format (previous slide) is referred to as the dual induction-SFL and will normally show three resistivity traces (units of ohm-m)

The trace coded ILD (induction log deep) the deepest resistivity measurement and correspond to Rt when invasion is not severe

The curve marked ILM (induction log medium) is an auxiliary measurement of intermediate depth of penetration and is highly influenced by the depth of invasion

The third curve, in this case marked SFLU (spherically focused log), is a measurement of shallow depth of investigation and reads closest to the resistivity of the invaded zone Rxo.

By combining the three resistivity measurements, it is possible, in many cases, to compensate for the effect of invasion on the ILD reading

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Log headings for three porosity devicesThe top two correspond to

two possible formats for simultaneous

density and neutron logsThe porosity is expressed as

a decimal (v/v) or in porosity units (p.u.), each of which corresponds to 1% porosity

The bottom is the sonic log format It is with the apparent transit

time Δt increasing to the left.

In all three presentations, the format is such that

increasing porosity produces curve deflections to the left

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matrix setting in neutron and density logsFor the neutron and density logs,

another point to be aware of is the matrix settingThis setting corresponds to

a rock type assumed in a convenient pre-interpretation that establishes the porosity

from the neutron and density device measurements

the matrix setting SS, means that the rock type is taken to be sandstoneIf the formations being logged are indeed sandstone,

• then the porosity values recorded on the logs will correspond closely to the actual porosity of the formation

if the actual formation matrix is different, say limestone, • then the porosity values will need to be shifted or corrected in

order to obtain the true porosity in this particular matrix

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An SP log over a clean section bounded by shalesshale sections

The intervals of high SP above 8,500 ft and

below 8,580 ft

The value of the typical flat response is called the shale base line

Sections of log with greater SP deflection (with a more negative value

than the shale base line) are taken as clean, or

at least cleaner, zonesOne clean section is

the zone between 8,510 and 8,550 ft

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A GR and caliper log over the same section as previous slideNote the similarity between

the GR trace and the SP trace

GR (solid) In the clean sections,

the gammy ray reading is on the order of 15 to 30 API units,

while the shale sections may read as high as

75 API units

the caliper (broken) It follows much of the same

trend as GR because the shale sections can

“wash out,” • increasing the borehole size

compared to the cleaner sand sections that retain their structural integrity

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An induction log over a water zone with a HC zone above itThe shallow, deep, and

medium depth resistivity curves are indicated. The zone below 5,300 ft

is possibly water, Assuming the resistivity of

the formation water is much less (i.e., the water is much more saline) than the resistivity of the mud

Mud resistivity effect: the shallow resistivity

curve, which for the most part stays around 2 ohm-m

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An induction log over a water zone with a HC zone above it (Cont.) At 5,275 ft,

a possible hydrocarbon zone ILD is much greater

than in the supposed water zone

However, this increase in resistivity may not be the result of hydrocarbon presence. A decrease in porosity

could produce the same effect for a formation saturated only with water

The real clue here is that even though the Rxo reading

has also increased (means the porosity has decreased), there is less of a separation between the Rxo and Rt curves than in the water zone. This means that

the value of Rt is higher than should be expected from the porosity change alone. By this plausible chain of reasoning, we are led to expect that this zone may contain hydrocarbons.

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Sample neutron and density logs the density-porosity estimate

(φd , or DPHI, on the log heading), in solid,

the dotted neutron porosity,

the compensation curve Δρ (or DRHO) (The auxiliary curve Δρ) indicates little borehole irregularity is the correction which was applied to the

density measurement in order to correct for mudcake and borehole irregularities

It can generally be ignored if it hovers about zero, as is the case at certain depths.

Note, once again, the built-in assumption that the matrix is sandstone.

Density and neutron derived porosity equality: the presence of liquid-filled sandstone is

confirmed. (for the 20 ft section below 700 ft)

Density and neutron derived porosity separation: caused by an error in the assumed matrix or

by the presence of clay or gas

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A neutron and density log exhibiting gas in the formationpresence of gas from

a comparison of the neutron and density logs. With gas in the pores the

formation density is less than with oil or water, so that the apparent density

porosity is higher.

At the same time the hydrogen content of gas is less than oil or water so the neutron porosity is

lower.

Thus, in the simplest of cases, gas is indicated in any zone in which the neutron porosity

is less than the density porosity.

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The signature of shale on a neutron and density combination logShale produces

the opposite effect [rather than gas] the neutron porosity

may far exceed the density porosity

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Neutron and density crossover caused by changes in lithologyAll of these generalities are

true only if the principal matrix corresponds to the matrix setting on the log.

The effect of having the wrong matrix setting on the log (or having the matrix change as a function of depth) is shown in Fig figure. Several sections show

negative density porosity. These are probably due to

anhydrite streaks, • which, because of their

much higher density, are misinterpreted as a negative porosity.

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An example of an LWD log in a horizontal well In track 1 is

the familiar GR along with three curves indicating

the time delay between drilling and the three types of measurements made;

depth track the tool rotation rate is there

Track 2 contains two types of resistivity

measurements, each with multiple

depths of investigation that overlay in this example.

The third track contains the LWD versions of

the neutron measurement (TNPH), the density measurement (ROBB), and the density correction (DRHB).

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A basic set of logs for performing a wellsite interpretationclean and possibly

permeable zones identificationan inspection of

the SP and GRfour clean, permeable

zones labeled A through D

resistivity readings are contained in the second track.What is the fluid in each

zone?the lowest resistivity

values =water

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electrical property measurements

An important component of the well logging suite is the measurement of

electrical properties of the formation. These measurements deal with

• the resistivity of the formation or

• the measurement of spontaneously generated voltages.

o These voltages are the result of an interaction between the borehole fluid and the formation with its contained fluids.

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spontaneous potential

Historically, the first logging measurements were electrical in nature. The first log was a recording of

the resistivity of formations as a function of depth and was drawn painstakingly by hand. Unexpectedly, in the course of attempting

to make other formation resistivity measurements, “noise” was repeatedly noted and was finally attributed to a spontaneous potential. • It seemed most notable in front of permeable formations.

Both of these measurements are still performed on a routine basis today.

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Resistivity

Resistivity is a general property of materials, as opposed to resistance,

which is associated with the geometric form of the material

the dimensions of resistivity are ohms-m2/m, or ohm-mThe units of its reciprocal, conductivity,

are Siemens per meter. In well-logging,

milli Siemens per meter (mS/m)

a material of resistivity 1 ohm-m with dimensions of 1 m on each side will have a total resistance, face-to-face,

of 1 ohm.

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Resistivity measurement

Thus a system to measure resistivity would consist of a sample of the material

to be measured contained in a simple fixed geometry.

If the resistance of the sample is measured, the resistivity can be obtained from the relation:

which becomes, using Ohm’s law:This constant k,

referred to as the system constant, converts the measurement of a voltage drop V, for a given current I , into the resistivity of the material.

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A schematic diagram of a mud cup for determination of its resistivity

A current, I , is passed through the sample of drilling fluid and the corresponding voltage, V, is measured.

the system constant can be calculated to be 0.012 m.

The resistivity, ρ, in ohm-m, is then obtained from the measured resistance R by:

a sample of salt water with a resistivity of 2 ohm-m in the chamber would yield a total resistance of 166 ohms

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Resistivity values

There are two general types of conduction: electrolytic and

the mechanism is dependent upon the presence of dissolved salts in a liquid • such as water

electronicExamples of electronic

conduction are provided by metals, which are not covered here

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Resistivity in different materials

Notice the range of resistivity variation for salt water, which depends on the concentration of NaCl.

Typical rock materials are in essence insulators.

The fact that reservoir rocks have any detectable conductivity is usually the result of the presence of electrolytic conductors in the pore space.

The conductivity of clay minerals is also greatly increased by the presence of an electrolyte.

In some cases, the resistivity of a rock may result from the presence of metal, graphite, or metal sulfides.

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sedimentary rocks resistivity

the resistivity of formations of interest may range from 0.5 to 103 ohm-m, nearly four orders of magnitude.

The conductivity of sedimentary rocks is primarily of electrolytic origin.

It is the result of the presence of water or a combination of water and hydrocarbons

in the pore space as a continuous phase

will depend on the resistivity of the water in the pores and the quantity of water present.

To a lesser extent, it will depend on lithology of the rock matrix, its clay content, and its texture (grain

size and the distribution of pores, clay, and conductive minerals).

will depend strongly on temperature

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Determination of the resistivity of an NaCl solution f(NaCl concentration, T)the resistivity

of saltwater (NaCl) solutions is a function of the

electrolyte concentration and

temperature

G/G is grains per

gallon

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1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007. Chapter 2 and 3