Introduction

• Formation evaluation is the process of using borehole

measurements to evaluate the characteristics of subsurface
formations
• It applies to many areas of engineering where various rock
properties are needed
• However, our efforts will be directed toward , the identification
and evaluation of commercial hydrocarbon-bearing formations
• Formation evaluation represents the expenditure of a
considerable sum of money each year
• In each individual well the evaluation cost may range up to
20% of the total well cost
Methods of Borehole Measurements
• A wide variety of in-situ measurements are available for evaluating
formations in an individual well. These measurements may be grouped into
four categories:
i. Drilling Operation Logs (mud logs): cuttings analysis, mud analysis. and
drilling data collection and analysis
ii. Core Analysis: qualitative measurements (visual lithology, presence of
shows. etc.) and quantitative measurements (porosity. permeability
formation factors, etc.)
iii. Wireline Well Logs: electrical (spontaneous potential (SP) nonfocused
current resistivity, focused current resistivity, induction), acoustic
(transit time, fullwave train, borehole televiewer), and radioactive
(gamma ray, neutron, density, neutron lifetime, spectral)
iv. Productivity Tests: formation tester. drill stem tests, and production
tests
• Obviously, not all of these measurements will be made in any single well
• Rather, a judicious selection of specific measurements is made in order to
completely identify and evaluate the commercially productive hydrocarbon-
bearing zones
• The problem is to select the minimum cost combination of measurements
providing a definitive evaluation
• Also, in evaluating commercially productive hydrocarbon zones, a wealth of
information of great value to petroleum geologists, geophysicists, and engineers
might be obtained
• A partial list of applications of borehole measurements is shown as follows. This
list includes some auxiliary applications of the data directed toward solving the
primary problem in formation evaluation:
1. Estimating recoverable hydrocarbons (primary application)
2. Estimating hydrocarbons in place (primary application)
3. Rock typing
4. Abnormal pressure detection
5. Evaluating rock stresses
6. Locating reservoir fluid contacts
7. Fracture detection
8. Identifying geologic environments
• Determination of recoverable hydrocarbons, or at least of hydrocarbons in place,
is the primary goal in the selection of measurements run in a specific well
• Any additional information generated from the array of data obtained is usually
considered a bonus
• The evaluation program is, therefore, designed to provide reliable estimates of
the following expressions for hydrocarbons in place:
Fig. 1: Typical water-oil relative permeability curve
Fig. 2: Typical porosity VS. permeability trends for different rock
types
• Recoverable hydrocarbon volume might be projected if an approximation of recovery
efficiency, ER , can be made so that Np = NER, where Np = cumulative produced oil in STB
• With proper selection of borehole measurements, it is possible to obtain quantitative
estimates of three of the parameters shown in the above equations : Shi (Soi or Sgi), Φ and
h
• Even if drainage volume, Ah, and recovery factor, ER, were unknown, an evaluation of the
three parameters Shi , Φ and h can provide information of value since Np=f(Shi,Φ and h )
• The most important of these parameters is hydrocarbon saturation, Sh. It represents not
only a volumetric quantity, but is also related to the ability of the rock to transmit fluids
• This can be seen in the typical relative permeability versus saturation curve shown in
Figure 1.
• As the hydrocarbon saturation, Sh approaches some critical or cut-off saturation value,
Sc , the ability of the hydrocarbons to flow decreases rapidly, and, conversely, the ability
of water to flow increases rapidly
• In a practical sense, therefore, a hydrocarbon saturation in excess of the critical
saturation must exist to have recoverable hydrocarbons
• Usually the critical saturation is about 50% and nearly always falls in the range
from 30% to 70%
• Most sedimentary rocks also exhibit a relationship between single phase
permeability. ka and porosity, Φ , as shown in Figure 2
• This leads to a second condition, cut-off porosity, which varies with rock type
• Cut-of f porosity is the minimum porosity above which an economically
acceptable single phase permeability is probable
• This means that only zones having porosity greater than cut-off porosity can be
considered when estimating recoverable hydrocarbons
• The summary of the basic concepts of formation evaluation is thus:
• The process of estimating hydrocarbon saturation, Sh,porosity, Φ, and producing
interval thickness, h, can be an arduous task
• Complications result because:
(1) desired parameters must be inferred from measurements indirectly,
(2) empirical relations of a statistical nature must be used, and
(3) economics must be considered
• First consider the problem of indirectness in obtaining the required parameters.
To obtain hydrocarbon saturation, Sh, we estimate water saturation, Sw, assuming
that Sh = 1 - Sw
• However, Sh, is in turn a function of five variables: bulk resistivity, Ri water
resistivity, Rw , porosity, Φ, and two empirical constants, m (cementation
exponent) and n (saturation exponent).
• These , in turn, may require other measurements
• If the acoustic log is used for estimating porosity it is necessary to account for the
effects of lithology, effective stress, and grain structure on the log-measured
value of interval transit time, t
• Next, to convert the indirect measurements to parameters of interest, it is
necessary to use empirical relationships, since theory cannot completely predict
the relations between the indirect borehole measurements and the parameters
needed in formation evaluation
• The statistical nature of the empirical relationships used will affect our choice of
evaluation tools
• Finally, economics influences the choice of evaluation tools
• Based on evaluation cost per-foot, cores may cost 500 times more, and "mud
logs" may cost five times more, than wireline logs.
• Productivity tests are usually quite expensive, costing thousands of dollars per
test
• Economically, it is desirable to design an evaluation program using wireline logs as
the principal source of information
• The ultimate objectives of well logging are the location of oil or gas formations
and their quantitative evaluation
• These require that the logging program give adequate information on:
(1) formation lithology,
(2) depth and thickness of productive zones,
(3) formation porosity and fluid saturations, and
(4) reservoir geometry and continuity through correlation with other wells, leading to a
determination of ·recoverable hydrocarbon or hydrocarbons in place
• To design an evaluation program for estimating recoverable hydrocarbons, it is
necessary to first consider past experience, theories, and supplementary
geological and geophysical data to establish the specific set of empirical relations,
which when solved will provide recoverable hydrocarbon volume, Np
• Secondly, enough measurements must be combined to solve these relations with
satisfactory accuracy at minimum cost
• To minimize costs the program ideally should:
1. Use wireline logs as basic tools where possible
2. Supplement with "mud logs" (cuttings samples and possibly borehole fluid logs)
3. Use cores for calibration of logs and for other geologic data
4. Use productivity tests to obtain Rt and assist-in evaluating important borderline cases
that cannot be satisfactorily resolved from the above
• Due to the great scope of information required from well logging operations, along with
the restrictions imposed by various borehole conditions and formation characteristics, a
wide variety of logging tools have been developed and are currently used
• Changes in the composition and character of formations that occur both geographically
and with depth require different logs in different areas and often in different sections of
the same well
• In order to understand these tool responses, however, it is first necessary to understand
the hostile environment under which these tools are operated
• All logs are affected in one way or another by the type of mud used.
• The factors with which we are generally concerned in well logging are the mud
properties (such as mud resistivity) and the water loss into the formation, along with its
MUD RESISTIVITY
• It is important to know mud resistivity, Rm, since it completes the circuit
between the logging tool and the formation.
• We can classify borehole muds into two groups, those that are conductive
and those that are non-conductive.
• The non-conductive muds include air, gas, and oil-base fluids having
infinite resistivity
• When logging in this fluid type, it is necessary to use a logging tool that
does not depend upon borehole conductance, such as the induction,
acoustic, or radioactive type logging devices
• Since this mud group is infinitely resistive, we are generally concerned
with those muds in the first category, namely the conductive water-base
type muds
• Current flow in conductive muds varies depending upon two factors-the
Function of Mud Type
• Mud conductivity depends first on water content, which is
approximately the same in all water-base muds and, second, upon the
number of dissociated ions in solution
• The number of dissociated ions varies greatly with mud type. Fresh-
water muds have few dissociated ions. This is particularly true if the
use of thinners and clay stabilizers is minimal
• Conversely, saltwater muds may contain large amounts of calcium,
magnesium, and sodium ions due to additives or contamination from
formation fluids
• It can be seen, therefore, that water-base muds may have widely
divergent resistivities depending upon the type of makeup water, the
types of additives used, and, in some cases , amount and type of
contamination from formation fluids
Function of Temperature
• It can generally be stated that mud resistivity, Rm, varies inversely with
temperature
• As temperature increases, mud resistivity decreases
• To measure mud resistivity, a uniform or homogeneous sample of the mud must
be obtained, preferably from the return line, or, if not available, then from the
mud pit
• A four-electrode resistivity cell is then used to measure resistivity at a particular
temperature
• This resistivity value must next be converted to mud resistivity at the
temperature existing opposite the various formations of interest
• The most desirable approach would be to measure mud resistivity at more than
one temperature
• The usual approach, however, is, to assume that the mud acts as a sodium
chloride solution and to use the sodium chloride resistivity chart to determine
• Of course, the fallacy in this method is readily apparent since all muds are not
sodium chloride solutions
• As mentioned previously, water-base muds may contain calcium, magnesium,
potassium, bicarbonate, carbonate, sulphate, etc
• Studies show that temperature-resistivity relations varied for the muds studied in
a linear manner as sodium chloride solutions
• In many cases the differences were small. However, some differences were quite
large and, therefore, application of the sodium chloride behaviour for
temperature corrections of mud resistivity may lead to relatively large errors
Downhole Temperature
• Temperatures recorded during well logging operations are usually the
bottomhole temperature, Tbh, and the flowline temperature, Tfl
• The borehole temperature at any depth can be determined using the
temperature gradient, gt, usually expressed in °F/100' where:
Where Dt = total Depth
• The borehole temperature at any desired depth, TfD, can then be calculated from:

Mud Filtrate Invasion

• The second consideration relative to the effects of mud influence on tool
response is that of mud filtrate invasion or water loss into the porous and
permeable formations
Fig. 3: Cross-sectlcn of porous and permeable formation with mud filtrate invasion
Fig.4: The borehole and surrounding zones
Table 1: The Logging Environment
• This process of filtrate invasion creates significant problems in log interpretation
but, at the same time, provides a unique method of log interpretation
• As the filtrate invades the formation, it creates a zone of varying resistivity, as
shown in Figures 3 and 4
• There are four distinct zones of resistivity:
• The first zone nearest the borehole is that of the mudcake, which has moderate
resistivity. This cake consists of highly compacted solids generally having very low
permeability (10-3 millidarcies) and a thickness generally between 1/3 to 3/4 in
• The second zone, the flushed zone, contains mud filtrate and, if an oil sand, will
also contain residual oil
• The filtrate saturation in this zone is usually referred to as Sxo
where: Sxo = 1 – Sor , if oil-bearing or Sxo = 1- Sgr , if gas-bearing
• The third zone is a transition zone saturated with a mixture of 'mud filtrate and
formation fluids
• The final zone is the undisturbed zone, which has true formation resistivity, Rt,
• Invasion process is related to formation and fluid parameters. Both porosity and
permeability are important parameters to describe the character of a formation
They also influence the invasion process heavily
• The depth of invasion ranges from less than one foot in high porosity formations
to perhaps 10 to 15 ft in low porosity formations
• For a high porous formation (ϕ=0.20), the volume in pore was large. More mud
filtrate was needed to replace the native fluids (water, oil or gas); thus, the
invasion rate decreased. The reverse is the case for a low porosity formation
• High value of permeability k means good flow ability; hence the greater the
permeability was, the deeper the invasion depth became. The reverse is the case
for a low K value
Resistivity Profile
• What is resistivity?
• Resistivity is measured in Ohm meter2/meter (Ωm2/m). It is the voltage required
to cause one amp to pass through a cube one meter long and one meter square.
• Typical formation resistivities lie between 0.5 and 1000 Ωm2/m.
• Resistivities in shaly sands typically lie between 0.5 and 50 Ωm2/m, whereas
carbonates are more resistive—between 100 and 1000 Ωm2/m
• Formation water resistivities range from 0.01 to 10 Ωm2/m (brine-fresh water)
and sea water clocks in at 0.35 Ωm2/m at 75°F
• To illustrate the effect of invasion on formation resistivity, two examples of
resistivity profiles are shown in Figures 5 and 6
• These profiles show the variation of resistivity with distance from the borehole
for both a water sand and an oil sand, respectively
• As shown, there is a considerable difference between the two profiles
Fig. 5: Resistivity profile-
water zone
Fig. 6: Saturation and
resistivity profile-oil
zone
• Specifically, the oil sand (see Fig. 6) contains a zone of low resistivity just inside the
undisturbed zone
• This zone occurs when the initial water saturation is low (less than about 50%) and
is referred to as the resistivity annulus
• It contains an abnormally high formation water saturation
• The creation of this annulus can be visualized as follows. The mud filtrate
penetrates the formation radially, sweeping the movable oil and formation water
ahead of it
• For beds that have a rather large oil saturation, the relative permeability to oil is
appreciably greater than that to water. Therefore, the oil moves faster, leaving a
zone enriched in formation water behind it. This flow is exceedingly smaIl
compared with the reservoir volume, and the saturation in the uncontaminated
zone remains undisturbed
• It seems likely that due to the effects of diffusion, capillary Pressure, gravity, etc.,
the existence of a well-defined annulus is a transitory phenomenon, although field
log experience seems to show that the annulus may exist when the logs are run
Diameter of Invaded Zone
• Factors affecting the diameter of the invaded zone include:
1. type of mud - mud filtrate invasion depends upon the water loss characteristics of the particular
mud
2. differential pressure between mud and formation - differential pressure between the
hydrostatic pressure of the mud column and formation fluid pressure, is highly important since
it is related to the amount of filtrate injected into the formation. A reasonable value for this
differential pressure is 100 psi. This is the pressure used in determining fluid lose in the mud cell
3. formation permeability - permeability has little bearing on the ultimate depth of invasion, but
it is related to the time it takes the filtrate to move a certain distance into the formation
4. Formation porosity - porosity, is the deciding factor in the depth of invasion. We can simply
state that as the porosity increases, the depth of invasion decreases
5. drilling process and exposure time - The dynamics of the drilling process and fluid circulation
alters the depth of the invasion since, with continued drilling, mud cake on a specific zone can
be partially or totally eroded off. If this occurs, the invasion process is again initiated to form a
new mud cake, and the invaded zone enlarges accordingly. With continued drilling and hence
longer exposure time, greater filtrate invasion can be expected
• 6. gravity segregation - Gravity segregation generally takes place after the mud
filtrate has been forced through the mud cake. This gravity segregation (normally
the lighter filtrate rises) definitely alters the invaded zone shape with time. The
mud filtrate is often less saline and, therefore, less dense than interstitial pore
water. Consequently, in the lower part of the formation , the mud filtrate raises
obliquely from the wall of the hole and the invasion is shallow , and In the upper
part, the mud filtrate accumulates below the upper boundary and the invasion is
deep . If filtration stops completely, all the filtrate eventually will gather along the
upper boundary and the invaded condition will disappear elsewhere in the bed.
Of course, the rate at which this takes place depends upon the vertical
permeability present within the bed and the difference in densities between the
fluids
• This rate can be approximated by the equation:
• Diameter of the invaded zone, dj, is difficult to determine since no tool has been
developed to measure it.
• It may be possible to approximate an electrically equivalent value for dj, using
combinations of resistivity logs having different depths of investigation. In
essence, this develops a resistivity profile that reflects dj