2. BASIC WELL LOG INTERPRETATION
3.1 INTRODUCTION
The continuous recording of a geophysical parameter along a borehole produces a geophysical
well log. The value of the measurement is plotted continuously against depth in the well. Well
logging plays a central role in the successful development of a hydrocarbon reservoir. Its
measurements occupy a position of central importance in the life of a well, between two
milestones: the surface seismic survey, which has influenced the decision for the well location,
and the production testing. The traditional role of wireline logging has been limited to
participation primarily in two general domains: formation evaluation and completion
evaluation.
The goals of formation evaluation can be summarized by a statement of four questions of
primary interest in the production of hydrocarbons:
Are there any hydrocarbons, and if so are they oil or gas?
First, it is necessary to identify or infer the presence of hydrocarbons in formations traversed
by the wellbore.
Where are the hydrocarbons?
The depth of formations, which contain accumulations of hydrocarbons, must be identified.
How much hydrocarbon is contained in the formation?
An initial approach is to quantify the fractional volume available for hydrocarbon
in the formation. This quantity, porosity, is of utmost importance. A second aspect is to
quantify the hydrocarbon fraction of the fluids within the rock matrix. The third concerns the
areal extent of the bed, or geological body, which contains the hydrocarbon. This last item falls
largely beyond the range of traditional well logging.
How producible are the hydrocarbons?
In fact, all the questions really come down to just this one practical concern. Unfortunately, it
is the most difficult to answer from inferred formation properties. The most important input is
a determination of permeability. Many empirical methods are used to extract this parameter
from log measurements with varying degrees of success. Another key factor is oil viscosity,
often loosely referred to by its weight, as in heavy or light oil.
Formation evaluation is essentially performed on a well-by-well basis. A number of
measurement devices and interpretation techniques have been developed. They provide,
3. principally, values of porosity, shaliness and hydrocarbon saturation, as a function of depth,
using the knowledge of local geology and fluid properties that is accumulated as a reservoir is
developed. Because of the wide variety of subsurface geological formations, many different
logging tools are needed to give the best possible combination of measurements for the rock
type anticipated. Despite the availability of this rather large number of devices, each providing
complementary information, the final answers derived are mainly three: the location of oil-
bearing and gas-bearing formations, an estimate of their producibility, and an assessment of the
quantity of hydrocarbon in place in the reservoir.
3.2 APPLICATIONS
In the most straightforward application, the purpose of well logging is to provide measurements,
which can be related to the volume fraction and type of hydrocarbon present in porous formations.
Measurement techniques are used from three broad disciplines: electrical, nuclear, and acoustic.
Usually a measurement is sensitive either to the properties of the rock or to the pore-filling fluid.
Uses of well logging in petroleum engineering. (Adapted from Pickett)
Logging applications for petroleum engineering
Rock typing
Identification of geological environment
Reservoir fluid contact location
Fracture detection
Estimate of hydrocarbon in place
Estimate of recoverable hydrocarbon
Determination of water salinity
Reservoir pressure determination
Porosity/pore size distribution determination
Water flood feasibility
Reservoir quality mapping
Interzone fluid communication probability
Reservoir fluid movement monitoring
4. 3.3 Well Log Interpretation: Finding the Hydrocarbon
The three most important questions to be answered by wellsite interpretation are:
1. Does the formation contain hydrocarbons, and if so at what depth and are they
Oil or gas?
2. If so, what is the quantity present?
3. Are the hydrocarbons recoverable?
3.4 INTERPRETATION PROCEDURE
The basic logs, which are required for the adequate formation evaluation, are:
1. Permeable zone logs (SP, GR, Caliper)
2. Resistivity logs (MFSL, Shallow and Deep resistivity logs)
3. Porosity logs (Density, Neutron and Sonic).
Generally, the permeable zone logs are presented in track one, the resistivity logs are run in
track two and porosity logs on track three.
Using such a set of logs, a log interpreter has to solve the following problems,
(I). Where are the potential producing hydrocarbons zones?
(II). How much hydrocarbons (oil or gas) do they contain?
First step: The first step in the log interpretation is to locate the permeable zones. Scanning the
log in track one and it has a base line on the right, which is called the shale base line. This base
line indicates shale i.e., impermeable zones and swings to the left indicate clean zones- e.g.,
sand, limestone etc. The interpreter focuses his attention immediately on these permeable
zones.
Next step: To scan the resistivity logs in track 2 to see which of the zones of interest gives
high resistivity readings. High resistivities reflect either hydrocarbons in the pores or low
porosity.
Next step: Scan the porosity logs on the track 3 to see which of the zones have good porosity
against the high resistivity zones. Discard the tight formations. Select the interesting zones for
the formation evaluation.
5. 3.5 FORMATION EVALUATION
Determining Geothermal Gradient
The first step involved in determining temperature at a particular depth is to determine the
geothermal gradient (gG) of the region. Temperature increases with depth, and the temperature
gradient of a particular region depends upon the geologic, or tectonic, activity within that
region. The more activity, the higher the geothermal gradient. Geothermal gradients are
commonly expressed in degrees Fahrenheit per 100 m (°F/100m).
If the geothermal gradient of an area is not known, then it can be determined by chart or by
formula.
gG= (BHT- Tms/TD) x100
Where:
BHT = bottom hole temperature (from header)
TD = total depth (Depth-Logger from header)
Tms = mean surface temperature
Determining Formation Temperature (Tf)
Once the geothermal gradient (gG) has been established, it is possible to determine the
temperature for a particular depth. This is often referred to as formation temperature (Tf).
Where:
Tms = mean surface temperature
gG = geothermal gradient
D = depth at which temperature is desired
Environmental Corrections
In actual logging conditions, porosity (Ø) and the "true" resistivity of the uninvaded zone (Rt)
cannot be measured precisely for a variety of reasons. Factors affecting these responses may
include hole size, mud weight, bed thickness, depth of invasion, and other properties of the
logging environment and formation. Many of these effects have strong impacts on analysis and
must be corrected prior to evaluating the formation. Several types of corrections and the tools
for which these corrections are necessary are illustrated in table 3.1
6. Table 3.1: Required Environmental Corrections
Correcting Resistivity for Temperature
Resistivity decreases with increasing temperature, and therefore any value of Rmf and/or Rw
determined at one depth must be corrected for the appropriate formation temperature (Tf)
where those values will be used to calculate water saturation (Sw). It is vital that formation
water resistivity (Rw) be corrected for temperature. Failing to correct Rw to a higher
temperature will result in erroneously high values of water saturation (Sw). Therefore, it is
possible to calculate a hydrocarbon-bearing zone as a wet zone if the temperature correction is
not applied.
Correction may be applied through the use of a chart (GEN-5) or an equation
(Arp's equation).
Where:
R2 = resistivity value corrected for temperature
R1 = resistivity value at known reference temperature (T1)
T1 = known reference temperature
T2 = temperature to which resistivity is to be corrected
k = temperature constant
k = 6.77 when temperature is expressed in °F
k = 21.5 when temperature is expressed in °C
7. Density porosity
Formation bulk density (ρb
) is the function of matrix density, porosity, and density of the fluid
in the pores (salt mud, fresh mud, or hydrocarbons). To determine density porosity, either by
chart or by calculation the matrix density and the type of fluid in the borehole must be known.
The formula for calculating the density porosity is:
Where;
ρma = matrix density of formation.
ρb = bulk density of the formation.
ρf = pore fluid density in the borehole.
Cross-Plot Porosity
There are a variety of methods--visual, mathematical, and graphical--used to determine the
cross-plot porosity of a formation. Porosity measurements taken from logs are rarely adequate
for use in calculating water saturation. There are two methods for the determination of
porosity:
1. Cross-Plot Porosity Equation
Where:
ΦD = density porosity
ΦN = neutron porosity
2. Cross- Plot Porosity from Chart
The proper Cross-Plot Porosity (CP) chart is determined first by tool type, and second by the
density of the drilling fluid.
8. SONIC POROSITY
Sonic Tool Cross-Plot Charts
The "Sonic versus Bulk Density" and "Sonic versus Neutron Porosity" charts may be
interpolated and extrapolated in the same manner as the "Bulk Density versus Neutron
Porosity" charts. These charts may be used as an alternative to the neutron-density cross-plots,
or an additional method for providing more information on the possible lithology of a
formation.
Wyllie-Time Average Equation:
Consolidated and compacted sandstones:
Unconsolidated sands:
Where:
∆tlog = travel time from the log.
∆tma = formation matrix travel time.
∆tf = fluid travel time
Cp = compaction factor.
Determining Formation Water Resistivity (Rw) by the Inverse Archie Method:
Determining a value for formation water resistivity (Rw) from logs may not always provide
reliable results; however, in many cases logs provide the only means of determining Rw. Two
of the most common methods of determining Rw from logs are the inverse-Archie method and
the SP method. Another method of Rw determination is by means of Hingle plot.
INVERSE ARCHIE METHOD: Rwa
Where:
Rt = resistivity of the uninvaded zone
Φ = porosity
9. Sw Calculations:
Water saturation may now be calculated for those zones that appear to be hydrocarbon bearing.
The water saturation equation for clean formations is as follows:
Archie's Equation
Where:
Sw = water saturation
n = saturation exponent
a = tortuosity factor.
Φ= porosity.
m = cementation exponent.
Rt = formation resistivity
Rw = formation water resistivity
Among the most difficult variables to determine, but one which has a tremendous impact upon
calculated values of water saturation (Sw). Often best obtained from the customer, but can be
obtained from logs under ideal conditions. Other sources include measured formation water
samples (DST or SFT), produced water samples, or simply local reservoir history.
Moveable Hydrocarbon Index (MHI)
One way to investigate the moveability of hydrocarbons is to determine water saturation of the
flushed zone (Sxo). This is accomplished by substituting into the Archie equation those
parameters pertaining to the flushed zone.
Where:
Rmf = resistivity of mud filtrate.
Rxo = resistivity of flushed zone.
10. Once flushed zone water saturation (Sxo) is calculated, it may be compared with the value for
water saturation of the uninvaded zone (Sw) at the same depth to determine whether or not
hydrocarbons were moved from the flushed zone during invasion. If the value for Sxo is much
greater than the value for Sw, then hydrocarbons were likely moved during invasion, and the
reservoir will produce.
An easy way of quantifying this relationship is through the moveable hydrocarbon index
(MHI).
11. SHALYSAND INTERPRETATION
The presence of shale (i.e. clay minerals) in a reservoir can cause erroneous results for water
saturation and porosity derived from logs. These erroneous results are not limited to
sandstones, but also occur in limestones and dolomites.
Whenever shale is present in the formation, porosity tools like, (sonic and neutron) will record
too high porosity. The only exception to this is the density log. It will not record too high a
porosity if density of shale is equal to or greater than the reservoir’s matrix density. In addition,
the presence of shale in a formation will cause resistivity logs to record lower resistivity.
Calculation of Vshale:
The first step in the shalysand analysis is the calculation of volume of shale from a gamma ray
log. Volume of shale from gamma ray log is determined by the chart or by the following
formulas:
Where:
IGR = gamma ray index
GRlog = actual borehole-corrected GR response in zone of interest
GRmin = minimum borehole-corrected GR response against clean zones
GRmax = maximum borehole-corrected GR response against shale zones
Determining Effective Porosity (Φe):
The second step of shaly sand analysis is to determine the effective porosity of the formation
i.e. determining porosity of the formation if it did not contain clay minerals.
Effective Porosity from Neutron-Density Combinations:
Φn-corrected = Φn - (Vcl x Φnsh) For Neutron
Φd-corrected = Φd - (Vcl x Φdsh) For Density
12. These values of neutron and density porosity corrected for the presence of clays are then used
in the equations below to determine the effective porosity ( effective) of the formation of
interest.
Determining Water Saturation (Sw) :( Indonesian Equation)
There are many different equations by which water saturation (Sw) of a clay-bearing formation
may be calculated. However, the most suitable equation is the Indonesian Equation, which is as
follow
Where:
Rt = resistivity of uninvaded zone
Vcl = volume of clay
Φe = effective porosity
Rcl = resistivity of clay
Rw = resistivity of formation water
