PARTE TERCERA
C.- LA CARRERA DE ARMAMENTOS
This and the following page outline a general interpretation flow which considers the availability of lithologic and other rock information in the determination of porosity. The more specific calculation methods which follow can be used within this general context.
Are two or more porosity Are two or more porosity
iques to determine logy and porosity.
scription and se the lithology
arameter.
iques to determine logy and porosity.
scription and se the lithology
arameter.
Is the lithology known?
(Go to the next page.) Are rock descriptions and/or measurements available?
Are two or more porosity measurements available?
Is a clay correction needed?
Are the c splot results and ck data in
agreement?
Use porosity crossplot techniques to determine
lithology and porosity.
Use the most likely estimate of lithology to calculate porosity.
Apply the clay correction.
measur ents available?
Use porosity crossplot techn
litho
Use rock de data to choo
matrix p Is a clay correction needed?
Apply the y correctio
Reconcile rock and log
Effective porosity, φe Depending on the
l
and the actual formation lithology,
Is the lithology known?
(Go to the next page.) Are rock descriptions and/or measurements available?
Are two or more porosity measurements available?
Is a clay correction needed?
Are the c splot results and ck data in
agreement?
Use porosity crossplot techniques to determine
lithology and porosity.
Use the most likely estimate of lithology to calculate porosity.
Apply the clay correction.
measur ents available?
Use porosity crossplot techn
litho
Use rock de data to choo
matrix p Is a clay correction needed?
Apply the y correctio
Reconcile rock and log
Effective porosity, φe Depending on the
l
and the actual formation lithology,
Openhole Interpretation
(Go to the previous page.) no yes (Go to the previous page.) no yes Is the lithology known?
Are rock descriptions and/or measurements available?
Is a clay correction needed?
Apply the clay correction.
Is a clay correction needed?
Apply the clay correction.
Reconcile rock and log
data.
Effective porosity,φe
yes no
yes no yes no
Are the porosity curve lithology (matrix) and fluid settings in agreement with the
known lithology?
Are the porosity curve lithology (matrix) and fluid settings in agreement with the
known lithology?
Compute the porosity using the matrix setting for the known
lithology.
Reconcile log, rock, and fluid data.
Compute porosities.
Is log-derived porosity in agreement with available rock
measurements?
yes no yes no
yes no Is the lithology known?
Are rock descriptions and/or measurements available?
Is a clay correction needed?
Apply the clay correction.
Is a clay correction needed?
Apply the clay correction.
Reconcile rock and log
data.
Effective porosity,φe
yes no
yes no yes no
Are the porosity curve lithology (matrix) and fluid settings in agreement with the
known lithology?
Are the porosity curve lithology (matrix) and fluid settings in agreement with the
known lithology?
Compute the porosity using the matrix setting for the known
lithology.
Reconcile log, rock, and fluid data.
Compute porosities.
Is log-derived porosity in agreement with available rock
measurements?
Openhole Interpretation
POROSITY FROM SINGLE MEASUREMENTS Density log
RhoFl = ρfl = fluid density (often assumed to be mud filtrate density)
Sonic log
Wyllie Time-Average Equation:
cp Bcp = compaction correction, where
0 . 100 ≥1
=DTShale Bcp
The Bcp factor was added to the equation when it was found that the equation gave highly optimistic porosity values in unconsolidated sands. DTShale is picked from a shale near the zone of interest. The correction factor is never less than 1.0.
Gardner-Hunt-Raymer Equation (Schlumberger Empirical Relation):
t
The above equation is an approximation of Schlumberger chart Por-3.
Neutron log
Except for the obsolete "Gamma Ray Neutron" tools, Neutron porosity is calculated by the acquisition software and is displayed directly on the log. This porosity is referenced to a specific lithology, usually limestone. Corrections to the porosity to account for the lithology actually present can be done through charts or appropriate algorithms.
OTE: It is important to use the chart or algorithm for the correct Neutron tool and acquisition ompany. Each tool has a unique lithologic response, and use of the wrong algorithm will result in erroneous porosity estimation.
N c
Openhole Interpretation
The old unts per second or API Units. In
s, increasing porosity is shown my movement of the curve to the left of the scale he newer tools which display porosity directly). These values can be converted to porosity through calibration to core data, or by rules
The core calibration and rules of thumb tend to apply geographic areas.
All Neutron tools can be run in cased holes to determine formation porosity. Corrections must be made for the pre
POROSITY FROM MEASUREMENT COMBINATIONS (CROSSPLOTS) sing two porosity measurements in an x-y plot (“crossplot”) tends to minimize some of the
ntal and lithologic effects which impact porosity estimation from individual tools, and etter estimates of porosity (and lithology) than by using a single porosity
To use the technique, the interpreter must assume the presenc two minerals may be used as long as they plot uniquely on the c
listed in the table below, lines for sandstone (quartz), limestone (calcite), and dolomite are shown.
The Neutron-De f the four possible crossplots, and is shown
here as an exam
© 1994 Halliburton er "gamma ray-neutron" tools will show response in co
these display (just like for t
of thumb which approximate the response.
only to specific reservoirs or over limited
sence of casing and cement.
U
environme produces b measurement.
e of some two-mineral pair. Any rossplot. For all the crossplots nsity crossplot is the most desirable o
ple.
Openhole Interpretation
To estimate the porosity of the point at (10, 2.50), a mineral pair must first be chosen. In this case, the pair is calcite-dolomite. The porosity is estimated by connecting the porosity values on each line and reading the porosity from the points location (in this case, about 9.5 percent or
termination section) summarizes the crossplot chniques. In general, the sonic-density crossplot is not recommended for porosity
etermination.
Most of unts of
0.095)
The table below (also shown in the lithology de te
d
the crossplots have algorithmic equivalents which are easier to use for large amo data.
Crossplot Advantages Limitations Neutron-Density Given two possible lithology
pair solutions, the porosity will remain relatively invariant between solutions.
In rough holes or in heavy drilling muds, the density data may be invalid.
The combination of neutron and density measurements is the most common of all porosity tool pairs.
Neutron-Sonic Given two possible lithology pair solutions, the porosity will remain relatively invariant between solutions.
The sonic is less sensitive to rough holes than the density.
The combination of sonic and neutron data (without the density) is not common.
Density (bulk density-Pe) Both measurements are made with the same logging tool; both will often be available.
The choice of lithology pair will have a significant effect of the estimation of porosity.
In rough holes or in heavy drilling mud, the data may be invalid.
The Pe measurement is relatively new, and will not be present in wells logged before about 1978.
Sonic-Density Best for identifying radioactive reservoirs, rather than predicting lithology and porosity:
Potential reservoirs will plot along the closely spaced lithology lines while shales will tend to fall toward the lower right of the plot. This can indicate the presence of radioactive
reservoirs which are intermingled with shales (which tend to have high radioactivity).
The choice of lithology pair will have a significant effect of the estimation of porosity.
The lithology lines are closely spaced, so any uncertainty in the measurements will produce large changes in the lithology and porosity estimates.
Openhole Interpretation
POROSITY FROM MEASUREMENT COMBINATIONS (QUICKLOOK) The Neutron and Density can be used in combination to determine porosity without using crossplot techniques. These are usually used as a “quicklook” technique, rather than a rigorous determination of porosity.
If the lithology and formation fluid are unknown:
φN
φ= 2+φD OH.16
Notes for use:
• The porosities should be recorded on limestone matrix in complex lithologies.
of the method in l yield slightly low porosity values (0 to 0.025)
g on the por .
• Use of the method in lues o ity which are very close to actual porosities.
If the lithology is known and th
• Use gas zones wil
dependin osity range and the value of Sxo
water or oil zones yields va f poros
e formation fluid is gas:
N
• The porosities should be recorded on, or corrected to, the actual formation lithology.
OF
T e general form of the equa ity
CALCULATION EFFECTIVE POROSITY tion to convert from total poros
h to effective porosity is:
φ = rosity measurement in a nearb shale
φ = porosity calculated from the measurement
shale value of the po y
Vshale = shale volume