FIELD OF THE INVENTION
[0001] The present invention relates generally to oil reservoirs. In particular, the present invention relates to
characterizing low resistivity pay zones.
BACKGROUND OF THE INVENTION
[0002] Clays along with pyrites, heavy minerals, conductive
minerals and salinity of sediments have been identified as
leading cause of low resistivity pay zones. For such lowresistivity
pay zones, an accurate determination of the
petrophysical parameters and the characterization of lowresistivity
pay zones has always proven difficult. For example,
it gets difficult to distinguish clay from shale with existing
wireline measurements for determining various petrophysical
parameters. The log analysis in low resistivity pays also gets
complicated due to presence of clay.
[0003] Further, there are situations when reasons for low
resistivity in pay zones or characterization of low resistivity
pay zones is desired in real time and at the drill site when the
drilling operations are ongoing or about to begin. In such
situations, waiting for analysis of logs and oil field data,
which is conducted at far off base laboratories, may not be an
efficient and economical way of collecting critical and time
sensitive information regarding low resistivity of pay zones.
[0004] Furthermore, log analysis that are carried out at
remotely located base laboratories are conducted by experts
using state of art equipment. Also, it is the core from the oil
2
field which gets analysed at the remote laboratories and not the
samples which are obtained from the drill sites.
[0005] Thus, in light of the above, there is a need for an
effective and efficient process for characterising low
resistivity pay zones. Further, there is a need for a process
for characterising low resistivity pay zones that can be carried
out at the drill site in real time. In addition, there is a need
for a process for characterising low resistivity pay zones that
just utilizes the samples obtained from the drill site.
SUMMARY OF THE INVENTION
[0006] A process for characterizing one or more low
resistivity pay zones in an oil field is provided. The process
comprises obtaining one or more samples from a drill site
comprising one or more low resistivity pay zones. The low
resistivity of the one or more pay zones may be caused due to
presence of at least one of: conductive clay minerals, pyrite,
and sediment salinity in the one or more pay zones. The process
further comprises characterizing, in real time and at the drill
site, the one or more low resistivity pay zones by conducting
one or more tests on the obtained samples. In an embodiment of
the present invention, the one or more tests comprise at least
one of: Cation Exchange Capacity (CEC) test for identifying
clays in the pay zone, test for estimating degree of pyritization
in the pay zone, and a test for estimating sediment salinity in
the pay zone.
[0007] In an embodiment of the present invention, the CEC
test for identifying clays in the pay zone comprises: taking 0.2
grams to 0.6 grams of the obtained sample in a conical flask;
hydrating the sample with 10 ml of water for 10 to 15 hours;
treating the hydrated sample with 0.5 ml of sulphuric acid;
heating the treated mixture for up to 10 mins; adding, stepwise,
3
excess of 0.01M methylene blue to the heated mixture; shaking
the mixture manually for 5 minutes and allowing the mixture to
settle down; and calculating concentration of unreacted
methylene blue at predefined wavelengths using an Ultra Violet
(UV) spectrophotometer. The predefined wavelength is one of: 664
nanometre (nm), 292nm, and 245nm. Further, the calculated
concentration of unreacted methylene blue provides CEC values
of type of clay content present in the sample. In an embodiment
of the present invention, the clay content comprises at least
one of: Kaolinite, Halloysite.2H20, Halloysite.4H20,
Montmorillonite, Illite, Vermicullite, Chlorite, Glauconite,
Attapulgite, Allophane.
[0008] In an embodiment of the present invention, the test
to estimate Degree of Pyritization (DoP) comprises: determining
quantity of pyrite iron (%Fepyrite) in the obtained sample;
determining quantity of reactive iron (%FeReactive) in the
obtained sample; and estimating the DoP using the equation DoP
= %Fepyrite / (%Fepyrite + %FeReactive). The pyrite iron is
determined by mixing 0.5 grams of the obtained sediment sample
with 25 ml of distilled water in a beaker; adding 15 ml of 20
percent zinc acetate solution and 285 ml of distilled water to
the mixture; heating the mixture; adding diluted zinc acetate
solution and 3 ml of 50 percent sodium hydroxide (NaOH) solution
to the mixture; filtering alkaline zinc acetate mixture through
Whatmann filter number 42 and discarding the filtrate; adding
45 ml of distilled water and 5 ml of concentrated nitric acid
to the residue; adding 5 ml of 20mM ethanolic o-phenantroline
(o-phen) solution and 5ml of 20mM of Ethylene Diamine Tetra
Acetic acid disodium salt dihydrate (EDTA) solution to stabilize
oxidation state of the solution; pipetting out 0.5 ml of the
solution and diluting it to 50 ml volume; and calculating
concentration of unreacted methylene blue at predefined
wavelengths using an UV spectrophotometer, wherein the
4
calculated concentration of unreacted methylene blue provides
concentration of the pyrite iron. The predefined wavelengths
comprise at least one of: 245, 254, 266 and 510nm.
[0009] The reactive iron is determined by mixing 0.5 grams
of the obtained sediment sample with 5 ml concentred
Hydrochloric Acid (HC1) in a beaker; heating the mixture at 70°C
for up to five minutes; quenching the reaction by adding 45 ml
of distilled water; adding 5 ml of 20mM ethanolic ophenantroline
(o-phen) solution and 5 ml of 20mM of EDTA solution
stabilize oxidation state of the solution; pipetting out 0.5 ml
of the solution and diluting it to 50 ml volume; and calculating
concentration of unreacted methylene blue at predefined
wavelengths using an UV spectrophotometer, wherein the
calculated concentration of unreacted methylene blue provides
concentration of the reactive iron. The predefined wavelengths
comprise at least one of: 245, 254, 266 and 510nm.
[0010] In an embodiment of the present invention, the test
for estimating sediment salinity comprises: obtaining 30-40
grams of sedimentary rock sample; hydrating the obtained sample
with distilled water for 24 hours; decanting 1 ml to 5 ml of the
hydrated sample; titrating the 1 ml to 5 ml of the hydrated
sample with a predefined quantity of AgNCb (Silver Nitrate),
wherein a brick red colour during the titration indicates
sediment salinity in the sample; and estimating the quantity of
the sediment salinity using the equation: sediment salinity
(ppm)= (volume of AgN03*normality of AgNO3*58500)/ volume of
obtained sample.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0011] The present invention is described by way of
embodiments illustrated in an accompanying drawing wherein:
5
[0012] FIG. 1 is a flowchart illustrating a process for
characterizing low resistivity pay zones in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] A process for characterizing one or more low
resistivity pay zones in an oil field is discussed herein. The
process comprises conducting one or more tests on one or more
samples obtained from a drill site comprising one or more low
resistivity pay zones. The outcome of the tests facilitates
characterization of the low resistivity pay zones.
[0014] The following disclosure is provided in order to
enable a person having ordinary skill in the art to practice the
invention. Exemplary embodiments are provided only for
illustrative purposes and various modifications will be readily
apparent to persons skilled in the art. The general principles
defined herein may be applied to other embodiments and
applications without departing from the spirit and scope of the
invention. Also, the terminology and phraseology used is for the
purpose of describing exemplary embodiments and should not be
considered limiting. Thus, the present invention is to be
accorded the widest scope encompassing numerous alternatives,
modifications and equivalents consistent with the principles and
features disclosed. For purpose of clarity, details relating to
technical material that is known in the technical fields related
to the invention have not been described in detail so as not to
unnecessarily obscure the present invention.
[0015] The present invention would now be discussed in
context of embodiments as illustrated in the accompanying
drawings.
6
[0016] FIG. 1 is a flowchart illustrating a process for
characterizing low resistivity pay zones. At step 102 one or
more samples are obtained from a drill site containing one or
more low resistivity pay zones. It may be apparent to a person
of ordinary skill in the art that the pay zone is a term used
to describe a reservoir that is producing oil or gas within a
particular wellbore. A pay zone is a reservoir or part of a
reservoir that contains hydrocarbons that can be extracted
economically. Generally, the oil bearing rocks in the pay zones
have a high resistivity and thus indicate presence of oil in the
reservoir. However, this high resistivity gets lower due to
presence of clay material, pyrite, conductive minerals and
sediment salinity in the oil bearing rocks. Due to this low
resistivity there is a possibility that these pay zones, despite
having oil bearing rocks, may get ignored as hydrocarbon zones.
Thus, understanding the causes of low-resistivity or
characterization of the pay zones becomes critical in efficient
and effective recovery of oil reserves that would otherwise be
left behind.
[0017] Further, after obtaining the samples from the pay
zones, the one or more low resistivity pay zones are
characterized by conducting one or more tests on the obtained
samples. In an embodiment of the present invention, the one or
more tests are conducted at the drill site and not at far off
or remotely located base laboratories. The one or more tests
further do not require high-end laboratory equipment and thus
may be conducted by non-expert people at the drill site. In an
embodiment of the present invention, the one or more tests
comprise a test for estimating Cation Exchange Capacity (CEC)
values for identifying clay mineral content, a test for
estimating Degree of Pyritization (DoP), and a test for
estimating sediment salinity.
7
[0018] The CEC test is useful for identifying clays and their
physical and chemical properties as clays have been identified
as a leading cause of low resistivity of the oil filled rocks.
Contribution of clay to low resistivity depends on the type,
volume and distribution of clay in the formation. Clays are
distributed in the formation as laminar shales, dispersed clays
and structural clays. Clay minerals have a substantial negative
surface charge that causes log resistivity values to fall. This
negative surface charge attracts cations such as Na+ and K+ when
the clay is dry. When the clay is immersed in water, cations are
released and thereby, increasing water conductivity.
[0019] Further, the CEC is the total capacity of a clay or
soil to hold exchangeable cations. Thus, conducting CEC tests
gives an indication of how much clay content is present in the
pay zone. In an embodiment of the present invention, a CEC value
less than 20 indicates low clay content, a CEC value between 20
and 30 indicates a moderate clay content and a CEC value more
than 30 indicates a high clay content in the pay zone. Further,
different types of clays have different CEC values. Thus, the
CEC test also facilitates in identifying the type of clay that
is responsible for low resistivity of the pay zone.
[0020] At step 104, the CEC test is conducted. The CEC test
begins with taking a predefined quantity of the obtained sample
in a conical flask at step 1. In an embodiment of the present
invention, the quantity of the sample may be from 0.2 grams to
0.6 grams. At step 2, the sample is hydrated with 10 ml of water
for 10 to 15 hours. At step 3, the hydrated sample is treated
with 0.5 ml of sulphuric acid. At step 4, the treated mixture
is heated for up to 10 mins. Thereafter, at step 5, 0.01 molar
(M) of methylene blue concentration is added in a stepwise manner
to the treated mixture. It may be apparent to a person of
ordinary skill in the art that methylene blue concentration may
8
be calibrated prior to using for the CEC test. Table 1
illustrates exemplary calibrated data for methylene blue
concentration for three different wavelengths i.e. 664 nanometre
(nm), 292nm, and 245nm.
S. No.
1
2
3
4
5
6
Sample
ID
Blank
0.25 ppm
0.63 ppm
1.27 ppm
2.55 ppm
5.10 ppm
Absorbance
(664 nm) Xmax
0.0033
0.0676
0.1612
0.3169
0.4869
1.1862
Absorbance
(292 nm) Amax
0.00
0.0221
0.0669
0.01448
0.2392
0.5646
Absorbance
(245 nm) Amax
0.00
0.00
0.0048
0.0324
0.0750
0.2301
Table 1
[0021] Thereafter, at step 6, the mixture is stirred or
shaken manually for 5 minutes and is allowed to settle down.
Finally, at step 7, an Ultra Violet (UV) spectrophotometer is
used to calculate concentration of unreacted methylene blue in
the settled mixture at a predefined wavelength. In an embodiment
of the present invention, the predefined wavelength may be one
of: 664 nm, 292nm, and 245nm. The readings from the calibration
data of methylene blue concentration along with readings from
UV spectrophotometer facilitate in determining CEC values of the
samples.
[0022] In an embodiment of the present invention, the result
of the CEC test for absorbance at a wavelength of 664nm has been
summarized in TABLE 2. As it can be seen from the test results,
for a sample of 0.520 grams a CEC value of 6.10 is determined.
This CEC value of 6.10 indicates presence of Kaolinite and
Halloysite.2H2O clays in the tested sample obtained from the
drill site. The CEC value of 6.10 further indicates that the
9
content of the identified clays is low. Further, for a sample
of 0.273 grams a CEC value of 23.8 is determined. The CEC value
of 23.8 indicates presence of Illite, Chlorite, Glauconite, and
Attapulgite clays clay in the tested sample. The CEC value of
23.8 further indicates that the content of the identified clays
is moderate. Furthermore, for a sample of 0.547 grams a CEC
value of 10.96 is determined. This CEC value indicates presence
of Kaolinite, Illite, and Chlorite clays. The CEC value of 10.96
further indicates that the content of the identified clays is
low. Finally for sample of 0.288 grams a CEC value of 26.9 is
determined. This CEC value indicates presence of clays like
Illite, Chlorite, Glauconite, and Attapulgite in the tested
sample. The CEC value of 26.9 further indicates that the content
of the identified clays is moderate. Thus, with the help of the
CEC test of obtained samples, the one or more low resistivity
pay zones may be characterized. The characterization gives the
reason for the low resistivity of the pay zone i.e. which clay
content and how much content of that clay is responsible for the
low resistivity of the pay zone.
Sample
ID
G-l
G-4
G-27
G-3 6
Weight
of
Sample
(mg)
0.520
0.273
0.547
0.288
Absorbance
(664 nm)
Xmax
0.318
0.5866
0.551
0.688
Dilution
Factor
25
25
25
25
Concentration
Obtained
1.27 ppm
2.6 ppm
2.4 ppm
3.1 ppm
Quantity
of
unreacted
MB
95.25 ppm
62 ppm
67 ppm
4 9.5 ppm
CEC
Value
6.10
23.8
10.96
26.9
Clay
determination
by interpreting
CEC results
Kaolinite and
Halloysite.2H20
Illite,
Chlorite,
Glauconite, and
Attapulgite
Kaolinite,
Illite, and
Chlorite
Illite,
Chlorite,
Glauconite, and
Attapulgite
TABLE 2
[0023] Another reason for low resistivity of pay zones is the
presence of small grain size and conductive minerals like Pyrite
(FeS2) . Pyrite is a common heavy mineral associated with marine
sedimentary rocks. It has a good electrical conductivity that
10
is usually comparable to, or even higher than the conductivity
of the formation water. The small grain size of the Pyrite can
result in low resistivity values over an interval, despite
uniform mineralogy and clay content. The increased surface area
associated with finer grains holds more irreducible water, and,
as with clay coated grains, the increasing water saturation
reduces the resistivity in the pay zones.
[0024] At step 106, the test for estimating DoP is conducted.
The test comprises determination of reactive iron and pyrite
iron in the pay zone. Once the reactive iron and the pyrite iron
are determined, the DoP can be obtained using the equation: DoP
= %Fepyrite / (%Fepyrite + %FeReactive) where FePyrite is the pyrite iron
and FeReactive is the reactive iron. The DoP indicates how much
pyritizable iron is present in the pay zone sediment that is
causing low resistivity in the pay zone. In an embodiment of the
present invention, the pyrite iron may be determined using the
following process. At step 1, 0.5 grams of the obtained sediment
sample is mixed with 25 ml of distilled water in a beaker. At
step 2, 15 ml of 20 percent zinc acetate solution and 285 ml of
distilled water is added to the mixture of step 1. At step 3,
the beaker is placed on a hot plate. At step 4, while stirring
the diluted zinc acetate solution, 3 ml of 50 percent sodium
hydroxide (NaOH) solution is added to the mixture. At step 5,
the resultant mixture is warmed but not boiled. At step 6, the
alkaline zinc acetate mixture is filtered through Whatmann
filter number 42 and the filtrate is discarded. At step 7, 45
ml of distilled water and 5 ml of concentrated nitric acid is
added to the residue. Thereafter, at step 8, 5 ml of 20mM
ethanolic o-phenantroline (o-phen) solution and 5ml of 20mM of
Ethylene Diamine Tetra Acetic acid disodium salt dihydrate
(EDTA) solution is added so that stability in oxidation state
may be attained. At step 9, 0.5 ml of the solution is pipette
out and is diluted to 50 ml volume. Finally, at step 10, readings
11
for the pyrite iron are taken using the UV spectrophotometer at
predefined wavelengths. In an embodiment of the present
invention, the predefined wavelengths may comprise 245, 254, 266
and 510nm.
[0025] In an embodiment of the present invention, the
reactive iron may be determined using the following process. At
step 1, 0.5 grams of obtained sediment sample is taken and 5 ml
concentred Hydrochloric Acid (HC1) is added to sample and is
kept at 70°C on a heating mantle for five minutes. At step 2,
the reaction is quenched by adding 45 ml of distilled water. At
step 3, 5 ml of 20mM ethanolic o-phenantroline (o-phen) solution
and 5 ml of 20mM of EDTA solution is added so that stability in
oxidation state may be attained. At step 4, 0.5 ml of the
solution is pipette out and is diluted to 50 ml volume. Finally,
at step 5, readings for reactive iron are taken using the UV
spectrophotometer at predefined wavelengths. In an embodiment
of the present invention, the predefined wavelengths may
comprise 245, 254, 266 and 510nm.
[0026] Once the content of the pyrite iron and the reactive
iron has been estimated, the DoP may be estimated using the
above equation. By conducting the DoP test at the drill site and
without high-end laboratory equipment, pyritizable iron present
in the pay zone sediment may be identified and thus, the one or
more low resistivity pay zones may be characterized. The
characterization gives the reason for the low resistivity of the
pay zone i.e. presence of pyritizable iron in the pay zone.
[0027] Yet another reason for low resistivity of pay zones
is salinity in the sedimentary rocks. At step 108, the test for
presence of sediment salinity in the pay zones is conducted. At
step 1, a predefined quantity of sample from the sedimentary
rocks is obtained. In an embodiment of the present invention,
the predefined quantity of the sample may vary from 30 to 40
12
grams. At step 2, the sample is then hydrated with distilled
water, with occasional stirring, and is left for 24 hours. In
an embodiment of the present invention, distilled water may be
taken in a ratio of 1:1 of the sedimentary rock sample and may
be taken in a ratio of 1:4 when the clays have a tendency to
swell. At step 3, the hydrated sample or content is then decanted
and is taken in a predefined quantity for titration process. In
an embodiment of the present invention, the predefined quantity
of the hydrated sample or content may be 1 to 5 ml. At step 4,
the predefined quantity of content is titrated with Silver
Nitrate (AgN03) with an indicator for brick red colour i.e. the
brick red colour during the titration indicates sediment
salinity in the sample. Finally at step 5, the sediment salinity
is estimated using the equation sediment salinity (ppm)= (volume
of AgN03*normality of AgNC>3*58500) / volume of obtained sample.
Thus, by conducting the sediment salinity test at the drill site
and without high-end laboratory equipment, the amount of
sediment salinity present in the pay zone sediment may be
identified and thus, the one or more low resistivity pay zones
may be characterized. The characterization gives the reason for
the low resistivity of the pay zone i.e. presence of sediment
salinity in the pay zone.
[0028] Thus, the present invention teaches an efficient way
to characterize low resistivity pay zones at the drill site
itself. Further, rather than analysing cores at the remotely
located laboratories, the present invention provides a process
for characterising low resistivity pay zones using the samples
obtained from the drill sites. The tests conducted at the drill
site gave amazing results, well correlated with the test logs,
and helped in characterizing the low resistivity pay zones.
[0029] While the exemplary embodiments of the present
invention are described and illustrated herein, it will be
13
appreciated that they are merely illustrative. It will be
understood by those skilled in the art that various
modifications in form and detail may be made therein without
departing from or offending the spirit and scope of the invention
as defined by the appended claims.

We claim:
1. A process for characterizing one or more low resistivity pay
zones in an oil field, the process comprising:
obtaining one or more samples from a drill site comprising
one or more low resistivity pay zones; and
characterizing, in real time and at the drill site, the one
or more low resistivity pay zones by conducting one or more
tests on the obtained samples.
2. The process as claimed in claim 1, wherein presence of at
least one of: conductive clay minerals, pyrite, and sediment
salinity in the one or more pay zones causes low resistivity
of the one or more pay zones.
3. The process as claimed in claim 1, wherein the one or more
tests comprise at least one of: Cation Exchange Capacity (CEC)
test for identifying clays in the pay zone, test for
estimating Degree of Pyritization (DoP) in the pay zone, and
a test for estimating sediment salinity in the pay zone.
4. The process as claimed in claim 3, wherein the CEC test for
identifying clays in the pay zone comprises:
placing 0.2 grams to 0.6 grams of the obtained sample
in a conical flask;
hydrating the sample with 10 ml of water for 10 to 15
hours;
treating the hydrated sample with 0.5 ml of sulphuric
acid;
heating the treated mixture for up to 10 mins;
15
adding, stepwise, excess of 0.01M methylene blue to
the heated mixture;
shaking the mixture manually for 5 minutes and
allowing the mixture to settle down; and
calculating concentration of unreacted methylene blue
at predefined wavelengths using an Ultra Violet (UV)
spectrophotometer, wherein the calculated concentration of
unreacted methylene blue provides CEC values of type of
clay content present in the sample.
5. The process as claimed in claim 4, wherein the predefined
wavelength is one of: 664 nanometre (nm), 292nm, and 245nm.
6. The process as claimed in claim 3, wherein the test to
estimate DoP comprises:
determining quantity of pyrite iron (%Fepyrite) in the
obtained sample;
determining quantity of reactive iron (%FeReactive) in the
obtained sample; and
estimating the DoP using the equation DoP = %Fepyrite /
(%FeP y r i t e + %FeReactive) .
7. The process as claimed in claim 6, wherein the pyrite iron is
determined by:
mixing 0.5 grams of the obtained sediment sample with 25
ml of distilled water in a beaker;
adding 15 ml of 20 percent zinc acetate solution and 285
ml of distilled water to the mixture;
heating the mixture;
16
adding diluted zinc acetate solution and 3 ml of 50 percent
sodium hydroxide (NaOH) solution to the mixture;
filtering alkaline zinc acetate mixture through Whatmann
filter number 42 and discarding the filtrate;
adding 45 ml of distilled water and 5 ml of concentrated
nitric acid to the residue;
adding 5 ml of 20mM ethanolic o-phenantroline (o-phen)
solution and 5ml of 20mM of Ethylene Diamine Tetra Acetic
acid disodium salt dihydrate (EDTA) solution to stabilize
oxidation state of the solution;
pipetting out 0.5 ml of the solution and diluting it to 50
ml volume; and
calculating concentration of unreacted methylene blue at
predefined wavelengths using an UV spectrophotometer,
wherein the calculated concentration of unreacted methylene
blue provides concentration of the pyrite iron.
8. The process as claimed in claim 6, wherein the reactive iron
is determined by:
mixing 0.5 grams of the obtained sediment sample with 5 ml
concentred Hydrochloric Acid (HC1) in a beaker;
heating the mixture at 70°C for up to five minutes;
quenching the reaction by adding 45 ml of distilled water;
adding 5 ml of 20mM ethanolic o-phenantroline (o-phen)
solution and 5 ml of 20mM of EDTA solution stabilize
oxidation state of the solution;
pipetting out 0.5 ml of the solution and diluting it to 50
ml volume; and
calculating concentration of unreacted methylene blue at
predefined wavelengths using an UV spectrophotometer,
17
wherein the calculated concentration of unreacted methylene
blue provides concentration of the reactive iron.
9. The process as claimed in claims 7 and 8, wherein the
predefined wavelengths comprise at least one of: 245, 254,
266 and 510nm.
10. The process as claimed in claim 3, wherein the test for
estimating sediment salinity comprises:
obtaining 30-40 grams of sedimentary rock sample;
hydrating the obtained sample with distilled water for 24
hours;
decanting 1 ml to 5 ml of the hydrated sample;
titrating the 1 ml to 5 ml of the hydrated sample with a
predefined quantity of AgN03 (Silver Nitrate), wherein a
brick red colour during the titration indicates sediment
salinity in the sample; and
estimating the quantity of the sediment salinity using the
equation sediment salinity (ppm)= (volume of
AgNC>3*normality of AgNO3*58500)/ volume of obtained sample.