A PROCESS FOR SEQUENTIALLY EXTRACTING FREE OIL AND
ADSORBED OIL FROM A CORE PLUG
FIELD OF THE INVENTION
[OOOl] The present invention relates generally to
extraction of oil from oil reservoirs. In particular, the
present invention relates to sequential extraction of free
oil and adsorbed oil from core plugs drilled from the oil
reservoirs.
BACKGROUND OF THE INVENTION
[0002] Geochemical investigations have opened up new
perspectives in petroleum system analysis. Based on these
investigations an attempt can be made to analyze and
understand oil filling history of the oil reservoirs. While
these studies or investigations are selective, as selected
reservoir core plugs may not be fully representative of the
entire reservoir, the findings are still very encouraging
and are open to further modifications and interpretations.
[0003] To investigate and determine the oil filling
history of a reservoir, recovery of separate free oil and
adsorbed oil is critical. The absorbed oil corresponds to
an initial oil charge that enters a reservoir before the
free oil charge or free oil. The free oil or producible oil
corresponds to a later charge which forces the adsorbed oil
or initial oil charge to enter smaller pores of source
rocks. The adsorbed oil, which in terms of its chemical
composition does not usually correspond to the free oil,
contains valuable information about charging or oil filling
history of the reservoir. Still, the adsorbed oil has
received comparatively less attention in terms of full
geochemical characterization, largely because of an
inadequate methodology of its extraction.
[0004] Earlier oil extraction studies on crushed and
ground rocks gave no convincing evidence that free oil and
adsorbed oil can be separated sufficiently to allow
discrimination of different oil charges. Further,
conventional techniques of extraction of oil from core
plugs provide only an average composition of residual or
adsorbed oil, which is mostly different from produced oil.
As a result of this, the information on the charging history
of a reservoir, which is contained in the adsorbed oil,.
gets diluted beyond detectability and thus, resulting in
poor analysis of the oil filling history of the reservoirs.
[OOOS] To overcome the limitations of the earlier
extraction techniques, generic sequential oil extraction
techniques have been adopted to extract the free oil and
adsorbed oil. The sequential oil extraction technique is a
non-destructive extraction method that yields both adsorbed
oil and free oil separately. The sequential oil extraction
technique theoretically reverses the process of natural
reservoir filling, such that the first oil recovered
represents the last oil charge and vice versa (i-e., first
in - last out). Therefore, the sequence in which free and
adsorbed oils are removed from the core may help
reconstruct the accumulation history of petroleum
reservoirs, opening up new perspectives in petroleum system
analysis.
[0006] However, sequential oil extraction techniques,
which are generic and are already known in the art, do not
give desired and convincing results. These sequential
extraction techniques are further inefficient as they
employ five to six steps to extract free and adsorbed oils.
Furthermore, in these sequential extraction techniques,
there are no predefined or pre-calculated amounts of the
extraction solvents because of which different fractions
are mixed for extraction of free oil and the adsorbed oil.
This mixing further contributes towards unconvincing
results.
[0007] Thus, in light of the above, there is a need of
an effective and efficient process for recovering or
extracting separate free oil and adsorbed oil from the core
plugs.
SUMMARY OF THE INVENTION
[OOOS] In an embodiment of the present invention, a
process for sequentially extracting recoverable oils from
a core plug is provided. The process comprises extracting
a first oil from the core plug by passing a first solvent
through the core plug. In an embodiment of the present
invention, the first solvent is a 93:7 volume mixture of
dichloromethane-methanol. The process further comprises
collecting 300-500 millilitres of the first solvent after
it has passed through the core plug. The collected first
solvent comprises the free oil. In an embodiment of the
present invention, the extracted first oil is 91-96% of
total recoverable oil in the core plug and exists as free
oil in the core plug. The process furthermore comprises
performing geochemical investigations on the extracted free
oil to determine amount of the free oil in the oil
reservoir.
[0009] The process further comprises extracting a second
oil from the core plug by passing a second solvent through
the core plug. In an embodiment of the present invention,
the second solvent is a 50:50 volume mixture of chloroformmethanol.
The process further comprises collecting 300-500
millilitres of the second solvent after it has passed
through the core plug. The collected second solvent
comprises adsorbed oil. In an embodiment of the present
invention, the extracted second oil is 4-9% of the total
recoverable oil in the core plug and exists as adsorbed oil
in the core plug. The process furthermore comprises
performing geochemical investigations on the extracted
adsorbed oil to determine amount of the adsorbed oil in the
oil reservoir.
[OOlO] The process further . comprises performing
geochemical investigations on the extracted free oil and
the adsorbed oil to analyze and determine oil filling
history of the oil reservoir.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[OOll] The present invention is described by way of
embodiments illustrated in an accompanying drawing wherein:
[0012] FIG. 1 is a flowchart illustrating a process for
sequentially extracting recoverable oils from a core plug
in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] A process for extracting free oil and adsorbed
oil from a core plug drilled from an oil reservoir is
discussed herein. The present invention provides for a
process that efficiently separates out or extracts 91-96%
of total recoverable oil in the core plug, existing as free
oil, by passing a first solvent through the core plug. The
process further efficiently separates out 4-9% of the total
recoverable oil in the core plug, existing as adsorbed oil,
by passing a second solvent through the core plug.
[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.
[0016] FIG. 1 is a flowchart illustrating a process for
extracting recoverable oils from a core plug drilled from
an oil reservoir in accordance with an embodiment of the
present invention. In an exemplary embodiment of the
present invention, the extraction of the recoverable oils
from the core plug is carried out using a Reservoired
Conditioned Core Flow (RCCF) apparatus. Thus, prior to
recovering the oils, the drilled core plug is first trimmed
to have desired dimensions as per the RCCF apparatus. The
core plug is then placed and fitted in extraction cell or
core plug holder of the RCCF apparatus. Thereafter,
extraction of recoverable oils from the core plug is
initiated. It may be apparent to a person of ordinary skill
in the art that the RCCF apparatus is well known in the art
and therefore, to maintain brevity of the specification,
has not been explained in great detail.
[0017] At step 102, a first solvent is passed through
the core* plug for separating or extracting a first oil from
the core plug. The first solvent is a 93:7 volume mixture
of dichloromethane-methanol. The first solvent flows
through the core plug and separates out the first oil. The
first solvent, containing the first oil, is then collected
at the outlet of the RCCF apparatus in a pre-calculated
amount. In an embodiment of the present invention, the precalculated
amount of the first solvent is in a range of
300-500 millilitres.
[0018] At step 104, a second solvent is passed through
the core plug for separating or extracting a second oil
from the core plug. In an embodiment of the present
invention, the second solvent is a 50:50 volume mixture of
chloroform-methanol. The second solvent flows through the
core plug and separates out the second oil. The second
solvent, containing the second oil, is then collected at
the outlet of the RCCF apparatus in a pre-calculated
amount. In an embodiment of the present invention, the precalculated
amount of the second solvent is in a range of
300-500 millilitres.
[0019] After the first oil and the second oil has been
extracted, geochemical investigations are performed on the
collected first solvent (containing extracted first oil)
and the collected second solvent (containing extracted
second oil). The outcomes of the geochemical investigations
performed on the first solvent and the second solvent are
further correlated with results of geochemical
investigations performed on crude oil produced from the
same reservoir from which the core plug has been drilled.
In an embodiment of the present invention, the correlation
facilitates in ascertaining whether the extracted first oil
(contained in the first solvent) and the second oil
(contained in the second solvent) correspond to free oil
and adsorbed oil respectively or not. The absorbed oil
corresponds to an initial oil charge or initial oil that
enters the oil reservoir before the free oil charge or free
oil. The free oil or producible oil corresponds to a later
charge which forces the adsorbed oil or initial oil charge
to enter smaller pores of source rocks. In another
embodiment of the present invention, the correlation
facilitates in determining how much free oil or producible
oil is available in the reservoir which can be extracted
or produced as crude oil from the reservoir. In other words,
geochemical investigations on the extracted first oil and
the second oil, in light of the produced crude oil from the
reservoir, facilitate in determining impact or role of the
free oil and adsorbed oil in accumulation of the producible
crude oil in the reservoir. This determination further
helps in analyzing the oil filling history of the
reservoir. In an embodiment of the present invention, the
geochemical investigations are performed using, without any
limitation, column chromatography, Gas Chromatography Mass
Spectrometry (GCMS) analysis, stable carbon isotopic
analysis, and bulk and gross characteristics analysis.
[0020] The geochemical investigations begin with
fractionation of the collected first solvent (containing
extracted first oil) and the second solvent (containing the
second oil), and produced crude oil from the reservoir into
saturate and aromatic compounds. In an exemplary embodiment
of the present invention, column chromatography is employed
for the fractionation and is done using a 1:l volume mixture
of alumina-silica gel by successive elution with petroleum
ether and benzene. In an embodiment of the present
invention, the outcome of the fractionation facilitates in
determining that the first solvent collected at the outlet
of the RCCF apparatus comprises 91-96% of total recoverable
oil in the core plug. It is further determined that the
second solvent collected at the outlet of the RCCF
apparatus comprises 4-9% of the total recoverable oil in
the core plug. It may be apparent to a person or ordinary
skill in the art that the total quantity of the recoverable
oil e , the first oil and the second oil, in the core
plug may be determined based on the size and weight of the
core plug drilled from the reservoir.
[0021] The fractionated saturate fraction is then
further separated into normal alkanes and
branched/cycloalkanes by urea adduction. The saturate
fraction of the oils of core extracts are then analyzed for
normal and isoprenoid alkanes distribution. In an exemplary
embodiment of the present invention the analysis for normal
and isoprenoid alkanes distribution is performed on Varian
CP-3800 Gas Chromatograph using Wall Coated Open Tubular
(WCOT) fused silica capillary column CP SIL 8 CB (60m x
0.32mm x 0.25pm), programmed from 80-300"~ @ 4"~/min with
a hold time of 30 minutes and nitrogen as carrier gas.
Thereafter, branched/cycloalkanes are analyzed. In an
exemplary embodiment of the present invention, the
branched/cycloalkanes are analyzed on GC-MS Perkin Elmer
Clarus-500 using fused silica capillary column DB-1MS (30m
x 0.25rnm x 0.25pm) and Helium as carrier gas. The initial
oven temperature is kept at 80" C with 2 minutes of hold
time, increased by 3"C/minute up to 300°C and final hold
time of 30 minutes.
LO0221 After the analysis of saturate fraction, the
aromatic fractions are analyzed for the distribution of
phenanthrenes. In an embodiment of the present invention,
the aromatic fractions are analyzed on GC-MS Perkin Elmer
Clarus-500 using fused silica capillary column DB-5MS (30m
x 0.25 mm x 0.25ym) and Helium as carrier gas. The initial
oven temperature is kept at 60°C for 2 minutes, increased
by 3"C/minute up to 300°C and final hold time of 40 minutes.
[0023] Once the GC-MS analysis has been performed,
stable carbon isotopic analysis is carried out to determine
impact of extracted oils on origin of hydrocarbons or crude
oil in the reservoir. In an exemplary embodiment of the
present invention, stable carbon isotopic analysis is
performed on VG IsoPrime Continuous Flow-Isotope Ratio Mass
Spectrometer interfaced'with Euro EA elemental analyzer,
equipped with a chromium oxide-silvered cobaltous-cobaltic
oxide oxidation reactor at 1020°c and a copper reduction
tube maintained at 650"~.
[0024] Finally, bulk and gross characteristics of the
oils extracted from the core plug are analyzed. In an
exemplary embodiment of the present invention, the bulk and
gross characteristics of the extracted oils are analyzed
using American Society for Testing and Materials (ASTM)
test methods. The results of the analysis indicate that the
bulk characteristics of the oil extracted at the first step
(102) are similar to that of the crude oil produced from
the reservoir i.e., high aliphatic contents, moderate
aromatic contents, and low Nitrogen Sulphur Oxygen (NSO)
contents. The results of the analysis further indicate that
the oil extracted at the second step (104) is characterized
by a sharp increase in NSO contents. Further, abundance of
saturated hydrocarbons in oils decrease from step one (102)
to step two (104) and abundance of polar compounds in oils
increase from step one (102) to step two (104). Thus, based
on the bulk and gross characteristics of the extracted
oils, it is ascertained that the first oil extracted in the
first step (102) of the sequential extraction process is
free oil or later charge. It is also ascertained that the
second oil extracted in the second step (102) of the
sequential extraction process is adsorbed oil or an early
charge.
[0025] Based on the outcomes of the geochemical
investigations various inferences may be drawn to determine
the oil filling pattern or history of the reservoir. It may
be apparent to a person of ordinary skill in the art that
such analysis and determination of the oil filling history
facilitates in exploration and exploitation of reservoir's
crude oil. In an embodiment of the present invention, it
may be inferred that petroleum accumulation in the
reservoir is a contribution of two distinct charges i-e.,
both free oil and adsorbed oil. In another embodiment of
the present invention, it may be inferred that petroleum
accumulation in the reservoir is based on the later charge
i.e., free oil. In yet another embodiment of the present
invention, it may be inferred that petroleum accumulation
in the reservoir is a result of continuous flow of
increasingly mature petroleum. In yet another embodiment
of the present invention, it may be inferred that petroleum
accumulation in the reservoir has some contribution from
late charge i. e., free oil. In yet another embodiment of
the present invention, it may be inferred that petroleum
accumulation in the reservoir has some contribution from
early charge i.e., adsorbed oil. In yet another embodiment
of the present invention, it may be inferred that petroleum
accumulation in the reservoir may have received two
petroleum charges of increasing maturity. Thus, based on
the inferences, the oil filling history of the reservoir
may be analyzed to understand petroleum system of the
reservoir or basin.
100261 Thus, the present invention teaches an efficient
way to distinctly separate out the free oil and adsorbed
oil from the core plug in just two steps and with two
solvents. This efficient separation of oils from the core
plugs facilitates in determining how much free oil or
producible oil can be extracted from the oil reservoir or
facilitates in determining amount of the free oil in the
oil reservoir. The separation of oils from the core plugs
further facilitates in determining how much oil in the
reservoir is absorbed oil (or dead oil or residual oil)
which cannot be extracted.
[0027] While the exemplary embodiments of the present
invention are described and illustrated herein, it will be
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 sequentially extracting recoverable oils
from a core plug, the process comprising:
extracting a first oil by passing a first solvent
through the core plug, wherein the extracted first oil is
91-96% of total recoverable oil in the core plug; and
extracting a second oil by passing a second solvent
through the core plug, wherein the extracted second oil is
4-9% of the total recoverable oil in the core plug.
2. The process as claimed in claim 1, wherein the first oil
is free oil.
3. The process as claimed in claim 1, wherein the first
solvent is a 93:7 volume mixture of dichloromethanemethanol.
4. The process as claimed in claim 1 further comprising
collecting 300-500 millilitres of the first solvent after
it has passed through the core plug, wherein the collected
first solvent comprises free oil.
5. The process as claimed in claim 1, wherein the second
oil is adsorbed oil.
6. The process as claimed in claim 1, wherein the second
solvent is a 50:50 volume mixture of chloroform-methanol,
7. The process as claimed in claim 1 further comprising
collecting 300-500 millilitres of the second solvent after
it has passed through the core plug, wherein the collected
second solvent comprises adsorbed oil.
8. The process as claimed in claim 1 further comprising
performing geochemical investigations on the extracted
first oil to determine amount of the first oil in the oil
reservoir.
9. The process as claimed in claim 1 further comprising
performing geochemical investigations on the extracted
second oil to determine amount of the second oil in the oil
reservoir.
10. The process as claimed in claim 1 further comprising
performing geochemical investigations on the extracted
first oil and the second oil to analyze oil filling history
of the oil reservoir.