System and Method for Delineating Reservoir Layer of Intermediate
Impedance
Field of the Invention
[OOOl] The present invention relates to a system and method for
delineating reservoir layer of intermediate acoustic impedance.
Background of the Invention
[0002] Conventional reservoir is a subterranean sedimentary rock
formation which has interconnected pore systems to store and permit
flow of hydrocarbon reserves. As such, identifying and delineating
presence of reservoirs in a subterranean formation is a prerequisite
to establish the presence of hydrocarbon reserves. In some
areas, presence of reservoir itself is indicative of presence of
hydrocarbons reserves. Potential of reservoirs is typically analysed
by building a model of the reservoir, using information that is
pertinent to its ability to store and transmit hydrocarbons.
Further, reservoir models are used to simulate production behaviour
of fluids (e.g. oil and gas reserves). The results of simulation are
used to identify additional exploratory location and to ascertain
optimal production techniques in order to maximize the production of
hydrocarbon reserves.
[0003] Reservoir layers are encased between various subterranean
layers, more often between the layers having similar impedance
values. Typically, reflected seismic signals from the subterranean
layer boundaries are acquired and analysed for identifying reservoir
layer in a subterranean formation. Acoustic impedance of the
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subterranean layers is a key parameter that translates into
reflected seismic signal and is used for identification of various
layers including reservoir layer. However, in some instances, the
subterranean layers encasing the reservoir layer have a significant
contrast in impedance by virtue of differences in their elastic
characteristics. For example, if the layers include a high impedance
shale layer and a low impedance coal layer, the encased reservoir
layer has an impedance lying in between impedances of coal and
shale. Conventional methods use reflection response of seismic
signals generated from boundary of high impedance shale layer and
reservoir layer and the boundary of reservoir layer and low
impedance coal layer to identify various subterranean layers. As
such, due to a limited frequency band of seismic data the reflection
responses do not depict any distinguishable difference that would
facilitate to interpret whether a reservoir layer between the shale
and coal is present or absent. Therefore, conventional methods fail
to interpret reservoir layer of intermediate impedance encased
between high impedance shale layer and low impedance coal layer. As
such, existing seismic data analysis techniques are not able to
delineate reservoir layer of intermediate impedance encased between
a high impedance shale layer and low impedance coal layer.
[0004] In light of the above, there is a need for a system and
method that facilitates delineation of reservoir layer having
intermediate impedance encased between high impedance shale layer
and low impedance coal layer.
Summary of the Invention
[0005] A system (100) for delineating reservoir layer of
intermediate acoustic impedance in a subterranean formation is
provided. In various embodiments of the present invention, the
system comprises a transition time measurement unit (104) configured
to measure transition time between a high acoustic impedance value
of an overlying layer to a low acoustic impedance value of an
underlying layer in the subterranean formation. The system further
comprises a transition time comparison unit (108) configured to
compare the measured transition time to a predetermined threshold
transition time. The predetermined threshold transition time is
determined by analysing impedance values of subterranean layers in
the absence of reservoir layer of intermediate impedance. Further,
the transition time comparison unit (108) is configured to determine
whether the measured transition time is higher or tending towards
the predetermined threshold transition time as a result of the
comparison. The system further comprises a mapping unit (110)
configured to analyse the result of comparison of the transition
time comparison unit (108) to determine presence of a reservoir
layer, the reservoir layer being encased between the high acoustic
impedance layer and the low acoustic impedance layer of subterranean
formation and having an intermediate acoustic impedance.
[0006] In an embodiment of the present invention, the transition
time measurement unit (104) is configured to estimate impedance
volume data of subterranean layers by performing seismic inversion
on seismic signals acquired from various layers of the subterranean
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formation. The transition time measurement unit (104) is further
configured to identify, based on the estimated impedance volume
data, impedance values that correspond to the high acoustic
impedance layer and the low acoustic impedance layer, respectively,
within a transition time interval. The transition time interval
represents transition of impedance values from high to low. Further,
the transition time measurement unit (104) is configured to
ascertain time taken to traverse between the impedance values of
high acoustic impedance layer and low acoustic impedance layer,
respectively. The transition time measurement unit (104) is further
configured to determine difference between the ascertained time
taken to traverse between the impedance value of the high acoustic
impedance layer and the impedance value of the low acoustic
impedance layer to measure the transition time from the high
acoustic impedance layer to the low acoustic impedance layer in the
subterranean formation.
[0007] In an embodiment of the present invention, the repository
(106) is configured to ascertain the predetermined threshold
transition time data based on predetermined data stored in the
repository. The predetermined data represents transition times
corresponding to different layers of the subterranean heterogeneous
formation having contrasting acoustic impedance values, in the
absence of a reservoir layer.
[0008] In an embodiment of the present invention, the mapping
unit (110) determines presence of the encased reservoir layer having
intermediate acoustic impedance, if, the measured transition time is
detected to be higher than the predetermined threshold transition
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time based on the results of comparison. In another embodiment of
the present invention, the mapping unit (110) is configured to
delineate the encased reservoir layer if the value of the measured
transition time traversing from the high impedance value of shale
layer to the low impedance value of coal layer is higher than the
predetermined threshold transition time. In another embodiment of
the present invention, the mapping unit (110) is configured to
estimate thickness of the delineated encased reservoir layer based
on difference between the measured transition time and the threshold
transition time. In yet another embodiment of the present invention,
the mapping unit (110) is configured to generate a reservoir facies
map based on sum of impedance values integrated over the transition
time interval. The reservoir facies map indicates presence of the
encased reservoir layer.
[0009] In an embodiment of the present invention, the high
acoustic impedance layer of the subterranean formation is a shale
layer. In another embodiment of the present invention, the low
acoustic impedance layer of the subterranean formation is a coal
layer.
[ OOlO] A method for delineating reservoir layer of intermediate
acoustic impedance in a subterranean formation is provided. In
various embodiments of the present invention, the method comprises
the steps of measuring transition time between a high acoustic
impedance value of an overlying layer to a low acoustic impedance
value of an underlying layer in the subterranean formation. The
method further comprises the steps of comparing the measured
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transition time, to a predetermined threshold transition time. The
predetermined threshold transition time is determined by analysing
impedance values of subterranean layers in the absence of reservoir
layer of intermediate impedance. The method further comprises
determining whether the measured transition time is higher or
tending towards the predetermined threshold transition time based on
the comparison. Further, the method comprises analysing the result
of comparison to determine presence of a reservoir layer, the
reservoir layer being encased between the high acoustic impedance
layer and the low acoustic impedance layer of subterranean formation
and having an intermediate acoustic impedance.
[ 0 0 11 ] In an embodiment of the present invention, measuring
transition time between the high acoustic impedance value of an
overlying layer to the low acoustic impedance value of an underlying
layer in the subterranean formation comprises estimating impedance
volume data of subterranean layers by performing seismic inversion
on seismic signals acquired from various layers of the subterranean
formation. The method also comprises identifying, based on the
estimated impedance volume data, impedance values that correspond to
the high acoustic impedance layer and the low acoustic impedance
layer, respectively, within a transition time interval. The
transition time interval representing transition of impedance values
from high to low. The method further comprises ascertaining time
taken to traverse between the impedance values of high acoustic
impedance layer and low acoustic impedance layer, respectively.
Further, the method comprises determining difference between the
ascertained time taken to traverse between the impedance value of
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the high acoustic impedance layer and the impedance value of the low
acoustic impedance layer to measure the transition time from the
high acoustic impedance layer to the low acoustic impedance layer in
the subterranean formation.
[0012] In an embodiment of the present invention, the
predetermined threshold transition time is determined by retrieving
predetermined data stored in a repository. The predetermined data
represents transition times corresponding to different layers of the
subterranean heterogeneous formation having contrasting acoustic
impedance values, in the absence of a reservoir layer.
[0013] In an embodiment of the present invention, presence of
the encased reservoir layer having intermediate acoustic impedance
is determined if the measured transition time is detected to be
higher than the predetermined transition time based on the results
of comparison. In another embodiment of the present invention,
presence of reservoir layer is determined by ascertaining if the
measured transition time of the acoustic waves traversing from the
high acoustic impedance layer to the low acoustic impedance layer is
higher than the predetermined threshold transition time. The method
further comprises estimating thickness of the encased reservoir
layer based on the measured transition time between the high
impedance value of shale and low impedance value of coal.
The method also comprises generating a reservoir facies
map based on sum of impedance values derived from seismic signals
integrated over the transition time interval. The reservoir facies
map indicate presence of the encased reservoir layer.
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Brief Description of the Drawings
[0015] The present invention is described by way of embodiments
illustrated in the accompanying drawings, throughout which, like
reference numerals indicate corresponding parts in the various
figures .
[0016] Fig. 1 is a block diagram of a system for delineating
reservoir layer of intermediate impedance in a subterranean
heterogeneous formation, in accordance with an embodiment of the
present invention;
[0017] Figs. 2A and 2B illustrate a flowchart of a method for
delineating reservoir layer of intermediate impedance in a
subterranean heterogeneous formation, in accordance with an
embodiment of the present invention;
[OOlS] Fig. 3 is an exemplary graphical representation of a
reservoir facies map, in accordance with an embodiment of the
present invention.
[0019] Fig.4A is an exemplary graphical representation
indicating presence of an intermediate reservoir layer in a
subterranean heterogeneous formation having transition time higher
than the threshold transition time, in accordance with an embodiment
of the present invention; and
[0020] Fig. 4B is an exemplary graphical indicating thickness of
an identified reservoir layer of intermediate impedance in a
subterranean heterogeneous formation, in accordance with an
embodiment of the present invention.
Detailed Description of the Invention
[0021] The invention provides for a system and method to
identify a reservoir layer of intermediate impedance encased between
subterranean layers of relatively high and low impedances. In
particular, the present invention provides for a system and method
to identify and map a reservoir layer of intermediate acoustic
impedance encased between high impedance shale layer and low
impedance coal layer. The reservoir layer encased between high
impedance shale and low impedance coal layer is identified on
transit time between the high impedance shale and low impedance coal
values on impedance volume data estimated after performing model
based seismic inversion of 'seismic signals. The invention identifies
presence of encased reservoir layer based on the enhancement on
transition time of acoustic impedance values of overlying shale
layer to underlying coal layer as compared to threshold value.
100221 The disclosure is provided in order to enable a person
having ordinary skill in the art to practice the invention.
Exemplary embodiments herein, are provided only for illustrative
purposes and various modifications thereof, 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. The
terminology and phraseology used herein, is for the purpose of
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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
herein. For purposes of clarity, details relating to the employed
techniques that are well known in the analogous fields of invention
have been briefly described or omitted so as to avoid ambiguity.
100231 The present invention would now be discussed in context
of embodiments as illustrated in the accompanying drawings.
[0024] Fig. 1 is a block diagram of a system for delineating
reservoir layer of intermediate impedance in a subterranean
heterogeneous formation. In various embodiments of the present
invention, the system 100 comprises an acquisition unit 102,
transition time measurement unit 104, repository 106, transition
time comparison unitl06, a mapping unit110 and a display unit 112.
[0025] Various embodiments of the present invention, may be
implemented via one or more computer systems. The computer system
includes at least one processing unit 114 and a memory 116 (Fig. 1).
Typical examples of processing unit 114 include a programmed
microprocessor, a micro-controller, a peripheral integrated circuit
element, and other devices or arrangements of devices that are
capable of implementing the steps that constitute various
embodiments of the present invention. In an embodiment of the
present invention, the memory 116 may store software for
implementing various embodiments of the present invention. The
processing unit 114, executes program instructions stored in the
memory 116. The computer system is not intended to suggest any
limitation as to scope of use or functionality of described
embodiments. In accordance with various embodiments of the present
invention, the functionalities of acquisition unit 102, transition
time measurement unit 104, repository 106, transition time
comparison unit 108, mapping unit 110 and display unit 112 of system
100 are executed by the processing unit 114, using instructions
stored in the memory 116.
[0026] In an embodiment of the present invention, the
acquisition unit 102 is an electronic device configured to receive
and process seismic signals from various layers in a subterranean
heterogeneous formation. In an exemplary embodiment of the present
invention, a seismic source (not shown) known in the art may be used
to generate acoustic waves that propagate into the subterranean
formation of the earth. Examples of seismic sources may include
vibrator unit, dynamite or an air gun. Each of the subterranean
layers may have different seismic responses to the acoustic waves.
Referring to Fig. 1, when the acoustic waves impinge the overlying
shale layer in a subterranean heterogeneous formation, a portion of
the acoustic waves is reflected back from the shale and reservoir
boundary as seismic signal 102a.The acquisition unit 102 collects
and stores the reflected seismic signal 102a. Subsequently,
remaining portion of the acoustic waves continues to traverse
through the subterranean formation and when the acoustic waves
impinge the underlying coal layer, a portion of the acoustic waves
is reflected back as seismic signal 102b from the reservoir and coal
boundary .The acquisition unit 102 collects and stores the seismic
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signal 102b from the coal layer and reservoir boundary. The
collection of seismic signals (102a, 102b) by the acquisition unit
102 is facilitated by a plurality of receivers (not shown) situated
at different depths of the subterranean heterogeneous formation. In
the absence of reservoir layer, the signal is reflected from the
shale and coal boundary.
roo271 Further, the acquisition unit 102 processes the seismic
signals (102a, 102b) received from shale and reservoir boundary and
coal and reservoir boundary to obtain data corresponding to various
parameters of the shale, reservoir, and coal layer, respectively.
Examples of various parameters include impedance, thickness,
density, velocity, acoustic impedance etc. of the layers. The
acquisition unit 102 processes the seismic signals (102a, 102b) to
derive wavelet formations, using known techniques. The wavelet
formations are representative of the seismic signals (102a,
102b).The acquisition unit 102 processes the wavelet formation of
the seismic signals (102a, 102b) using data stored in the repository
106. The data in the repository 106 is obtained from various
subterranean layers under varied geographical, environmental and
topographical conditions. The obtained data corresponds to various
parameters of the subterranean layers. Examples of various
parameters include acoustic and density logs of various subterranean
layers. The repository 106 thereafter, processes the data to obtain
reflectivity information of the subterranean layers and stores the
data as reference data. In an exemplary embodiment of the present
invention, after the acquisition unit 102 generates the wavelet
formation representative of seismic signals (102a, 102b), the
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acquisition unit 102 retrieves the reference data from the
repository 106. The acquisition unit 102 generates a seismic
response using the wavelet formation of seismic signals (102a, 102b)
and the retrieved data from the repository 106. Seismic response is
indicative of reflection of acoustic waves from the layer boundaries
propagating within the subterranean region and are used to estimate
the acoustic impedance at various depths within the subterranean
heterogeneous formation. In an exemplary embodiment of the present
invention, the seismic response may be generated by convolving the
reflectivity data derived from acoustic and density logs stored in
the repository 106 and wavelet formation derived from the seismic
signals (102a, 102b).
[0028] As discussed previously, the acoustic waves propagating
through the earth encounter subterranean heterogeneous formation
having non-uniform thickness, density, and velocity. As such, when
acoustic waves traverse through subterranean heterogeneous formation
consisting of overlying shale layer and underlying coal layer
encasing the reservoir layer, the acoustic impedance across the
subterranean heterogeneous formation varies by virtue of the
difference in elastic characteristics of the shale, reservoir, and
coal layers. This variation in acoustic impedance is inflicted in
the reflected seismic signals generated by the acquisition unit 102,
as discussed previously. Transition time between shale and coal
impedance values on inverted seismic data exists even in the absence
of an encased reservoir layer due to a limited band of frequency of
seismic data. Further, presence of an encased reservoir layer
results in an increase in the transition time from the impedance
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value of overlying shale layer to the impedance value of underlying
coal layer. Furthermore, the transition time from the high impedance
shale layer to the low impedance coal layer also varies based on the
thickness of reservoir layer of intermediate impedance encased
between the high impedance shale layer and the low impedance coal
layer i.e. greater the thickness of encased layer, higher is the
transition time.
[0029] Referring to Fig. 1, in an exemplary embodiment of the
present invention, the transition time measurement unit 104 is
configured to measure the transition time between impedance value of
the overlying shale layer and the impedance value of underlying coal
layer. The transition time measurement unit 104 is an electronic
device configured to access the acquisition unit 102 to process data
related to seismic signals. The seismic signals indicate a change in
acoustic impedance of the subterranean layers at various depths of
the subterranean layers. The transition time measurement unit 104,
performs an inversion on the seismic signals to estimate impedance
volume of the subterranean layers. Thereafter, the transition time
measurement unit 104 uses the impedance volume to identify high
impedance values of shale and low impedance values of coal as well
as an impedance transition time interval within a subterranean
formation. The transition time measurement unit 104 then measures
the time within the transition time interval for the impedance
values that are close to high shale impedance value and that are
close to low coal impedance value. Subsequently, the transition time
measurement unit 104 calculates the time difference between the high
impedance value of shale and low impedance value of coal within the
15
transition time interval in order to determine the transition time
from the high impedance shale layer to the low impedance coal layer.
[0030] In an embodiment of the present invention, transition
time comparison unit 108 is an electronic device configured to
receive measured transition time data from the transition time
measurement unit 104. The transition time comparison unit 108 is
further configured to retrieve from the repository 106, a threshold
transition time between shale and coal impedance values of
subterranean layers on impedance volume derived from inversion of
seismic data in the absence of reservoir layer of intermediate
impedance. The transition time comparison unit 108 retrieves
threshold transition time stored in the repository 106 corresponding
to an impedance contrast between the overlying shale layer and the
underlying coal layer and determines whether the transition time is
higher than the threshold transition time. In an exemplary
embodiment of the present invention, the threshold transition time
is -4ms, for a typical seismic band and impedance values of shale
and coal, which may marginally decrease with increase in seismic
frequency band or vice versa. Presence of reservoir layer of
intermediate impedance enhances the transit time beyond the
threshold value, which in this exemplary embodiment is -4ms.
[0031] In an embodiment of the present invention, mapping unit
110 is an electronic device configured to receive results of
comparison between the measured transition time and the threshold
transition time from the transition time comparison unit 108. The
mapping unit 110 analyses the data received from the transition time
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comparison unit 108 in order to determine the presence of a
reservoir layer encased between the overlying shale layer and the
underlying coal layer. If, it is determined from the results of
comparison received from the transition time comparison unit 108,
that the transition time from the high impedance shale layer to the
low impedance coal layer is higher than the threshold transition
time, then presence of an encased reservoir layer having
intermediate acoustic impedance is inferred by the mapping unit 110.
Conversely, if it is determined from the results of comparison
received from the transition time comparison unit 108, that the
transition time between shale and coal impedance values of
subterranean layers on impedance volume is equal or tends towards
the threshold transition time, then absence of an encased reservoir
layer having intermediate acoustic impedance is inferred by the
mapping unit 110.
[0032] Further, the mapping unit 110 is configured to generate a
reservoir facies map to indicate presence of an encased reservoir
layer having intermediate acoustic impedance from data received from
the transition time comparison unit 108. In order to generate a
reservoir facies map displaying the encased reservoir layer, the
mapping unit 110 calculates the difference between the high
impedance value of shale and low impedance value of coal within the
transition time interval to represent the reservoir layer on a
reservoir facies map. The mapping unit 110 also determines sum of
impedance values integrated over the transition time interval
between the high impedance shale value of shale and low impedance
values of coal of subterranean layers; higher total impedance
17
corresponds to higher values of transition time interval. Fig.3 is
an exemplary graphical representation of the reservoir facies map
generated by adding impedance values derived from seismic impedance
volume collected over the transition time. The reservoir facies map
displays the presence of an encased reservoir layer. Transition time
values higher than the threshold transition time are indicative of
presence of an encased reservoir layer on the map. While, values
equivalent to or tending towards the threshold values indicate
absence of an encased reservoir layer and are graphically
represented in green color forming background of the map.
[0033] In an embodiment of the present invention, the display
unit 112 is configured to communicate with the mapping unit110 to
receive and display the reservoir facies map (as shown in Fig. 3) on
a user interface configured on an electronic device (not shown).
Example of an electronic device include, but is not limited to,
laptop, netbook, tablet, mobile phone, Personal Digital Assistant
(PDA), Personal Computer (PC) and any other handheld computing
device. As shown in exemplary graphical representation (Fig. 4A) the
display unit indicates the presence of an intermediate reservoir
layer having transition time higher than the threshold transition
time. Fig. 4A depicts impedance log data obtained from sonic and
density well logs along with other well logs, and also impedance log
filtered in the seismic frequency band. The figure displays that
presence of an encased reservoir layer of intermediate impedance in
a subterranean formation can be inferred from the transition time
i.e. from higher transition time than the threshold transition time.
Further, as shown in Fig. 4B the display unit 112 also indicates the
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thickness of the reservoir layer encased between the high impedance
shale layer and the low impedance coal layer. Fig. 4B displays
synthetic transit time response of 2D impedance model reservoir
layer of varying thickness encased in high impedance coal layer and
low impedance shale layer. The synthetic transit time response is
derived by filtering the 2D impedance model in seismic frequency
band. As evident from Fig. 4B, thickness of an encased reservoir
layer having intermediate impedance can be inferred from the
transition time between the high impedance value of shale and the
low impedance value of coal. Also, absence of an encased reservoir
layer is indicated in Fig. 4B.
roo341 Figs. 2A and 2B illustrate an exemplary flowchart of a
method for delineating a reservoir layer of intermediate impedance,
in accordance with an embodiment of the present invention.
[0035] At step 202, seismic signals from an overlying
subterranean reservoir boundary and underlying subterranean
reservoir boundary are acquired. In an embodiment of the present
invention, the overlying layer from which the seismic signal is
acquired is a high impedance shale reservoir layer boundary and the
underlying layer from which the seismic signal is acquired is a low
impedance reservoir coal layer boundary.
[0036] At step 204, the seismic signals acquired at step 202 are
processed. In an exemplary embodiment of the present invention,
seismic signals are processed to generate a wavelet formation
representative of the seismic signals using known techniques.
Subsequent to the generation of the wavelet formation representative
19
of seismic signals, reference data from a repository is retrieved.
Thereafter, a seismic response using the wavelet formation of
seismic signals and the retrieved data from the repository is
generated. The seismic response is generated by convolving the data
retrieved from the repository and the wavelet formation derived from
the seismic signals. Seismic response is indicative of acoustic
waves propagating within the subterranean region and are used to
predict the acoustic impedance at different points within the
subterranean layers. As such, seismic response is used to estimate
acoustic impedances at various depths within the subterranean
formation consisting of shale and coal layers.
[0037] At step 206, transition time between high impedance value
of overlying shale layer to low value of underlying coal layer
within a transition time interval is measured. In an embodiment of
the invention, seismic inversion is performed on the acquired and
processed seismic signals. The inverted seismic signals are used to
estimate the impedance volume of various subterranean layers.
Thereafter, the impedance volume is used to identify high impedance
shale layer and low impedance coal layer by identifying impedance
values that are close to high impedance values of shale and by
identifying low impedance values that are close to low impedance
values of coal at different depths within a subterranean formation.
Subsequently, the difference between the time of high impedance
value of overlying shale layer to low value of underlying coal
layer within the transition time interval is calculated, which in
turn determines the transition time of high impedance value of
shale layer to the low impedance value of coal layer.
[0038] At step 208, a check is performed to determine if the
transition time between the high impedance value of shale layer to
the low impedance coal layer is higher than the threshold transition
time. In an embodiment of the present invention, the repository
stores a predetermined threshold transition time for acoustic waves
traversing between subterranean layers having different acoustic
impedance. The predetermined threshold transition time is based on
predetermined data which represents transition time in the absence
of reservoir layer. In an embodiment of the invention, the threshold
transition time is retrieved from the repository and compared to the
calculated transition time between high impedance value of shale to
low impedance value of coal layer.
[0039] If it is determined that the transition time between the
high impedance value of overlying shale layer to the low impedance
value of underlying coal layer is higher than the threshold
transition time stored in the repository, then, at step 210, it is
inferred that a reservoir layer of intermediate impedance is encased
between the high impedance shale layer and the low impedance coal
layer. In another embodiment of the invention, amount of difference
between the threshold transition time stored in the repository and
the transition time of the acoustic wave traversing between the high
impedance shale layer and the low impedance coal layer is used to
determine the thickness of the encased impedance layer.
[ 0 0 4 0 ] At step 212, in an embodiment of present invention, a
reservoir facies map is generated based on sum of impedance values
integrated over the transition time interval. In an embodiment of
the present invention, the reservoir facies map displays the
presence of an encased reservoir layer. Transition time values
higher than the threshold transition time are indicative of presence
of an encased reservoir layer on the map. Further, values equivalent
to or tending towards the threshold values indicate absence of an
encased reservoir layer and are graphically represented in green
color forming background of the map (Fig. 3) . In an embodiment of
the invention, the reservoir facies map showing the encased
reservoir layer is displayed on a user interface configured on an
electronic device (not shown), including but not limited to, laptop,
netbook, tablet, mobile phone, Personal Digital Assistant (PDA),
Personal Computer (PC) and any other handheld computing device.
[0041] If it is determined that the transition time between the
overlying high impedance value of shale layer to the underlying low
impedance value of coal layer is tending towards the threshold
transition time stored in the repository, then, at step 214, it is
inferred that a reservoir layer of intermediate impedance is not
encased between the high impedance shale layer and the low impedance
coal layer.
[0042] Advantageously, the various embodiments of the present
invention facilitate identification of reservoir layer encased
between a high impedance overlying shale layer and a low impedance
underlying coal layer.
We Claim:
1. A system (100) for delineating reservoir layer of intermediate
acoustic impedance in a subterranean formation, the system
comprising:
a transition time measurement unit (104) configured to
measure transition time between a high acoustic impedance
value of an overlying layer to a low acoustic impedance value
of an underlying layer in the subterranean formation;
a transition time comparison unit (108) configured to:
compare the measured transition time to a predetermined
threshold transition time, the predetermined threshold
transition time is determined by analysing impedance values of
subterranean layers in the absence of reservoir layer of
intermediate impedance; and determine whether the measured
transition time is higher or tending towards the predetermined
threshold transition time as a result of the comparison; and
a mapping unit (110) configured to analyse the result of
comparison of the transition time comparison unit (108) to
determine presence of a reservoir layer, the reservoir layer
being encased between the high acoustic impedance layer and
the low acoustic impedance layer of subterranean formation and
having an intermediate acoustic impedance.
2. The system as claimed in claim 1, wherein the mapping unit
(110) determines presence of the encased reservoir layer
having intermediate acoustic impedance if the measured
transition time is detected to be higher than the
predetermined threshold transition time based on the results
of comparison.
3. The system as claimed in claim 1, wherein the transition time
measurement unit (104) is configured to:
estimate impedance volume data of subterranean layers by
performing seismic inversion on seismic signals acquired from
various layers of the subterranean formation;
identify, based on the estimated impedance volume data,
impedance values that correspond to the high acoustic
impedance layer and the low acoustic impedance layer
respectively within a transition time interval, the transition
time interval representing transition of impedance values from
high to low;
ascertain time taken to traverse between the impedance
values of high acoustic impedance layer and low acoustic
impedance layer respectively; and
determine difference between the ascertained time taken
to traverse between the impedance value of the high acoustic
impedance layer and the impedance value of the low acoustic
impedance layer to measure the transition time from the high
acoustic impedance layer to the low acoustic impedance layer
in the subterranean formation.
4. The system as claimed in claim 1, wherein a repository (106)
is configured to ascertain the predetermined threshold
transition time data based on predetermined data stored in the
24
repository, the predetermined data representing transition
times corresponding to different layers of the subterranean
heterogeneous formation having contrasting acoustic impedance
values, in the absence of a reservoir layer.
5. The system as claimed in claim 1, wherein the mapping unit
(110) is configured to delineate the encased reservoir layer
if the value of the measured transition time traversing from
the high impedance value of shale layer to the low impedance
value of coal layer is higher than the predetermined threshold
transition time.
6. The system as claimed in claim 5, wherein the mapping unit
(110) is configured to estimate thickness of the delineated
encased reservoir layer based on the measured transition time
between the high impedance value of shale and the low
impedance value of coal.
7. The system of claim 3 and 5, wherein the mapping unit (110) is
configured to generate a reservoir facies map based on sum of
impedance values integrated over the transition time interval,
wherein the reservoir facies map indicates presence of the
encased reservoir layer.
8. The system as claimed in claim 1, wherein the high acoustic
impedance layer of the subterranean formation is a shale
layer.
9. The system as claimed in claim 1, wherein the low acoustic
impedance layer of the subterranean formation is a coal layer.
10. A method for delineating reservoir layer of intermediate
acoustic impedance in a subterranean formation, the method
comprising:
measuring transition time between a high acoustic
impedance value of an overlying layer to a low acoustic
impedance value of an underlying layer in the subterranean
formation;
comparing the measured transition time, to a
predetermined threshold transition time, the predetermined
threshold transition time is determined by analysing impedance
values of subterranean layers in the absence of reservoir
layer of intermediate impedance;
determining whether the measured transition time is
higher or tending towards the predetermined threshold
transition time based on the comparison; and
analysing the result of comparison to determine presence
of a reservoir layer, the reservoir layer being encased
between the high acoustic impedance layer and the low acoustic
impedance layer of subterranean formation and having an
intermediate acoustic impedance.
11. The method as claimed in claim 10, wherein determining
presence of the encased reservoir layer having intermediate
acoustic impedance comprises, determining if the measured
transition time is detected to be higher than the
predetermined transition time based on the results of
comparison.
12. The method as claimed in claim 10, wherein measuring
transition time between the high acoustic impedance value of
an overlying layer to the low acoustic impedance value of an
underlying layer in the subterranean formation comprises:
estimating impedance volume data of subterranean layers
by performing seismic inversion on seismic signals acquired
from various layers of the subterranean formation;
identifying, based on the estimated impedance volume
data, impedance values that correspond to the high acoustic
impedance layer and the low acoustic impedance layer
, respectively, within a transition time interval, the
transition time interval representing transition of impedance
values from high to low;
ascertaining time taken to traverse between the impedance
values of high acoustic impedance layer and low acoustic
impedance layer respectively; and
determining difference between the ascertained time taken
to traverse between the impedance value of the high acoustic
impedance layer and the impedance value of the low acoustic
impedance layer to measure the transition time from the high
acoustic impedance layer to the low acoustic impedance layer
in the subterranean formation.
13. The method as claimed in claim 10, wherein determining
the predetermined threshold transition time comprises:
retrieving predetermined data stored in a repository, the
predetermined data representing transition times corresponding
to different layers of the subterranean heterogeneous
formation having contrasting acoustic impedance values, in the
absence of a reservoir layer.
14. The method as claimed in claim 10, wherein determining
presence of reservoir layer comprises:
ascertaining if the measured transition time of the
acoustic waves traversing from the high acoustic impedance
layer to the low acoustic impedance layer is higher than the
predetermined threshold transition time; and
estimating thickness of the encased reservoir layer based
on the measured transition time between the high impedance
value of shale and the low impedance value of coal.
15. The method as claimed in claim 12, wherein a reservoir
facies map is generated based on impedance values derived from
seismic signals integrated over the transition time interval,
the reservoir facies map indicating presence of the encased
reservoir layer.
16. The method as claimed in claim 10, wherein the high
acoustic impedance layer of the subterranean formation is a
shale layer.
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17. The method as claimed in claim 10, wherein the low
acoustic impedance layer of the subterranean heterogeneous is
a coal layer.