Process for characterising the evolution of an oil or gas reservoir over time
Disclosed is a process for characterising the evolution of a reservoir over a time lapse comprising the steps of: providing a base and a plurality of monitor surveys of the reservoir, each having a set of seismic traces at different times; deriving an optimal regularization weight-map from a combination of at least three surveys; and using the optimal regularization weights to invert and obtain an improved time lapse seismic image between pairs of seismic surveys.
1. A process for characterising evolution of a reservoir over a time lapse by analyzing changes of seismic data wherein n surveys of the reservoir exist, where n>2, each survey of the n surveys having been taken at a different time within said time lapse and comprising a set of seismic traces, the process comprising:
inverting, via a computer, one or more seismic traces for each of the set of seismic traces from multiple survey pairs taken from the n surveys to obtain an estimate of the changes having occurred in the time lapsed between surveys of each survey pair, said inversion being performed by minimising an ill-posed function which includes at least one regularisation term to impose constraints on inverted parameters, the regularization term comprising a regularization weight;
repeating, via a computer, the inverting for a plurality of regularisation weights;
determining, via a computer, mismatches of terms of the ill-posed function in terms of a regularisation weight of the plurality of regularisation weights; and
using the determined mismatches to determine an optimal regularisation weight for the at least one regularisation term that represents a best compromise between modelling the changes and imposing the constraints.
2. The process according to claim 1 wherein said function to be minimised is dependent on relative velocity changes, said function modelling time shifts in a seismic response of reflectors and associated changes in reflectivity between surveys of each survey pair.
3. The process according to claim 1 wherein said ill-posed function is defined as:
C
=
b
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-
m
_
1
2
2
+
b
_
-
m
_
2
2
2
+
m
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1
-
m
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2
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+
b
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m
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3
2
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1
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m
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…
and said at least one regularisation term; and
where b is a base seismic trace from a first of said n surveys, and m n are subsequent traces from subsequent ones of said n surveys on which a shaping operator has been applied to compensate for time shift and 4D amplitude effects in order to match the base seismic trace.
4. The process according to claim 3 wherein a regularisation term occurring for each pair of surveys is added to the ill-posed function and thus minimised during inversion.
5. The process according to claim 3 wherein terms of the ill-posed function are weighted differently.
6. The process according to claim 3 wherein the ill-posed function is computed with a window restricted to contain a majority of 4D effects.
7. The process according to claim 1 wherein said inversion is carried out for interval time shift (time strain).
8. The process according to claim 7 wherein said ill-posed function is defined as:
C
=
b
_
-
m
_
1
2
2
+
b
_
-
m
_
2
2
2
+
m
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1
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m
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+
b
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1
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…
and said at least one regularisation term; and
where b is a base seismic trace from a first of said n surveys, and m n are subsequent traces from subsequent ones of said n surveys on which a shaping operator has been applied to compensate for time shift in order to match the base seismic trace.
9. The process according to claim 8 wherein terms of the ill-posed function are weighted differently.
10. The process as claimed in claim 1 further comprising the step of using resultant data to aid hydrocarbon recovery from said reservoir.
11. A computer program residing on a non-transitory computer-readable medium, comprising computer program code means adapted to run on a computer all the steps of the process of claim 1 .
12. Apparatus specifically adapted to carry out all the steps of the process of claim 1 .