Process for characterising the evolution of an oil or gas reservoir over time

1. A method for characterising evolution of a reservoir by determining a seismic wavelet which links observed seismic data to a sequence of reflectivities, said method comprising:

obtaining seismic data representing seismic changes which have occurred between a first time and a second time defining a production period over which hydrocarbons have been extracted, said seismic data comprising a plurality of seismic traces; and

performing an optimisation operation simultaneously on said plurality of seismic traces so as to optimise for said seismic wavelet, said optimization operation comprising simultaneously optimising for said seismic wavelet and reflectivity change data occurring between said first time and said second time;

extracting the time-lapsed changes in the seismic traces collected over the production period;

integrating the time-lapsed changes in the seismic traces with production data;

using the integrated time-lapsed changes in the seismic traces and the production data to manage at least one of extraction of oil and gas from the reservoir and injection of other fluids into the reservoir; and

wherein said optimisation operation is performed without using known reflectivity data as an input.

2. The method as claimed in claim 1 wherein said optimisation comprises minimising a cost function comprising a measurement of a difference between the seismic data and a convolution of the seismic wavelet and reflectivity change data.

3. The method as claimed in claim 1 wherein each reflectivity change described by said reflectivity change data is defined by two parameters describing positions of two reflectors defining the change in reflectivity and a single amplitude parameter representing a magnitude of a measured amplitude change at the positions of said two reflectors, the amplitude change being equal in magnitude and of opposite sign at these positions.

4. The method as claimed in claim 1 comprising the steps of:

providing a base survey of the reservoir with a set of seismic traces at said first time;

providing a monitor survey of the reservoir, taken at said second time, with a set of seismic traces associated to the same positions as in the base survey; and

determining said seismic data from said base survey and monitor survey.

5. The method as claimed in claim 4 wherein said step of determining said seismic data comprises aligning said monitor and base surveys; and subtracting said base survey from said monitor survey.

6. The method as claimed in claim 4 wherein said base and monitor surveys have been performed at a location remote from a well, or adjacent to a horizontal or sub-vertical well.

7. The method as claimed in claim 4 , wherein said base and monitor surveys have been performed at a location of interest for the wavelet being determined.

8. The method as claimed in claim 1 comprising a scaling step so as to determine an absolute value for amplitude of the seismic wavelet.

9. The method as claimed in claim 8 wherein said scaling step comprises defining a scaling factor such that ΔS=αω*1/αΔR, where α is the scaling factor, ΔS is the seismic data, w is the seismic wavelet and ΔR is a reflectivity change data.

10. The method as claimed in claim 8 wherein said scaling step comprises convolving an unscaled wavelet with actual reflectivity measurements taken from another location of the reservoir.

11. The method as claimed in claim 8 wherein said scaling step comprises using travel-time information to estimate the actual reflectivity measurements.

12. The method as claimed in claim 11 wherein said actual reflectivity measurements are determined from a product of measured relative velocity changes and a factor based on an estimated ratio between relative velocity changes and relative density changes.

13. The method as claimed in claim 8 wherein said scaling step comprises directly inferring maximum reflectivity changes from production history data.

14. The method as claimed in claim 1 further comprising the step of using results of said method to aid hydrocarbon recovery from a reservoir.

15. A computer program comprising computer readable instructions which, when run on suitable computer apparatus, cause the computer apparatus to perform the method of claim 1 .

16. A computer program carrier comprising the computer program of claim 15 .

17. An apparatus specifically adapted to carry out the steps of the method as claimed claim 1 .