Successful Time Lapse Seismic Pilot on Al Shaheen field (Offshore Qatar): Analysis and Practical Applications in Reservoir Monitoring

Abstract

The Al Shaheen field, offshore Qatar, is one of the world’s oil largest carbonate fields currently at a production plateau of 300 000bopd with more than 300 active wells. It comprises a stacked sequence of thin layered Lower Cretaceous reservoirs. The objective of the paper is to illustrate the applicability and demonstrate the economic impact of 4D information by revisiting data acquisition, work overs, appraisal wells around monitor acquisition time. Time lapse seismic survey (4D seismic) is a geophysical technique consisting in acquiring 3D seismic over the same area at different times. Following a conclusive 4D feasibility study, a pilot monitor survey was shot in 2015 to be compared to a base survey shot in 2007 (first oil in 1994). Aside from seismic acquisition repeatability and processing, successful 4D analysis was highly dependent on extracting meaningful 4D attribute, integration and collaboration of different geoscience disciplines. 4D analysis led to the following conclusions: - 4D seismic response is broadly consistent between Al Shaheen carbonate reservoirs - 4D signal associated with gas saturation changes is easily observable in a very reduced time frame: - Sg increase associated with gas exsolution (due to unsupported production) or gas injection (WAG) - Sg decrease associated with gas production / re-dissolution due to unsupported production before base monitor and support between base and monitor. - 4D signal associated with water saturation increase is mostly limited to non-uniform sweep such as early water breakthrough issue. - 4D signal associated with pressure decrease is not directly observed as quickly associated with gas exsolution whereas 4D signal associated with pressure increase is limited to producers in depletion mode converted around base monitor time into water injectors. Current applications in reservoir management include: - Identification of undrained / poorly supported areas based on non-uniform 4D signal associated with gas saturation changes - Identification of early water breakthrough issue location along water injector - reservoir surveillance plan strategy - influencing workover strategy - optimizing appraisal well location in order to sample sweep efficiency or investigate inter reservoir communication. Despite 4D has been proven a successful technique in clastic environment, its applicability to carbonates fields is more challenging and depends first on rock physics and also seismic quality. Nevertheless, the 2015 seismic pilot results proved the 4D value particularly in reservoir management and consequently validated a full field 4D OBN monitoring strategy with first survey to be executed in 2019.