Forecasting Of Ekofisk Reservoir Compaction And Subsidence By Numerical Simulation

Abstract

To evaluate alternative reservoir management plans and to assess needs for modifying offshore facilities, a computational procedure has been developed to simulate the reservoir compaction and subsidence processes observed at the Ekofisk field in the Norwegian North Sea. The procedure involves combining the results of two axially symmetric, 2-D finite element calculations so that in plan view the Ekofisk reservoir is treated as an elliptically-shaped body. Reservoir compaction is assumed to be driven by reductions in pore pressure, with input information deduced from reservoir simulators used for reservoir management. Porosity distributions and geometrical features of the reservoir are based on porosity logs from some sixty wells drilled prior to the onset of significant compaction. Mechanical properties for the reservoir rock (chalk) are based on laboratory compaction data; for the overburden they are based on data from vertical seismic profiles and compaction tests. The simulation procedure has been applied to several reservoir management scenarios, three of which are considered in the paper. One is a pressure maintenance scenario involving aggressive gas injection to mitigate reservoir compaction and subsidence while the others are depletion scenarios which involve a waterflood that was planned prior to discovery of subsidence to enhance production, but also will serve to maintain reservoir pressure. The good agreement between calculations and recent measurements provides a basis for confidence in future projections. Hence, the prediction of about 6m of subsidence by the year 2011 for depletion reservoir management scenarios suggests that the recently completed operation to elevate the central platforms of the Ekofisk field by 6m represents a long-term solution to the subsidence problem.