To evaluate alternative reservoir management plans and to assess needs for modifying off-shore 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, two-dimensional 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 severa1 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 limited gas injection. All three scenarios involve a waterf100d that was planned prior to discovery of subsidence to enhance production, but also will serve to maintain reservoir pressure.
Simulation runs provide information about displacements and stress changes throughout the zone modelled, but the paper will concentrate on displacements of the seafloor (where reliable measurements have been made), compaction of the reservoir, and changes in stress just above the reservoir. Calculated values for subsidence at the center of the subsidence bowl for the 1986-1987 time period are in good agreement with measurements (3.6 to 3.8 m); calculated and measured bowl shapes also are in agreement. Predicted values for subsidence in the year 2011 (the end of the current 1ease period) are 5.9 and 6.1 m for the two depletion scenarios and 4.2 m for the pressure maintenance scenario. Comparisons between measured and calculated subsidence rates (which are both in the mid-20 to mid-3D cm/year range for the 1986–1987 time period), and correlations of subsidence rates with changes in gas-injection rates, indicate that changes in production or injection schedules are detected rather quickly (within few months) on the seafloor.
The good agreement between calculations and recent measurements provides a basis for confidence in future projections. Hence, the prediction of about 6 m 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 6 m represents a long-term solution to the subsidence problem.
In late 1984, it was discovered that the seafloor at the Ekofisk oil field in the Norwegian sector of the North Sea was subsiding as a consequence of reservoir compaction induced by fluid removal. This discovery prompted the immediate initiation of a variety of technical programs to study the physical phenomena having bearing on the problem.
