Numerical simulation of shear-induced compaction in the ekofisk reservoir

Early efforts to simulate the reservoir compaction and subsidence phenomena for the Ekofisk oil and gas field (located in the Central Graben of the Norwegian North Sea) by using finite-element numerical methods, with pore collapse as the sole mechanism for compaction of the reservoir chalk, were quite successful in that agreement between measured and calculated subsidence magnitudes was good. In time, however, measured subsidence rates tended to be higher than calculated rates. This observation, coupled with field data that indicated deviatoric stresses in the reservoir had become very large, led to a decision to incorporate an additional compaction mechanism (referred to here as shear-induced compaction) into the compaction/subsidence simulation model for Ekofisk. The present paper describes the effort to develop and implement a constitutive formulation to treat this compaction mechanism in the reservoir compaction/subsidence model for Ekofisk.