Rocksalt creep, uncertainties, and their implications for surface subsidence above a producing rocksalt-capped reservoir

Many hydrocarbons reservoirs are capped by thick layers of rocksalt, a material that creeps when a shear stress is applied to it. Pumping leads to compaction of the reservoir material but also induces shear stresses inside rocksalt. These need to relax, leading to time-dependent ground deformations and surface subsidence. To quantify creep-induced subsidence we have created simplified yet realistic Finite Element models of medium-sized rocksalt-capped reservoirs that include key aspects of the subsurface geometry and material properties layering. These show that creep-induced subsidence can be a significant proportion of the total subsidence and so rocksalt caprocks need to be carefully modeled. We discuss uncertainties relating to the low-stress creep behavior of rocksalt and show the subsidence evolution predicted by the use of different constitutive models for rocksalt creep. Another source of uncertainty is the poor characterization of the rocksalt body's physical properties spatial distribution. Here we touch upon rocksalt grain-size and chemical composition as two such major uncertainties and discuss the effects this can have on the overall subsidence response. We finish with the conclusion that engineers who are faced with the task of predicting the subsidence evolution have to resort to calculating bounds for the resultant response based on a number of possible scenaria for rocksalt body behavior.