Constitutive modeling of rocks and numerical simulation of oil reservoirs exhibiting pore collapse

The problems related to reservoir compaction and surface subsidence caused by the pore collapse occurring in reservoir rocks were analysed through the development of a numerical simulator.;A generalized constitutive model was developed to characterize the stress-strain and volumetric response of rocks. The material parameters employed by the model were determined using simple laboratory data and Simplex optimization procedure. To validate the model, an experimental program was conducted on Berea sandstone to procure data along different stress paths with special emphasis on eliminating the end effects. In view of the stress conditions existing in a depleting reservoir and due to non-availability of data on actual reservoir rocks along different stress paths, the constitutive model was modified to obtain a simple and practical model, called the pore collapse model, for describing the non-linear behavior of reservoir rocks exhibiting pore collapse.;Both versions of the constitutive model were implemented in an existing non-linear finite element simulator, MICROFEM. MICROFEM was extensively modified to include the axisymmetric idealization of the domain and special loading conditions such as those associated with gravity loading, in situ pore pressure, and pore pressure depletion. The modified version of the simulator, called as NARCASS, was tested through successful simulation of laboratory tests representing field loading conditions.;Using the developed simulator, a detailed analysis was conducted for the Ekofisk field. Comparisons between available field data and simulator results indicated that the simulator can be used successfully to predict subsidence, subsidence rate, subsidence bowls, and stress ratio in the reservoir. Based on the results obtained, recommendations are made to elevate the platform and to perform a comprehensive study on the stability of casings to avoid problems during the future operations of the field.;The simulator can be used during the planning stages of the reservoir development to assess the field changes associated with the production and to determine the effects of various depletion scenarios and enhanced recovery techniques such as gas injection and water flooding on the field deformations.