The use of reservoir simulation coupled with geomechanics to model physical phenomena such as compaction, subsidence or shear failure etc. has been increasing. Different methods of coupling have been investigated by numerous researchers: fully implicit coupling, iterative coupling and explicit coupling. So far, the iterative coupling method appears to be the most preferable method for field-scale simulation. In this method, the porosity can be expressed as a function of pressure, temperature and mean stress. Since the parameters of the porosity function are evaluated in the geomechanics module, they are lagged and are kept unchanged (explicit formulation) during the solution of the reservoir flow equations. Therefore, the solution is computed less accurately than expected.

In this paper, a modification of the porosity formula for the iterative coupled method is described. The improvement includes the use of additional derivatives for the parameters to give a more implicit formulation of porosity with respect to pressure and temperature. In the modified porosity formula, the second order terms of compressibility are included. This modification makes the coupling implicit and gives a more accurate solution.

Some test cases will be performed to demonstrate how the modified porosity formula yields better solutions than the unmodified one.

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