Abstract

SUMMARY The solution of the poroelastic equations for predicting land subsidence above productive gas/oil "elds may be addressed by the principle of virtual works using either the e!ective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the "nite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate di!erent local forces on the element nodes. The two formulations are brie#y reviewed and discussed for triangular and tetrahedral "nite elements. They are shown to yield di!erent results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modi"cation to both formulations is suggested which allows to correctly solve the problem of a "nite reservoir with an in"nite pressure gradient, i.e. with a pore pressure discontinuity on its boundary.