Land surface uplift above compacting overconsolidated reservoirs

An investigation on land surface deformation resulting from the compaction of producing overconsolidated gas/oil reservoirs is presented. If the depleted formation is significantly less compressible than the surrounding medium the vertical reservoir shrinkage is relatively smaller than the horizontal one, thus generating a possible swelling of the overburden and a consequent pronounced decrease of land subsidence or even a surface uplift. Such an effect may also cause an additional land settlement during the post-productive recovery phase over fields which exhibit a stiffer behavior in expansion. In the present paper the surface displacements caused by the depletion of an overconsolidated disk-shaped reservoir are analysed by a finite element model taking into account the influence of the disk burial depth and areal extent, the Poisson ratio of the medium, and the underburden stiffness. The results obtained with a more realistic three-dimensional setting are also discussed to address the possible effects of an irregular reservoir geometry.