Overburden modeling above a compacting reservoir using a trap door apparatus

Abstract

The consequences of large reservoir compaction on the surface subsidence and the stress path of the overburden formations were investigated with physical modeling experiments where the compacting reservoir was modeled with a circular retracting trap door (TD) under a deformable overburden of sand. Tests with various overburden heights showed the formation of shear bands starting almost vertically at the TD edges and converging successively to the symmetry axis. A shallow and a deep formation mechanism were identified. In the shallow mechanism, the shear bands reach immediately the upper surface and thus the TD displacement is felt immediately at the surface as subsidence. In the deep mechanism, at low TD displacements the shear bands meet initially each other forming an arch and thus only part of the TD displacement is felt at the surface. At higher TD displacements, additional shear bands form above the initial shear band and eventually reach the surface. After that, the TD displacement is directly felt at the surface as subsidence. The TD stress drops rapidly to a minimum value, which appears to be independent of the overburden height.