Combining acoustic P- and S-wave measurements and microstructural analysis to determine the in-situ state of stress of a porous sandstone reservoir
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Stress changes within a porous sandstone reservoir due to gas extraction may lead to reservoir compaction and corresponding subsidence, as observed for the Groningen gas field. Therefore, a good understanding of the field stress state is necessary for an accurate prediction of reservoir compaction. This study addresses the question if the in-situ state of stress of a porous sandstone reservoir can be determined by measuring acoustic wave velocities during true triaxial experiments. Experiments are performed on Bleurswiller sandstone (~25% porosity). P- and S-wave velocities are monitored during two loading cycles, with an initial deviatoric loading- and unloading stage (σ1max = 85 MPa, σ2 = σ3 = 15 MPa), to induce permanent deformation and create a stress imprint, succeeded by hydrostatic loading- and unloading (σ1max = σ2 = σ3 = 117 MPa). Acoustic data are combined with microstructural analysis of sectioned samples, to quantify microcrack densities and orientations before and after deformation. Whereas microcrack analysis does not show increased crack densities after the experimental procedure, extreme porosity reduction indicates pore collapse as possible responsible mechanisms for most observed inelastic deformation. During hydrostatic loading, the previously applied maximum stress is characterized by stable minimum S-wave velocity anisotropy values, together with P-wave and S-wave velocity anisotropy rate of change values showing minima approaching zero. This result indicates that the maximum stress of the applied deviatoric stress during triaxial loading can be retrieved by introducing a subsequent hydrostatic stress. Adopting the same procedure, core damage from a Rotliegend sample can be minimized during hydrostatic loading and the in-situ state of stress of the Rotliegend reservoir can be determined.