Analysis of compaction phenomena due to water injection in reservoirs with a three-phase geomechanical model

In coastal regions, land subsidence that results from industrial pumping of underground fluids, such as methane, is documented by in situ surveys. Laboratory characterization of gas bearing formations has also been published, which complements knowledge of reservoir compaction due to variation of fluid pressures. Gas withdrawal is reproduced in the laboratory by injecting water under a constant uniaxial or hydrostatic load to simulate the overburden. The water injection experiments provoke plastic compaction of the samples. This paper proposes a new modelling of the observed compaction of samples, as well as the changes in compressibility and size of the elasticity domain during water injection. The conceptual framework relies on the mechanics of unsaturated weak rocks, provided that the subsidence phenomenon concerns a three-phase material with solid minerals, liquid water and gas. The proposed constitutive model called ACMEG-s describes the water retention capability of the studied soils and their mechanical behaviour. Consequently, the elasto-plastic volumetric changes within the porous medium incorporate the effects of water saturation and capillary pressure, or suction. The yield locus is formulated such that the shape of the yield limit depends on suction to model the apparent added stiffness that results from low water saturation. The modelling framework, based on generalising the effective stress principle to three-phase media, also provides an elasto-plastic explanation of the well-known "wetting pore collapse" phenomenon. The ACMEG-s model shows consistent understanding of compressibility changes as the quantity of retained water varies. The successive phases of isotropic compression and uniaxial mechanical compaction are used for the model calibration. Interestingly, the phases of plastic compression during the water injection are captured accurately, which indicates that the model is applicable to reservoir-related subsidence studies.