A carbonate reservoir has an average thickness of 90 feet and an average porosity of 24% (with localized porosity stringers up to 32%). After 25 years production, it experienced a reduction in pore pressure from 4,300 psi to 2,000 psi. Subsequent modeling indicated that continued production would reduce the reservoir's pore pressure to 600 psi. This led to concerns related to increase in effective stresses, which could result in reservoir compaction and subsequent surface subsidence.
To assess this risk, ADCO performed a geomechanical testing campaign using carbonate plugs with high porosity (25-30%). Because of the heterogeneous nature of the reservoir, it was not possible to use conventional testing methods, which would require a set of five to ten "identical" plugs. Moreover, conventional multi-stage triaxial tests could not be used to evaluate cohesion and friction angle parameters of high-porosity plugs because the yield cap was reached before the critical state line. Consequently, a dedicated multi-stage triaxial testing method was developed, which consisted of performing a series of loading/unloading cycles starting from an isotropic cycle, followed by deviatoric-stress cycles with confining pressures decreasing from one cycle to the next one, until the critical state line is reached.
These innovative tests were used to evaluate the parameters of a capped constitutive law by back-analyzing the stress-strain curves of all cycles in one operation, which gave very good matches between measured and back-analyzed curves, with high-confidence estimation of the constitutive law parameters. This allowed developing trends of Young's modulus, cohesion, and friction angle versus porosity. However, the point characterizing the in situ stress was sometimes located outside the envelope, meaning that some plugs were damaged before testing.