Experimental characterization of the deformation behavior of a gas-bearing clastic formation: soft or hard rocks? A case study

This paper presents a case study of the geotechnical characterization for subsidence analyses of an Italian clastic rock gas reservoir at about 1200–1400 m sub sea level. A coherent and exhaustive dataset including lab data and well log measurements allowed to address the main aspects of the rock material characterization. The a priori qualitative inspection of the cores showed significant variability in the quality of rock material, typical feature of soft rocks. However, data from lab triaxial compressive tests showed good quality rock material characterized by high Young’s modulus values primarily affected by lithological differences between reservoir and non-reservoir facies in general and, for sandstone reservoir facies in particular, by cementation and grain sides. In relation to scale effects, the discrepancy between stiffness specimens and stiffness rock mass can be mainly ascribed to formation heterogeneities in terms of lithologies organized in thin layers with centimeter thickness below the resolution of the conventional logs. Finally, the decay of specimen stiffness in relation to induced strain was also inferred: the effects of induced strain from very small to small strain, which represents the range of strain amplitude induced by gas production in a clastic formation, is almost negligible, thus the adoption of constant formation stiffness values during reservoir production life is a realistic assumption for subsidence predictive analyses.