Geomechanical behavior of Cambrian Mount Simon Sandstone reservoir lithofacies, Iowa Shelf, USA

Abstract

The Mount Simon Sandstone (Mt. Simon), a basal Cambrian sandstone underlying much of Midwestern US, is a target for underground CO2 storage and waste injection which requires an assessment of geomechanical behavior. The range of depositional environments yields a heterogeneous formation with varying porosity, permeability, and mechanical properties. Experimental deformational behavior of three distinct Mt. Simon lithofacies was examined via axisymmetric compressional testing of core samples. Initial yielding was confirmed with acoustic emissions in many tests and failure envelopes were determined for each lithofacies. Evolution of elastic moduli with stress and plastic strain was determined by use of unload–reload cycles, which permit separation of total measured strains into elastic and plastic strains. The Upper Mt. Simon lithofacies yields at higher shear stresses compared to two “Lower” lithofacies, with little modulus degradation with plastic strain. Lower Mt. Simon lithofacies are weaker and deform plastically with modulus degradation. This range in constitutive response is quantified with an elasto-plasticity model. Based on these results, Mount Simon Sandstone would likely deform elastically during CO2 injection and storage, with large pore pressure increases (∼8–9MPa above hydrostatic) predicted to initiate plastic yielding. Nonetheless, near-wellbore damage could result in weaker lithofacies during injection and/or brine extraction.