The Influence of Super-critical CO2 Saturation on the Mechanical and Failure Properties of a North Sea Reservoir Sandstone Analogue

Abstract

Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-critical state into deep saline aquifers. As CO2 is injected, stress changes due to fluid pressure and temperature changes may result in deformation and potentially may result in microseismicity if existing faults are reactivated. Microseismicity may provide additional information on in situ stress conditions and the progression of the CO2 plume. To assess failure criteria in the reservoir, we must know the mechanical response of reservoir sandstone due to injection of super-critical CO2 (scCO2) under reservoir stress conditions. This includes fracturing processes at the microstructural scale as well as at the macroscopic scale by reactivation of existing fractures. We conducted three triaxial tests on a sandstone analogue to deep North Sea reservoir rock: once pressurized, samples were saturated with either scCO2, brine, or a brine-scCO2 mixture. Each sample was then deformed axially to induce a through-going fracture, which was then reactivated—first by increasing the pore pressure under anisotropic stress conditions, then by axial reloading with a constant pore pressure. Throughout testing, we monitored acoustic emissions (AE) and measured ultrasonic P-wave velocities. AE were located within the sample, and their relative magnitudes were calculated, as well as their source mechanisms for a subset of high signal-noise ratio events. We observed:

• Higher strength and stiffness for the sample saturated only with scCO2 than for samples containing brine, and a brine-scCO2 mixture.
• No clear difference in fracture strength for samples with different fluid saturations.
• Higher energy events released from fracturing of the stronger and stiffer scCO2-saturated rock.
• For all samples, the AE response during fracture reactivation was more energetic and had a higher frequency content at higher effective stresses (by axial loading) than at lower effective stresses (by pore pressure increase).

We found fluid saturation to have only a slight influence on the mechanical and failure properties. We observed no significant weakening with scCO2 saturation that might otherwise cause undesired inelastic deformation within the reservoir.

Keywords: North Sea, sandstone, super-critical CO2, laboratory testing, acoustic emissions, microseismicity