Effect of pore fluid chemistry on uniaxial compaction creep of Bentheim sandstone and implications for reservoir injection operations

Highlights

• Uniaxial creep tests on sandstone to study rock response to fluid injection.

• Creep rate and AE rate increased with increasing sample porosity and applied stress.

• Creep enhanced going from SC fluids to dry to saturated conditions, with higher pH.

• Stress corrosion cracking is enhanced at higher stresses, water activity and/or pH.

• SC/acidic fluid injection into quartz-rich reservoirs likely decreases creep rates.

Abstract

In the transition to a sustainable energy system, natural gas may be an interim source for relatively low-carbon energy production. However, hydrocarbon production worldwide is leading to reservoir compaction and, consequently, surface subsidence and induced seismicity, hampering the potential of natural gas. Reservoir compaction may potentially be mitigated by fluid injection. Fluid injection into porous subsurface reservoirs is also required in other technologies envisioned in a sustainable energy system, such as geothermal energy production and temporary storage of renewable energy. However, fluid injection into porous reservoirs may create a chemical disequilibrium between the pore fluid and host rock, potentially activating fluid–rock interactions that can cause compaction of the reservoir. These chemically activated fluid–rock interactions are not well-understood, and, therefore, we performed uniaxial compaction experiments at 35, 75 and 100 MPa effective stress, employing samples of Bentheim sandstone saturated with supercritical phases (i.e. ${\text{N}}_2$, CO₂, wet-${\text{N}}_2$ and wet-CO₂), distilled water and aqueous solutions (i.e. 3.7 pH HCl solution, AMP solution and AlCl₃ solution), as well as low-vacuum (dry) conditions. Creep strain and acoustic emissions (AEs) accumulated with increasing stress and sample porosity. While saturation with supercritical fluids produced slightly less creep strain than dry conditions, flooding with distilled water doubled the creep strain. The acidic solutions inhibited compaction creep compared to distilled water saturation. AE activity and microstructural analysis revealed that microcracking controlled deformation, presumably via stress corrosion cracking. While the supercritical fluids may have dried crack tips, distilled water likely reduced the stress required for Si-O bond breakage. The acidic solutions inhibited microcracking through, presumably, a change in surface energy. Our results suggest that fluids devoid of water, with low water content or acidic in nature can be injected into quartz-rich porous reservoirs without increasing reservoir compaction rates.