Biogrouting with microbially-induced calcium carbonate precipitation: Correlating injection overpressures with observed microstructural response in permeable sandstone

Sandstones with high permeability are prominent reservoirs for hydrocarbons and for geological carbon sequestration. However, high-permeability zones favor preferential flow, promote fluid short-circuiting, and reduce sweep efficiency, especially where multi-phase fluids are displaced and produced. Microbially-induced calcium carbonate precipitation (MICP) has been shown to seal rock fractures and pores. However, few studies have assessed its effectiveness in plugging high-permeability zones within porous rocks. We examine the effectiveness of MICP treatment for sealing highly permeable porous sandstone through grouting, as measured by X-ray computed tomography (X-CT). Results show that injection pressure and the amount of calcium carbonate (CaCO 3 ) precipitation increase from 50 kPa to 800 kPa and by 0.16 g/cm 3 , after 8 cycles of MICP-grouting, while porosity and permeability concomitantly decrease by 31% and 63%, respectively. Such changes in permeability are nonlinear with increasing grouting cycles, despite the injection volume being constant in each cycle. Specifically, three distinct stages of pressure evolution were observed during constant rate injection: (1) an initial stage with constant peak pressure; (2) a gradual pressure rise; and (3) an exponential increase followed by a rapid decline. Quantitative analysis of resulting pore structures using X-CT imaging indicates a distinct evolution in isolated and connected pores. The accumulation of precipitates reduces the macroscopic pore volume by tightening and sealing pore connectivity, thereby increasing the volume of relatively isolated pores. These phenomena, together with the accelerated rise in injection pressure at a constant injection rate, suggest the blockage of preferential flow paths and a resulting increase in tortuosity, highlighting the potential of biogrouting as a flow diverter for sealing high-permeability zones in sandstone.