The impact of spatial heterogeneity on porosity-permeability evolution during microbially-induced carbonate precipitation (MICP) in sandstone

Microbially-induced carbonate precipitation (MICP) has shown great promise in a variety of subsurface applications including CO2 sequestration and immobilization of groundwater contaminants. Previous studies at the laboratory scale have demonstrated how variables like injection strategy and cementation media composition can be manipulated to tune the amount and spatial localization of carbonate precipitates. To date, however, few lab-scale studies have utilized unfractured, consolidated, natural samples, and these samples have been relatively spatially homogenous. Since bacterial attachment may be influenced by differences in fluid velocity and the abundance and morphology of precipitates may be shaped by chemical microenvironments, a robust investigation of how spatial heterogeneity in the pore space influences porosity-permeability relationships is needed. In this study, Sporosarcina pasteurii is used to induce porosity and permeability reduction in natural sandstone cores of similar initial porosity and permeability but varying degrees of heterogeneity. Porosity change due to precipitation is resolved spatially using X-ray computed microtomography (XCT) and scanning electron microscopy (SEM) with backscatter (BSE) and energy-dispersive X-ray spectroscopy (EDX). Using porosity and permeability values at three time points, porosity-permeability relationships are derived and compared between spatially homogeneous and heterogeneous cores. Subsequently, implications for CO2 sequestration applications are investigated by repeating the characterization and correlation process during dissolution of the precipitated carbonates by acidified brine.