Microbially mediated plugging of porous media and the impact of differing injection strategies

Ureolysis-driven calcite precipitation is investigated as a means of reducing the porosity of soils and rocks in order to control or prevent fluid flow in the sub-surface. For this, unidirectional flow-through experiments in sand columns were carried out examining the extent and distribution of the filled pore space as a function of varying injection strategies. When the ureolytic bacteria Sporosarcina pasteurii and the cementation fluid were injected at the same time (parallel injection), a heterogeneous calcite fill along the column occurred, where most calcite precipitated close to the inlet area. In contrast, when S. pasteurii was injected first, followed by the cementation fluid (staged injection), a more homogeneous distribution developed. Moreover, greater porosity was reduced (up to 54%) using staged injection compared to 34% for parallel injection. Detailed backscattered electron imaging showed that during parallel injection most injected bacteria accumulated near the inlet, while during staged injection, bacteria were distributed more evenly throughout the columns. Furthermore, calcite crystals formed during repeated and staged injection exhibited microstromatolitic textures where calcite growth layers were marked by embedded bacterial cells. This highlighted the importance of repeated bacterial injection to ensure the continuation of ureolysis and calcite precipitation. Furthermore, this texture indicated that bacterial immobilisation within sand is enhanced in the presence of freshly precipitated calcite surfaces, thereby creating a positive feedback effect (i.e., inducing higher rates of ureolysis and calcite precipitation).