Experimental investigation of microbially induced carbonate precipitation in sandstone cores under in-situ North Sea temperature and pressure conditions

Bio-grouting using ureolytic microorganisms has been developed over the past decade for civil engineering applications including: (i) sealing fractures in rock, (ii) sealing cracks in cement, (iii) reducing the permeability of porous media  and (iv) soil stabilisation and (v) repair of concrete and stone. This study investigates the potential application of microbially induced carbonate precipitation (MICP) within the oil and gas industry. To deploy MICP in a well abandonment context a more in-depth knowledge of the influence and performance under elevated subsurface pressures and temperatures is required.

Batch experiments investigated the ureolytic activity at subsurface temperatures ranging from 20-90°C and fluid pressures from 1-13MPa for up to 2hrs exposure time. Strong evidence of increased ureolytic activity was observed in specimens at temperatures of 60°C and above, but with increasing exposure time ureolytic activity ceased. In comparison increased fluid pressures had little influence on ureolytic activity. Our results imply that the bacterial cell protects the enzyme from denaturation at elevated temperature conditions.

A second set of experiments consisted of multiple injections of the treatment fluids in a fine-grained sandstone sampled from the Brent sandstone formation of the Dunlin oilfield in the North Sea. With a focus on simulating the in-situ environmental conditions, we set-up a high pressure high temperature system consisting of a HPLC pump, water bath, Hassler core-holder with pressure capabilities of up to 2400psi and a temperature rating of 90°C with high sensitivity pressure transducers and a backpressure regulator. The cores were exposed to realistic North Sea subsurface temperatures of 20, 50, 60°C and fluid pressures of 442, 1326, 1621psi according to their corresponding depths: at 1000ft, 3000ft and 3667ft.

The study investigated the influence of the pressure and temperature conditions on (i) permeability reduction, (ii) distribution of CaCO₃ precipitates via X-CT imaging and (iii) mineralogy via FE-EPMA coupled with EDX/WDX spectroscopy.

Permeability reductions in the coarse-grained sandstones of 5 orders of magnitude were achieved in all the subsurface temperature-pressure combinations tests. Micro xCT scans indicate that CaCO₃ precipitation occurred closer to the inlet as the temperature and pressure increased, due in part to the higher ureolytic activity at higher temperatures and the lower solubility of CaCO₃ at higher temperatures. At elevated pressure and temperature conditions the energy barrier to transform from a calcite dominated system could be overcome and formed predominantly aragonite.

This study has demonstrated the potential for deploying MICP at subsurface conditions in oil and gas applications. The biotechnology itself could be used to seal off reservoir formations in mature oil and gas assets, repair fluid migration pathways or act as an environmental wellbore barrier element and therefore could ultimately reduce the number of well barriers required to be installed during plugging and abandonment.