TOWARDS REDUCING WELLBORE LEAKAGE VIA A NOVEL WELLBORE BIOMINERALIZATION TECHNOLOGY BASED ON MICROBIALLY INDUCED CARBONATE PRECIPITATION (MICP)

Wellbore leakage, from fractured hydrocarbon wells, can lead to stray oil and gas migration resulting in greenhouse gas emissions, risk of impaired potable groundwater quality and costly repairs. A promising repair technology is, 'microbially induced carbonate precipitation’ (MICP), a method of carbonate formation by bacteria. It has been previously applied in wellbore leak reduction at small aperture fractures where cement based sealing options such as the, 'cement squeeze method' are not applicable. However, it offers only shallow depth of repair based on the metabolic limits of the bacteria so far employed. The use of MICP under deep depth wellbore conditions at high temperatures, pressures and anaerobic conditions has yet to be explored.

This research presents preliminary lab scale experiments screening extremophiles of the genera Geobacillus, Anoxybacillus and Gracilibacillus for the ability to survive and undergo carbonate precipitation in deep wellbore-like conditions in comparison to well studied MICP species of the genera Bacillus and Sporosarcina.

In nutrient bench tests, species of Geobacillus were found to grow to the highest densities (OD₆₀₀ = 2 – 4) and in the broadest conditions (40 – 75ºC, pH 5 – 10, 0 – 5% salinity) both aerobically and anaerobically compared to those of the other genera considered. Two species, Geobacillus subterraneus (DSM 13552) and Geobacillus thermodentrificans (DSM 465) could produce carbonate sediment, supported as calcium carbonate (CaCO₃), by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), via anaerobic denitrification. Crystals were confirmed to be calcite by X-ray diffraction (XRD) and of the size (10 – 85 µm) for microscale (1 – 50 µm) fracture sealing. Finally, treatment of concrete pieces with G. thermodenitrificans had CaCO₃ precipitate along their surfaces in 8 hours. This indicates fractured concrete could be sealed by surface MICP at high heat.

This study has shown that select species of the genus Geobacillus can preferentially survive and form carbonates, found as CaCO₃, on concrete surfaces at certain deep depth wellbore-like conditions. This gives promise for MICP as a sealing technology of wellbore fractures at deep depths. Next steps will evaluate the efficacy of the technique by simulating a repair procedure under a core-flood system.