VISUALIZING AND QUANTIFYING BIOMINERALIZATION IN A WELLBORE ANALOG REACTOR

The keystone of subsurface fluid storage is the ability to inject and keep fluids deep in underground aquifers for extended periods of time. Fluid migration in these systems can cause failure of operations and lead to unwanted fugitive emissions. Pressure gradients between the storage reservoir and the surface can create a driving force for fluid migration to the surface, requiring a reliable seal to maintain containment of subsurface fluids. A great risk for seal failure lies in and around the wellbore; defects between the wellbore cement in the annulus and the well casing are of particular concern. While current oil field remediation technologies, such as cement injections, are effective for a majority of defects, they can falter with defects small enough to hinder high viscosity fluid injection. Microbially induced calcite precipitation (MICP) is a new technique that uses low viscosity fluids and microorganisms (~2 µm diameter) to seal these small defects.

In the MICP process, ureolytic microorganisms hydrolyze urea into carbonate and ammonia. When this process occurs in the presence of fluids rich in calcium, the carbonate can react with calcium ions to form calcium carbonate. This calcium carbonate can then precipitate and form a seal that is capable of bridging small fluid migration pathways. To better understand the capability of MICP in the subsurface, an analog reactor modeled closely after an injection well used for MICP field demonstrations was developed to simulate the downhole geometry and flow dynamics seen in the wellbore. Defects of 0.2 mm to 1.5 mm can be created in the cement annulus and a transparent outer casing allows for visual observation of calcium carbonate formation in the annulus during experiments. Recent MICP treatments performed in this analog reactor resulted in a three order of magnitude permeability reduction of a flow channel approximately 25.4 mm in width and 0.8 mm in depth due to the creation of a calcium carbonate seal. Our results show the promise of the MICP technology to complement existing well-cement remediation technologies, sealing small defects and improving the integrity of down-hole gas storage systems.