Using Low-Field Nuclear Magnetic Resonance and X-Ray Computed Microtomography Imaging to Explore Potential of Microbially-Induced Calcium Carbonate Precipitation Treatment to Seal Shale Fractures

ABSTRACT Microbially-induced calcium carbonate precipitation (MICP) is a biological process in which microbially produced urease enzymes convert urea and calcium into solid calcium carbonate (CaCO3) deposits. Studies have shown that MICP can be used to seal fractures in shale, raising the possibility of applying this technology to restimulate fracking wells by plugging underperforming fractures. For this and other applications to become a reality, non-invasive tools are needed to determine how effectively MICP seals shale fractures under subsurface conditions. In this study, a 2.54 cm wide and 5.08 cm long Marcellus shale core with a single, ∼1 mm wide fracture held open by sand "proppant" underwent MICP-treatment at 60°C until reaching three orders of magnitude permeability reduction. Low-field nuclear magnetic resonance (LF-NMR) and X-Ray computed microtomography (μ-CT) techniques were used to assess the extent of biomineralization within the fracture. These tools revealed that while CaCO3 precipitation occurred throughout the fracture, there was preferential precipitation around proppant, and the core sealed at the effluent end before filling most of the fracture. Both tools were able to independently calculate of the amount of solid biomineral formed inside the fracture. This study found that the distribution of proppant within the shale fracture was an important parameter controlling the degree of biomineralization. INTRODUCTION Background Shale is a fine-grained, sedimentary rock often found adjacent to layers of sandstone and limestone up to thousands of feet below ground surface (bgs). The unique characteristics of this formation have led it to play multiple roles in the ever-changing energy landscape of the 21st century. Possessing relatively high porosity but extremely low permeability, shale has been targeted for geologic carbon sequestration and nuclear waste disposal (Washburn, 2014). Many shale deposits also contain rich quantities of hydrocarbon fuels, which can be extracted from very tight pore spaces with hydraulic fracturing, also known as fracking. All of these applications highlight a need for methods to control permeability to ensure the long-term safety and utility of these reservoirs. For example, there may be a need for a method to plug fractures that arise in shale used for storage purposes to mitigate leakage. There is also motivation to seal fractures created in fracking wells to prevent contamination of aquifers by fracking fluids or potentially restimulate the well to enhance natural gas recovery (i.e. "frack/re-frack").