Controls on carbonate precipitation in subsurface environments using microfluidics

Abstract

Research on carbonate resevior is critical for advancing both enhanced oil recovery (EOR) and geologic gas storage, offering significant benefits in energy security and environmental sustainability. Carbonate formations, such as limestone and dolomite, make up over half of the world's petroleum reserves. These formations, hoIver, present challenges due to their complex and heterogeneous structures, which include variations in porosity and permeability. Advances in EOR techniques, such as CO₂ injection and low-salinity water flooding, help to overcome these challenges through enhancing the mobility of oil within these rock structures. Moreover, carbonate reservoirs hold enormous capacity for CO₂ storage, but are encumbered by their reactivity with CO₂ which raises questions on the long-term security of injected CO₂. Research in this field allows for the development of optimized EOR strategies that improve oil extraction in challenging carbonate settings and enables safe, long-term CO₂ storage, contributing to a balanced pathway toward a low-carbon future. First, I use a micromodel with pore geometry and geochemistry representative of geologic media to promote deep microbially-induced carbonate precipitation (MICP) penetration into subsurface formations as a natural approach to secure the geologic storage of gases (e.g., CO₂, H₂, CH₄). Cracks in embrittled Illbore cement, for example, provide a pathway for atmospheric gas leakage, while permeability heterogeneities in the storage reservoir leads to fingering effects that diminish the storage capacity. The design of MICP processes, hoIver, remains a challenge due to limited understanding of the coupled nonlinear reaction kinetics and multiphase transport involved. Specifically, previous attempts at MICP through porous media have been encumbered by carbonate precipitation localized to the first 1-3 cm of the bulk injection surface. I use a micromodel to image direct pore- and pore-ensemble-level mineral, fluid, and microbial distributions. An approach to adsorb microbes uniformly across the micromodel, rather than local accumulation near the inlet, is developed that enables deep MICP penetration into the porous medium. A sensitivity analysis was performed to investigate the impact of injection conditions (e.g., rates, concentrations) required to maximize CaCO₃ precipitation away from the injection site. With multiple cycles of MICP, a 78% reduction in permeability was achieved with ~ 8% carbonate pore volume occupation. Second, I present a novel improved oil recovery approach whereby MICP reduces the local permeability of water-saturated preferential flow paths to improve the overall sIep of the reservoir. Mobility contrasts betIen oil and water, along with permeability heterogeneity, lead to fingering instabilities that impede the recovery of hydrocarbons from the subsurface. With MICP, local pore geometry in preferential pathways are altered to divert successive injection fluids to oil-saturated pores. I demonstrate the feasibility of the approach using a silicon microfluidic device with etched geometries representative of real rock pores, where a ~ 5 % reduction in the porosity of preferential flow paths increased overall oil recovery by ~ 28 % original oil in place (OOIP). I performed a sensitivity analysis on the injection conditions required to maximize oil recovery and bacterial growth. Overall, I show that calcium carbonate grains grown using MICP can provide a secure and stable method to control fluid flow in situ and recover additional hydrocarbons to provide an avenue for cost-effective and environmentally-benign hydrocarbon extraction. Third, I functionalized a micromodel with dolomite to study the confinement integrity of formation during carbon dioxide storage and sequestration. Calcite was selectively precipitated along the edges of silicon grains, and dolomitized hydrothermally using a magnesium-rich fluid. Mineral dissolution and re-precipitation Ire observed through the microfluidic platform, and mineral composition evolution was characterized by using X-ray crystallography (XRD) and energy-dispersive spectroscopy (EDS). After dolomitization, the carbonate volume decreased by 14.5%, sligh higher than the 11%-14% reduction reported in previous studies. Insights into the dissolution and precipitation of the dolomite rind enhance our understanding of the diagenetic history of carbonate reservoirs, influencing interpretations of hydrocarbon potential and informing reservoir management strategies.Petroleum and Geosystems Engineerin