Beneficial Use of Produced Water and Carbon Dioxide in Depleted Oil Reservoirs: In-Situ Microbial Conversion of Crude Oil to Methane

Abstract

Produced water from oil and gas reservoirs contains high levels of organic chemicals and heavy metals whose concentrations, in many cases, exceeds USA drinking water standards. To prevent the contamination of underground sources of drinking water, produced water from oil and gas reservoirs is commonly disposed into deep highly porous and permeable media such as saline aquifers. However, produced water also contains indigenous microbial communities that are well adapted to deep underground conditions. This property of petroleum produced water can be used to convert remaining crude oil (mostly n-alkanes) in depleted oil reservoirs to methane (CH4) gas. This can be done by stimulating the activity of methanogenic and crude oil-degrading microbial communities present in produced water by combining the supply of protein-rich matter and CO2.

The goal of my research was to numerically assess the feasibility of microbial conversion of CO2 and crude oil to CH4 in depleted oil reservoirs. To achieve this goal, I developed a non-iterative fugacity-activity thermodynamic model to predict the mutual solubility of CO2-CH4-H2S-N2 gas mixtures and brine, and also expanded the capability of a thermodynamic model to calculate the solubility of CO2-CH4-H2S-H2 gas mixtures in brine. After that, I included these thermodynamic models into multiphase-multicomponent fluid flow simulators TOUGH2 and TOUGHREACT. The flow capabilities of the developed TOUGH2 and TOUGHREACT modules were verified by comparing their simulation results with other reliable multiphase simulation programs including the ECO2N module of TOUGHREACT, the EOS7C module of TOUGH2, and CMG-GEM©. The final step was to apply relevant changes in the reactive part of the developed TOUGHREACT module, named CO2Bio. The CO2Bio can simulate the multiphase-multicomponent reactive transport of CO2-CH4-H2S-H2 gas mixtures and brine under biotic conditions. Using CO2Bio, a simulation study was conducted in a section of Cushing oil reservoir in Oklahoma. The results of this simulation showed that it is feasible to produce CH4 from biodegradation of the remaining crude oil in depleted oil reservoirs.