Phase behavior and compositional simulation of solvent EOR processes in unconventional reservoirs
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Recent advances in hydraulic fracturing and horizontal drilling technologies applied to unconventional resources have enabled very large increases in oil and gas production in the United States. While the initial production rate from new wells is often high, the rate of decline during the first year of production is also very high. The ultimate hydrocarbon recovery using current technology is estimated to be between 5 to 10%, which is very low compared to conventional oil reservoirs. These challenges have increased the demand for enhanced oil recovery (EOR) in unconventional reservoirs. Unconventional reservoirs have very different properties than conventional reservoirs such as extremely low permeability, wide and complex pore size distributions, high total organic carbon (TOC), and high heterogeneity. New cost effective EOR methods consistent with these properties are needed. This research presents a new framework for phase behavior modeling and compositional simulation of solvent EOR in unconventional reservoirs (also called tight oil reservoirs). Several new physical models were developed for this purpose. The new developments include a three-phase capillary pressure model, a phase stability method for multi-component mixtures with capillary pressure, coupled three-phase flash and capillary pressure models, and a new oil characterization method for organic-rich reservoirs. These models improve petrophysical, thermodynamic, and transport modeling of unconventional reservoirs. All new models were implemented in UTCOMP, an equation-of-state compositional reservoir simulator. The simulator was used for design and optimization of solvent EOR processes in organic-rich tight oil reservoirs. The oil recovery using cyclic huff-n-puff injection of a variety of solvents such as natural gas, CO₂, methanol and dimethyl ether (DME) was compared. DME has the best performance among all solvents considered in this research. At reservoir conditions, DME mixes with both water and hydrocarbon phases. This helps to remove water blockage as well as retrograde condensate in gas-condensate reservoirs. DME can also extract part of the bitumen in the rock, which does not flow by itself due to its extremely high viscosity. Since the recovery rate of DME is very high, it can be recycled and injected back into the reservoir to reduce its net cost.