| Description |
Shale plays have revolutionized oil and gas production in the United States. In the last decade, many shale gas and liquid plays have been explored and developed in the US and elsewhere. Prospective shales consist of a complex organic component known as kerogen which is a precursor to oil and gas. Shales have pores with dimensions in the range of nanometers in the organic and inorganic constituents. The presence of organic matter and nanometer pores affect the thermodynamic properties of fluids in these rocks. A hypothesis has been proposed and proved through modeling and experiments to account for the influence of kerogen on thermodynamic properties of hydrocarbon fluids. Kerogen preferentially absorbs hydrocarbons and subsequently swells in volume. This splits oil in liquid-rich shale plays into two phases â€" a retained phase and a free phase, both of which remain in equilibrium. The retained and free phases together form in-situ oil; equilibrium of in-situ oil with gas was studied to investigate the effect of kerogen on saturation pressures of oils in shales. Results indicate a bubble point suppression between ~ 4150 kPa and ~ 16350 kPa from an original value of 28025 kPa for produced Eagle Ford oil. This is attributed to the presence of kerogen. This suppression depends on the type and level of maturity of the kerogen. The confinement of hydrocarbon fluids in the nanometer pores present in shales also changes the behavior of these fluids. Pore-wall â€" fluid interactions become dominant at the nano-scale and conventional equations of state(EOS) fail to include the effect of these confined state interactions. Gibbs Ensemble Monte Carlo simulations were performed in this work to investigate the thermodynamic properties of pure components and fluid mixtures in confined pores. Suppression of critical densities and critical temperature of confined decane, decaneâ€"methane, and decaneâ€"carbon-dioxide was observed from the bulk properties. This leads to changes in the saturation pressures of fluids in the confined state. Experiments on kerogen isolated from a shale and oil were performed with a differential scanning calorimeter and a thermogravimetric analyzer. These experiments complimented the modeling results and thus, verified the effect of kerogen and hydrocarbon fluid confinement observed in the models. Finally, for gas-rich shales, a carbon dioxide injection as the most effective method was evaluated for enhanced production of gas sorbed in kerogen. Molecular modeling indicates that the carbon dioxide can replace methane sorbed in the kerogen and the kerogen matrix decreases in volume during this process. The carbon dioxide shows higher retention in the kerogen than methane, indicating the viability of enhanced gas recovery and carbon dioxide sequestration. |
