The Effect of Kerogen-Hydrocarbons Interaction on the PVT Properties in Liquid Rich Shale Plays

Abstract In liquid rich shale plays, the hydrocarbons may remain associated with the parent kerogen. During catagenesis of kerogen in the geologic history, the kerogen is converted to hydrocarbons. A fraction of the generated hydrocarbons is expelled from the residual kerogen matrix, while the rest is retained within the kerogen matrix. In liquid rich shale plays, the hydrocarbons retained within the kerogen plays a key role in estimated ultimate recovery (EUR). However, the vapor-liquid equilibrium between oil and gas in shales is affected by the presence of kerogen. The conventional compositional simulators ignore this kerogen-fluid interaction in calculation of pressure-volume-temperature (PVT) properties which leads to vastly different results. The swelling of kerogen in the presence of solvents has been experimentally studied in previous works. These experiments suggest that the hydrocarbons expelled from the kerogen structure remain in a multicomponent equilibrium with the kerogen matrix. Molecular Dynamics Simulations (MDS) techniques have been used in the current paper to confirm the swelling behavior of kerogen in presence of solvents. The retained hydrocarbons stay in the dissolved form within the kerogen matrix. Considering the cross-linked nature of kerogen, there have been studies of the equilibrium behavior between the kerogen matrix and the expelled hydrocarbons using an extended Flory-Rehner Regular Solution theory model. The effect of this equilibrium between the expelled fluids and the kerogen matrix on PVT properties has been studied in the current paper. The vapor liquid equilibrium between oil and gas in the expelled hydrocarbons has been solved together with the kerogen-expelled hydrocarbons equilibrium. The kerogen-fluid interaction in liquid rich shales affects important reservoir fluid properties such as saturation pressures, gas oil ratios (GOR) and formation volume factors leading to changes in produced GOR. A thermodynamic model has been proposed in the current study to calculate changes in above properties due to the kerogen-fluid interaction. The PVT properties calculated with the proposed model account for the kerogen-fluid interaction and will significantly change the predicted recoveries from liquid rich shale plays