Investigation of gas solubility and its effects on natural gas reserve and production in tight formations

Abstract

In this study, the natural gas solubility in brine and its influence on natural gas reserve and production in a tight gas reservoir have been investigated by the integration of laboratory experiments, phase equilibrium calculations, and numerical evaluation. Experimentally, a visualized PVT cell has been used to measure natural gas solubility at various pressures, temperatures, and salinity. Theoretically, the solubilities of natural gas and natural gas components are calculated using the phase equilibrium theory and equation of state (EOS), which are calibrated by the experimental data first. A sensitivity analysis is implemented to investigate the effects of pressure, temperature, and salinity on natural gas solubility. Subsequently, with the assistance of numerical simulation, the influence of the dynamic gas solubility on the natural gas reserve and production is examined in a tight gas reservoir in Middle Permian Shihezi Formation, Ordos Basin, China. The natural gas solubility is found to be 2.91 m3/m3 at initial reservoir conditions, which contributes to an increase of 1.33% in natural gas reserve. For natural gas and formation brine with specific composition, the gas solubility decreases gradually with the reservoir pressure reduction in the production process. The production prediction indicates that the cumulative gas production from tight formation requires calibration if the dynamic gas solubility is considered. In the field-scale simulation, the cumulative gas production after 10 years is 0.52% higher than the case not considering the dynamic natural gas solubility.