Gas trapping and mobilization through water influx in natural gas reservoirs.

摘要

Residual gas saturation is important in determining recovery from a gas reservoir, with water influx. This research addressed residual gas saturation, and other variables, resulting from imbibition tests in porous media, which included spontaneous and forced imbibition; co-current and counter-current imbibition; primary and secondary imbibition; water imbibition and oil imbibition. Several hundred water and oil imbibition experiments were performed on 47 core plugs, consisting of sandstone and carbonate samples. Concurrently, automated experimental rigs were developed, employing a balance in one case, and NMR in the other. The experimental techniques were critically evaluated with regard to repeatability and accuracy.; The residual gas saturation, and gas saturation as a function of imbibition time, liquid distribution in different pore sizes, and relative wettability were investigated. A new Critical Capillary Number for gas-liquid system is postulated. A simple procedure was developed for determining wettability from imbibition data. Capillary pressure and relative permeability curves were extracted from the experimental data, and used in 1D and 3D numerical simulations of co-current and counter-current laboratory imbibition tests. The simulations showed the important role of gas compressibility and verified the residual gas saturation. Additionally, a gas reservoir with strong water influx was simulated, and methods for improving gas recovery were examined via many sensitivity studies. It is concluded that because of gas compressibility, residual gas saturation is important in low pressure systems, such as laboratory experiments, but is not important in a gas reservoir, unless the abandonment pressure is close to the initial pressure.