Prediction model of water-soluble gas content in a high-pressure and high-temperature water-soluble gas reservoir

In high-temperature, high-pressure (HTHP), water-soluble gas reservoirs, a significant amount of natural gas is dissolved in the formation water. During development, with changing reservoir pressure, the natural gas in the formation water will be released. The released natural gas will carry the formation water to form gas-liquid two-phase flow into the porous media of the gas reservoir, which results in premature water breakthrough, thus changing the law of formation water invasion. In order to determine the influence of water-soluble gas on formation water invasion, it is important to study the water-soluble gas content of water-soluble gas reservoirs. Based on experimentation and existing modeling, a prediction model was established for water-soluble gas content in HTHP water-soluble gas reservoirs, and the influence of water-soluble gas content on water invasion was studied. From the results, the following conclusions can be drawn. (a) The content of water-soluble gas increases with increasing pressure, first decreasing and then increasing with increasing temperature. (b) In addition to temperature and pressure, the factors affecting the content of water-soluble gas include the gas components and the mineralization degree of formation water. (c) The existing prediction model has the problems of large error and inconsistency with the measured value, (average error being 33.92%), especially lacking consideration of how changing carbon dioxide content in the gas composition affects the water-soluble gas content. (d) Considering the influences of temperature, pressure, gas components, and formation water mineralization on the prediction model of water-soluble gas reservoirs, the calculation results of the model are more in line with the actual circumstances of gas reservoirs, with the average error being 6.81% and the accuracy of water-soluble gas content prediction being increased five times. (e) Through case analysis, it is shown that the release of water-soluble gas has a great impact on the water breakthrough law of gas wells. Considering the water-soluble gas, the water breakthrough time of gas wells is relatively early. Results show that the water-soluble gas content prediction model for HTHP water-soluble gas reservoirs can predict the water-soluble gas content of water-soluble gas reservoirs more accurately, laying a foundation for understanding the water invasion law of water-soluble gas reservoir development and developing such a gas reservoir efficiently.