An optimal calculation method for the geological reserves of shale gas reservoirs

Evaluating the gas in place accurately is one important part of the development planning of shale gas reservoirs, and its theories and methods are being improved continuously, but there are still some deficiencies. The adsorbed gas in shale gas reservoirs not only contains methane, but also a certain percentage of ethane and other hydrocarbon gases, so the adsorbed gas reserves shall be calculated by using the multi-component adsorption model. In addition, there is natural gas in a great number of natural fractures, which is not negligible. What’s more, some gas is dissolved in the kerogen, and greater deviation will be induced if the dissolved gas is neglected. The multi-component adsorption model is used, the porosity occupied by the adsorbed phase, the free gas in fractures and the dissolved gas in kerogen are taken into account, and a modified calculation method for the gas in place of shale gas reservoirs is established. Case analysis indicates that the percentages of fracture free gas and dissolved gas in the total reserves are 10.41% and 7.05%, respectively. The adsorbed gas reserves calculated by using the traditional method is lower, the matrix free gas reserves is higher and the total reserves is lower. In order to evaluate the gas in place of shale gas reservoirs rationally, it is necessary to adopt the multi-component adsorption model, and take into consideration the porosity of the adsorbed phase, as well as the free gas in fractures and the dissolved gas in kerogen.