Estimation of gas-in-place content in coal and shale reservoirs: A process analysis method and its preliminary application

Abstract

The gas-in-place content in coal and shale reservoirs is one of the key parameters, which impacts the exploitation potential of natural gas resources. Though its estimation has been paid close attention in the last decades, there is not an effective method that can be utilized to obtain reliable gas content data (including total, adsorbed, and free gas contents) in coal and shale reservoirs. In this paper, a process analysis method is proposed, which suggests dividing the coring and on-site desorption processes into five stages and comprehensively considers the gas release behaviors of only free gas and both adsorbed and free gas during different stages. Significantly, the two key parameters of time zero and lost gas time were precisely determined. The release of free gas can be described by the equation of gas state. The release of both adsorbed and free gas can be described by the Langmuir isothermal adsorption equation, the equation of gas state, and Fick’s first law. As a case study, the gas-in-place contents of two cores obtained from coal and shale reservoirs, respectively, were estimated. Prior to the application of the method, a series of operations involving the coring, on-site canister desorption, and reservoir characterization should be performed. Then, the total gas and the adsorbed and free gas contents can be obtained using this method combined with the on-site desorption data. The method proposed in this paper has a wide application potential in the estimation of coalbed methane and shale gas resources.