Estimating Free-to-adsorbed Ratio Based on Acoustic Amplitude Attenuation and Porosity in Shale Gas Reservoirs

Abstract

Gas content is currently determined using either direct measurements, such as on-site desorption canister tests, adsorption isotherm experiment, measurements of porosity and saturation used in conventional gas content tests, or via indirect prediction such as regression-based approach or calculations using measured dada and wireline logs. Generally, the free gas content can be calculated in terms of porosity, gas saturation, in-situ temperature and pore pressure. And adsorbed gas content can be calculated based on adsorption isotherm experiment. However, there are some drawbacks involved with these methods, such as high costs, limited samples. In this paper, a simple and effective method of estimating free-to-adsorbed gas content ratio is proposed for shale gas reservoirs. Firstly, the adsorption isotherm and acoustic amplitude measurement joint experiment is designed. Meanwhile we get the Langmuir isotherm, we can also get the acoustic amplitude at different pore pressures. Then the empirical relationship between gas content, acoustic amplitude attenuation coefficient and porosity can be acquired by the experimental data. And the new method is applied well in YS shale gas field, and has good agreement with the results by the traditional method.