Simulation of Gas Desorption and Geomechanics Effects for Unconventional Gas Reservoirs

Abstract

Abstract Shale gas resource plays an increasingly important role in energy supply worldwide. Hydraulic fracturing of horizontal wells is crucial for economic production of shale gas. Gas in shale reservoirs is mainly composed of free gas within fractures and pores and adsorbed gas in shale matrix. With gas production, more gas may be released or desorbed because of substantial pressure depletion, especially during late time of gas production. However, hydraulic fractures may close to some degree based on proppant quality and shale hardness, resulting in decreasing gas production. Impacts of gas desorption and geomechanics in hydraulic fractures, i.e., stress-dependent propped fracture conductivity, on ultimate gas recovery are not clearly understood and systematically investigated. Additionally, most reservoir modeling work in the literature have usually ignored the gas desorption and geomechanics effects together. Therefore, it is absolutely critical to study and evaluate the impacts of gas desorption and geomechanics on gas recovery for different shale reservoirs. In this paper, we perform history matching with two field gas production data from Barnett Shale and Marcellus Shale, and first analyze the positive contribution of gas desorption and the negative effect of geomechanics on gas production, respectively, and then compare these two effects on gas production with the purpose of identifying which effect is dominant in the whole process of gas production. Furthermore, we numerically study the effect of gas desorption on gas recovery with available laboratory data of Langmuir isotherm from five different shale formations including Barnett Shale, New Albany Shale, Eagleford Shale, Marcellus Shale, and Haynesville Shale. The impact of different fracture spacing on gas desorption is considered. Also, we use the method of Design of Experiment (DoE) to perform sensitivity studies with six uncertain parameters such as reservoir permeability, bottom hole pressure (BHP), fracture conductivity, initial reservoir pressure, porosity, and fracture spacing to screen insignificant parameters and obtain critical parameters that control this process. This paper enables operators to develop an early better understanding of the effects of gas desorption and geomechanics on shale gas well performance, and provides insights into history matching and optimization of hydraulic fracturing treatment design for shale gas production.