Where gas is produced from a shale formation: A simulation study

Gas production from shale reservoirs has increased rapidly in the past decade, becoming an important source for natural gas supply. For tight shale formations, horizontal wells combined with multi-stage hydraulic fracturing is the key to realize commercial production. The multi-scale flowing spaces and complex flowing mechanisms of shale gas reservoirs pose challenges to elucidating their production behaviors. Existing modelling methods are not able to distinguish gas production from different regions or different forms (adsorbed-gas/free-gas). Therefore, they cannot accurately assess how much produced gas is from desorption effects and whether gas stored in the unstimulated region will support production. Answers to these questions will improve our understanding of production mechanisms and facilitate the optimization of development plans. In this study, we proposed a general component subdivision modelling method to separately model the flowing behavior of gas stored in different forms or different regions. As an example the method is incorporated in the compositional model in the UNCONG simulator. Applying this method, a conceptual model based on Fuling Jiaoshiba shale was built, and contributions from adsorbed-gas/free-gas inside of the stimulated reservoir volume (SRV) and outside of the SRV were analyzed. Sensitivity studies on fracture spacing were also performed. The results show that, for the Fuling case, when the matrix permeability is 10(-4) mD, 78% of total production comes from the free-gas inside of the SRV after about 20 years of production, and free-gas outside of the SRV and adsorbed-gas inside of the SRV also help to sustain long-term production. However, when the matrix permeability is as low as 10(-5) mD, almost all production comes from free-gas inside of the SRV within 20 years of production. Sensitivity studies show that network fractures play an important role to increase production. In the final case study, a more efficient way for component subdivision modelling was demonstrated. This study provides insights for shale gas production and its controlling factors, which can be utilized to optimize fracturing design and resources assessment. (C) 2017 Elsevier B.V. All rights reserved.