Pressure transient characteristics of multi-stage fractured horizontal wells in shale gas reservoirs with consideration of multiple mechanisms

The approach of multi-stage fractured horizontal wells (MFHWs) has been proven an efficient technology for successful development of shale gas reservoirs. Together with multiple gas flow mechanisms, the existence of multiple hydraulic fractures further complicates the gas flowing problem in shale gas reservoirs. Understanding pressure transient dynamics of MFHWs in shale gas reservoirs is of great importance to provide a perception into long-term pressure dynamics forecast as well as to estimate relevant reservoir and fracturing parameters. This paper presents a comprehensive seepage model to investigate the characteristics of pressure transient responses of a horizontal well intercepted by multiple finitely conductive hydraulic fractures in shale gas reservoirs, incorporating desorption and diffusive flow in shale matrix. Point source theory, Laplace transformation and numerical discrete method are employed to solve the mathematical model. Type curves are plotted by using Stehfest inversion method. Characteristics of type curves are analyzed, and flow regimes are identified. Finally, a sensitivity analysis is conducted to screen influential parameters to the dynamics of MFHWs in shale gas reservoirs. Results from this work could provide insights into the pressure transient dynamics of MFHWs during shale gas production, reservoir parameters estimation and fracturing parameters optimization. INTRODUCTION Shale gas reservoirs have been a growing source of natural gas development worldwide. Typically, shale gas reservoirs have extremely low matrix permeability and clusters of natural fractures. As reported by Javadpour (Javadpour, 2007) the permeability of shale is in the microto nanodarcy range. Gas in shales is stored not only as free gas but also as adsorbed gas, thus gas flow in shales is believed to be a combined result of multiple mechanisms, including desorption, diffusion and viscous flow. These complicated storage and transport mechanisms present challenges to the petroleum industry. Furthermore, the very low matrix permeability of shale makes un-stimulated production of shale gas quite difficult. MFHW has been proven an efficient technology for successful development of shale gas. Together with multiple gas flow mechanisms, the existence of multiple hydraulic fractures further complicates the gas flowing problem in shale gas reservoirs. Several researchers have studied the performance of MFHWs in unconventional reservoirs. Larsen and Hegre (Larsen and Hegre 1991 and 1994) studied pressure transient behavior in a horizontal well with transverse fractures and provided a description of its pressuretransient flow regimes. Raghavan et al. (Raghavan et al. 1997) discussed the effects of number, location and orientation of fractures on pressure transient responses in conventional reservoirs. Kobaisi et al. (Kobaisi et al. 2006) proposed a hybrid numerical/analytical model to represent the pressure transient response of a finiteconductivity fracture intercepted by a horizontal well. Medeiros et al. (Medeiros et al. 2008) simulated the effects of matrix permeability, fracture spacing and well spacing on pressure and pressure derivative responses in tight gas reservoirs. Some work has also been done to incorporate the effect of gas desorption in shale gas reservoirs. Freeman et al. (Freeman et al. 2009) and Cheng (Cheng 2011) employed a simulator to study the flow regimes for a multiple-fractured horizontal well in shale gas reservoirs and the effect of gas desorption was briefly discussed. Imad et al. (Imad et al. 2011) presented a composite model to model production from a multiple-fractured horizontal well in shale gas reservoirs. They introduced a desorption compressibility in the partial differential equation to account for the desorbed gas. But the diffusion mechanism in shale gas reservoir is not considered. Wang (Wang 2014) presented a model for MFHWs in shale gas reservoirs with consideration of multiple mechanisms, but hydraulic fractures in his