Abstract
Although a multi-stage hydraulically fractured horizontal well in a shale reservoir initially produces gas at a high production rate, this production rate declines rapidly within a short period and the cumulative gas production is only a small fraction (20–30%) of the estimated gas in place. In order to maximize the gas recovery rate (GRR), this study proposes a multi-parameter optimization model for a typical multi-stage hydraulically fractured shale gas horizontal well. This is achieved by combining the response surface methodology (RSM) for the optimization of objective function with a fully coupled hydro-mechanical FEC-DPM for forward computation. The objective function is constructed with seven uncertain parameters ranging from matrix to hydraulic fracture. These parameters are optimized to achieve the GRR maximization in short-term and long-term gas productions, respectively. The key influential factors among these parameters are identified. It is established that the gas recovery rate can be enhanced by 10% in the short-term production and by 60% in the long-term production if the optimized parameters are used. Therefore, combining hydraulic fracturing with an auxiliary method to enhance the gas diffusion in matrix may be an effective alternative method for the economic development of shale gas.
Highlights
As shale gas reservoirs typically have extremely low permeability, their flow behaviors are very different from those in conventional gas reservoirs
The sensitivities of the seven variables are quantitatively and synchronously evaluated. These results may be helpful in the choice and design of variables or parameters to maximize gas recovery rate (GRR), providing a guide for the exploration and development of shale gas reservoirs
It is noted that the equation for the gas flow in a hydraulic fracture is implemented through a weak form in the code
Summary
As shale gas reservoirs typically have extremely low permeability, their flow behaviors are very different from those in conventional gas reservoirs. Wang et al [22,23] used RSM to investigate the sensitivities of seven parameters including structural parameters, geomechanical parameters, in-situ field stress parameter (stress difference) and operational parameter (injection rate) on maximizing stimulated reservoir volume (SRV) Their studies proved the feasibility of RSM in shale gas production and hydraulic fracturing modeling. An optimization algorithm will be introduced for multi-stage hydraulically fractured shale gas horizontal wells This algorithm combines the RSM for objective optimization and the fully coupled hydro-mechanical fracture equivalent continuum-dual porosity model (FEC-DPM) for forward computations [14]. The sensitivities of the seven variables are quantitatively and synchronously evaluated These results may be helpful in the choice and design of variables or parameters to maximize GRRs, providing a guide for the exploration and development of shale gas reservoirs
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