This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 181365, “Production Pressure-Drawdown Management for Fractured Horizontal Wells in Shale Gas Formations,” by Ankit Mirani, University of Houston; Matteo Marongiu-Porcu, SPE, Schlumberger; HanYi Wang, SPE, The University of Texas at Austin; and Philippe Enkababian, SPE, Schlumberger, prepared for the 2016 SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September. The paper has not been peer reviewed.
The primary objective of this study is to address all known causes of productivity declines in unconventional shale gas formations with horizontal multifractured wells and to develop
a fully coupled geomechanical/flow simulation model to simulate these production conditions. The model mimics the effect of depletion-induced in-situ stress variations on productivity by taking into account several phenomena, such as stress-dependent matrix and natural-fracture permeability and reduction in fracture conductivity because of fracture-face creep and proppant crushing, deformation, and embedment.
Reservoir Matrix. The gas-storage mechanism in a shale gas reservoir is conceptually different from that of a conventional reservoir. Natural gas is adsorbed on the organic matter present in shales and, in some cases, on certain clay minerals. This adsorbed-gas layer further constricts the area for flow through the pore structure, thus reducing the effective pore radius. As the pressure in the pores reduces, gas is desorbed and the effective pore radius increases, resulting in an increase in the apparent matrix permeability. It has been suggested that desorption of methane might be partially responsible for the relatively long and flat production tails that have been observed in some shale reservoirs.
Fig. 1 shows the effects of different mechanisms on permeability evolution. Clearly, shale gas long-term performance forecasting will be affected severely if these effects are not properly considered.
Hydraulic Fractures. Hydraulic fractures are generally kept open and conductive by proppant. After exposure to confining stress for a long period of time, proppant can become deformed, causing degradation of its mechanical and physical properties. This leads to reductions in fracture width and fracture-pack permeability because of intermixing and plugging.
Additionally, the interaction between the proppant and fracture surfaces under the same confining stress can result in embedment, decreasing the fracture aperture and impairing conductivity.