Transient-rate analysis of stress-sensitive hydraulic fractures: Considering the geomechanical effect in anisotropic shale

Production from multistage-fractured horizontal wells (MFHWs) in shale reservoirs causes stress changes that further influence the conductivities of hydraulic fractures. Moreover, many shale rocks are strongly anisotropic. The objective of this study is to semianalytically model hydrocarbon-flow dynamics in reservoirs with MFHWs. The effects of stress-sensitive hydraulic fractures and shale anisotropy are considered. First, this study explores the relationship between principal-stress and pore-pressure changes in anisotropic shale. Second, an exponential correlation is further incorporated to describe the fracture conductivities vs. pore-pressure changes in anisotropic shale. The exponential correlation is validated by matching experimental data of fracture conductivities vs. effective stress. The fracture compressibility d_f in the exponential equation is stress-dependent rather than constant. Next, this study discretizes each hydraulic fracture into several source segments. For each segment in each timestep, pressure distribution is calculated with source/sink functions. Both the stress field and the hydraulic-fracture conductivities are updated according to the pressure distribution with the previously mentioned correlations before starting the next timestep. In addition to the constant-bottomhole-flowing-pressure condition, nonconstant bottomhole pressure (BHP) in real-field cases can also be entered for this semianalytical model. The model is validated by comparing its results with numerical simulations. A series of type curves q vs. t is generated on the basis of model calculations. The type curves are applied to investigate the effects of initial fracture conductivity F_ci, initial fracture compressibility d_fi, declining rate of fracture compressibility b, shale anisotropy, and the BHP profiles on MFHW transient-rate behavior. To maximize the hydrocarbon production, the BHP profile must be adjusted on the basis of fracture stress-sensitive characteristics. The semianalytical model is used to analyze two field cases with different p_wf profiles under the influence of stress-sensitive hydraulic fractures.