Semi-analytical Modeling of Multi-stage Fractured Horizontal Wells Coupled with Geomechanics: Considering Hydraulic Fracture Stress-sensitivity and Shale Anisotropy

Abstract

Production from multi-stage fractured horizontal wells (MFHWs) in shale reservoirs causes stress changes which further influence the conductivities of the hydraulic fractures. Moreover, many shale rocks are strongly anisotropic. The objective of this study is to develop a fast semi-analytical model for MFHWs with coupling geomechanics and fluid flow. The effects of stress-sensitive hydraulic fractures and shale anisotropy are considered. This study first uses an exponential correlation between principal stresses and pore pressure changes in anisotropic shale. Then the correlation is applied to match experimental data of fracture conductivities vs. effective stress. The fracture compressibility in the exponential equation is stress-dependent rather than constant. Next, this study discretizes each hydraulic fracture into several slab source segments. For each segment in each time step, pressure distribution is calculated with source/sink functions. Then both stress field and hydraulic fracture conductivities are updated according to the pressure distribution with the aforementioned correlations before starting next time step. Calculations of production rates with this model generate series of type curves for rate transient analysis of MFHWs with stress-sensitive hydraulic fractures. First of all, the production rate of a MFHW with stress-sensitive hydraulic fractures decreases faster than that with constant fracture conductivities at early-stage production. Secondly, the hydraulic fracture compressibility decreases with the increasing effective stress. Such change of the fracture conductivity is determined by the initial value and declining rates of fracture compressibility. Thirdly, shale anisotropy caused by kerogen leads to a larger increase of effective horizontal stresses when the reservoir pressure drops during production. Moreover, in terms of stress-sensitivity, fracture designs with improved conductivity are more resistant to the loss of fracture conductivity than large-volume designs for longer fracture length but lower conductivity.