Influence of magnitude and permeability of fracture networks on behaviors of vertical shale gas wells by a free-simulator approach

Abstract

After stimulated reservoir volume (SRV), complex fracture networks can be generated around shale gas (SG) vertical wells. Due to intricate flow characteristics, performance forecasts of those wells highly depend on numerical simulators. To lighten the burden of numerical computation work, a free-simulator approach was proposed in this work.To facilitate methodology development, the SG formation was assumed to be made up of two systems (reservoir/fracture system) and four media (shale matrix, unreacted nature fractures, reacted nature fractures or micro fractures (MF), and hydraulic fractures (HF)). Based on that, a mathematical model comprised of reservoir equations, fracture equations, and mass balance equations was developed for vertically fracture-network wells (VFNW). Using Laplace transformation and Pedrosa's substitution, equations in the mathematical model were solved. Type curves were generated and flow regimes were identified. Case study and sensitivity analysis were also conducted.Results from this study show that the possible flow regimes for a VFNW in SG reservoirs includes (1) wellbore storage and skin effect, (2) bilinear flow, (3) formation linear flow, (4) pseudo boundary-dominated flow, (5) bi-radial flow, (6) Knudsen diffusion flow, and (7) pseudo radial flow. As the magnitude of fracture networks increases, the formation linear flow lasts longer and the pseudo boundary-dominated flow occurs later. As the permeability of fracture networks increases, the pseudo boundary-dominated flow becomes more obvious. Additionally, it is found that pressure depletion in the SG reservoir decreases with the increase of magnitude or permeability of fracture networks.This work provides a novel respective on performance forecast on vertical wells with fracture networks, which has considerable significances on fracturing optimization as well as productivity estimation for SRV.