Steady-state optimization model and practical design of the fracture network system in tight sand gas reservoirs

• A simulation model of seepage field in tight gas fracture mesh is established.
• The best fracture mesh is small-to-medium with a high conductivity main fracture.
• We studied the process conditions to form small-to-medium scale fracture mesh.

Based on steady-state seepage theory in porous media and the Galerkin method, a finite element model of seepage field in a stimulated reservoir volume of a horizontal well was established. The relationship between fracture conductivity and production under different fractured drainage volumes was studied. For shale gas reservoir, the productivity will increase as the fractured drainage volume enlarges. However for tight sand gas reservoir, this study revealed that the fractured drainage volume should be maintained in the mid-small scale (0.1≤FCI≤0.25) with highly conductive main fractures. For certain fracture network morphology, there exists a critical value or optimal value of SRV curve. The production increase slows down when SRV exceeds its critical value. Cluster spacing and number of clusters for horizontal well fracturing can be determined by critical value of SRV and FCI value. The paper studied the operational conditions for the creation of small and medium-scale fracture network. Firstly, analytical solution of pressure inside the fracture is derived from fracture dynamics of rocks. Pump rate of fracturing fluid, viscosity, rock dynamic fracture toughness, elastic modulus, Poisson's ratio, in-situ stress, natural fracture parameters are input parameters to this analytical solution. Optimized pump rate for small to medium-scale fracture network was derived from the dynamic expanding condition of fracture network.