The flowback and production analysis in sub-saturated fractured shale reservoirs

The initial water saturation of shale reservoirs is in general lower than the irreducible water saturation. However, the hydraulic fracture network region is highly saturated by fracturing fluid, which results in a two-phase flow inside fractured region. A long-time two-phase flow period is always observed in the fields even after a high flowback rate, which cannot be explained by existing models. In this paper, we develop a 2D gas-water flow model coupling multi-scale discrete fractures and continuum framework to describe the flowback process of a multi-stage fractured horizontal well in shale gas reservoirs. The model considers the non-linear flow mechanism of free gas, desorption of adsorbed gas and the co-existence of single gas flow and gas-water flow. The capillary pressure model and relative permeability model are both employed with different parameter values for matrix and fractures, respectively. The permeability of naturally fractured shale is described using a fractal model. To overcome non-convergence problem of finite element method for solving two-phase flow in cross fractures, the model is solved by finite element method and finite volume method. The model is verified through a simple two-phase flow simulation and a single gas phase simulation with some simplifications. Then the model is used for history matching as well as production prediction of a well in Changning block, Sichuan, China. The simulation results show that the high water saturation induced skin factor in the vicinity of hydraulic fractures significantly delays the gas production for a long time because some of the fracturing fluids become irreducible water and lower the gas phase permeability. Due to the great influence of fracture interference on water flow, the fracturing fluids concentrate on regions near hydraulic fractures and their junctions. The long-term gas production of the complex fracture network is slightly higher than that of the simple fracture network, which is different with the simulation results of single gas flow model.