Development of a three-dimensional, three-phase coupled model for simulating hydraulic fracture propagation and long-term recovery in tight gas reservoirs.

摘要

In the past decades, development of tight gas reservoirs has become more important. These low permeability reservoirs need to be stimulated effectively with hydraulic fracturing to produce economically. Stimulation design has improved with better understanding these unconventional reservoirs, advances in modeling and study of flow mechanisms.;Conventional fracture propagation models predict fracture geometry based on fracture fluid mechanics, rock mechanics, petrophysics and empirical/analytical leak-off models. Reservoir flow simulators are then used to evaluate post-fracture well performances. These approaches are called de-coupled modeling. It is a challenge to couple these two processes, particularly when dealing with large amounts of input data. Furthermore decoupled modeling is a time-intensive job that requires a coordinated effort from stimulation and reservoir engineers. This approach may not work in low-permeability reservoirs because the hydraulic fracture propagation is complex, fracture fluid leak-off is pressure/reservoir/fracture dependent and there are changes in in-situ stress and permeability during and after a fracture treatment. It has been recognized that fluid loss can be computed directly by solving the multiphase flow equations in porous media. Such an approach is more general and does not have many of the assumptions in decoupled models. Models based on this approach are called coupled models. Hydraulic fracturing is an integrated process of injection of fracture fluid, fracture propagation, proppant transport, cleanup and multi-phase flow through the reservoir. Available coupled models are not fully integrated as they were developed to simulate just one or two of these steps.;The main objective of this research is to develop an integrated coupled model which is capable of fully simulating reservoir flow, fracture propagation, proppant distribution, flowback, long term gas recovery and resulted stress change through a stationary reservoir/stress grid system. The model uses a three-dimensional, three-phase finite difference reservoir flow simulator coupled with a finite difference geomechanics model where both are applied on the same grid system. The model has been validated with published data in the literature.;Using the developed model, parametric studies have been carried out to quantify important factors affecting fracture and recovery processes such as injection rate, treatment volume, proppant type, flowback rate and flowing bottom hole pressure (FBHP). The model enables us to simulate and compare different scenarios and suggest the optimized hydraulic fracturing design. The new findings lead to better understandings of hydraulic fracturing and well performances in tight gas reservoirs.