Hydraulic and natural fractures are the key channels for shale gas to flow at economic rates. Fracture growth in shale gas reservoirs appears to be much more complex compared to most conventional reservoirs as massive multistage, multicluster hydraulic fracturing treatments are executed at many shale gas formations. It becomes crucially important to develop advanced approaches to model and simulate such a complex system to better understand the recovery mechanisms and optimize stimulation designs of shale gas reservoirs.
Most existing shale gas models are idealized dual-porosity models. The objective of this work is to model the actual network of hydraulic and pre-existing fractures based on geological interpretations and microseismic mapping results. The Discrete Fracture Modeling (DFM) approach is applied to represent each fracture individually and explicitly. This requires unstructured gridding of the fracture-matrix system using Delaunay triangulation and transmissibility evaluation between each pair of adjacent cells. The near-well effects are modeled in detail by refining the unstructured 3D grid to the point where we fully resolve stimulated fractures.
Simulations have been performed based on the detailed model of an synthertic shale gas reservoir with considering various mechanisms including adsorption/desorption, matrix-fracture transfer, and non-Darcy effects. Sensitivity studies by varying the production rate, pressure and hydraulic fracture parameters are conducted to provide guidance on optimizing stimulation and production schemes. Furthermore, to extend the application of this technique to large models, the Multiple Subregion (MSR) upscaling procedure is proposed to enable much quicker simulation runs. The high degree of accuracy provided by this technique is demonstrated by comparing the solution of the upscaled model with the corresponding fine-grid solution for a synthetic case.