Unconventional gas resources from tight and shale gas reservoirs have received great attention in the past decade and become the focus of petroleum industry. Shale gas reservoirs have specific characteristics, such as tight reservoir rock with nano-Darcy permeability. Multi-stage hydraulic fracturing is required in such reservoirs to create very complex fracture networks to connect a huge reservoir area to the wellbore effectively. During hydraulic fracturing, an enormous amount of water is injected into the formation, and only a part of the injected water (25-60%) can be reproduced during a flowback and long production period. A major concern with hydraulic fracturing is water blocking effect in tight formation due to high capillary pressure and the presence of water sensitive clays. High water saturation in the invaded zone near the fracture face may reduce greatly gas relative permeability and impedes gas production.
In this paper, we will consider numerical techniques to simulate water invasion or formation damage during hydraulic fracturing and its impact on the gas production in shale-gas reservoirs. Two-phase flow simulations are considered in a large stimulated reservoir volume (SRV) containing extremely-low permeability tight matrix and multi-scale fracture networks including primary hydraulic fractures, induced secondary fractures and natural fractures.
To simulate water blocking phenomena, it is usually required to explicitly discretize the fracture network and use very fine meshes around the fractures. On one hand, the commonly used single-porosity model is not suitable for this kind of problem, because a large number of gridblocks is required to simulate the fracture network and fracture-matrix interaction. On the other hand, a dual-porosity model is not suitable either, because of large block sizes and long transient duration with ultra-low permeability matrix. In this paper, we study the MINC (Multiple INteracting Continuum) type method and use a hybrid approach between matrix and fractures to simulate correctly fracturing fluid invasion and its backflow under hydraulic fracturing. This approach allows us to quantifying with satisfactory the fracturing water invasion and its formation damage effect in the whole SRV.