Keshen gas field, located in Kuqa Depression of Tarim Basin, northwest China, is an ultra-deep (7000m), high pressure and high temperature fractured tight sandstone gas reservoir with low permeability of 0.09mD and strong stress anisotropy about 30 MPa in horizontal stress contrast. For economic development, hydraulic fracturing is necessary in this reservoir.
To understand the mechanism of hydraulic fracturing and evaluate the fracability and fracture this reservoir efficiently, an integrated research has been conducted. We established a geomechanical model which described vertical and horizontal distribution of the geomechanical parameters in this reservoir. A fracturing experimental simulation was conducted with large size rock samples to analyze the interactive relationship between natural fractures and hydraulic fractures during fracturing. We also carried out a sensitive study to comfirm the key parameters for optimizing fracturing treatment design. Finally a new fracablity index calculation method suitable for fractured tight sandstone was built.
It is shown that for fractured tight sandstone reservoir with strong stress anisotropy like Keshen, shear deformation of natural fractures is a key factor of creating fracture area with high permeability. And the evolution of fracture area during fracturing experienced three processes. (1)Fracturing fluid extends along natural fracture at the initial fracturing stage; (2)At a certain injection pressure, slight shear deformation happens along the plane of natural fracture. As injection pressure increases, fracturing fluid breaks through a weak point of natural fracture and propagates along the direction of maximum horizontal stress; (3)A new set of fractures with combination of tensile-opening and shearing are formed, which are caused by changing of in-situ stress field around natural fractures. These three processes happen alternately and eventually form an ideal fracture area. It is also found that post-fracturing productivity has a close correlation with the shear deformation of natural fractures. At the axis of structure or fault developed area, natural fractures with strong potential shear deformation trend are more easily stimulated, and productivity can be high after fracturing. By contrast, fractures at structure saddle or steep part have weak potential shear deformation trend and are difficult to be stimulated, and thus productivity would likely be lower. Accoding to the mechanism of fracturing, we considered that the fracability of fractured tight sandstone is a function, and the parameters are in situ stress, shearing slip deformation, brittleness and fracture toughness.
Mechanisms of hydraulic fracturing and key factors effecting well productivity after fracturing in Keshen reservoir have been found through this study. Hydraulic fracturing treatment design and execution were conducted based on this study. After fracturing, the production performance (open flow potential) of stimulated wells increased by four-fold on average.