Reservoir performance in tight sandstone is influenced by geomechanical behavior of natural fractures profoundly. Keshen gas field, located in Kuqa Depression of Tarim Basin, northwest China, and the pay zone is naturally fractured sandstone undergone strong tectonic activities of squeezing and thrusting. To understand key factors to productivity in this reservoir, an analysis of geomechanical response of natural fractures was presented that elaborated the relationship between productivity and geomechanical characteristics of fractured tight gas reservoir.
To evaluate the geomechanical response of natural fractures quantitatively, a comprehensive laboratory testing program (covering more than 130 core samples from 7 wells) was performed, 1D geomechanical model was built for 23wells which described distribution of the mechanical properties and in situ stress in Keshen reservoir. The normal stress and shear stress of each fracture plane were calculated to compare the relative mechanical response of natural fractures across the anticline structure. We then simulated mechanical response of natural fractures under different pore pressure according to Mohr-Coulomb failure criterion, and obtained the correlation between geomechanical response and open-flow potential of wells.
The results illustrated that well productivity in Keshen reservoir was closely related to the geomechanical response of natural fractures. At the crest of structure and faulting area, the favorable combination of in situ stress and natural fracture strike resulted in high shear-to-normal stress ratio and high fracture conductivity. On the other hand, at the saddle and flanks of this structure, the ratios were lower and so did for well conductivity. The difference of well productivity for the two cases could be up to forty times. And simulation under different pore pressure showed that the proportion of natural fractures reaching critical stress state in the wellbore affected the flow potential directly. In other words, some wells developed natural fractures with strong shear deformation potential corresponding to a high productivity. And then with the depletion, the dynamic change of mechanical response of natural fractures was also closely related to productivity. Taking 3 new wells for example, as pore pressure of reservoir decreased by 10MPa, productivity of 2 wells dropped since the shear-to-normal stress ratio of 70% natural fractures reduced, their productivity was lower than adjacent wells even after fracturing. The productivity of the third well maintained high because the shear-to-normal stress ratio of all natural fractures increased.
It was revealed that the geomechanical response of natural fractures was a controlling factor of well productivity in Keshen tight sandstone gas reservoir. The information obtained from this study provided critical input for reservoir stimulation and development for the gas field.