Producing natural gas from shale gas reservoirs presents a great challenge to the petroleum engineers because of the low permeability nature of this type of gas reservoirs. Large scale and expensive hydraulic fracturing operations are often required for enhancing gas well productivity. Due to the shally characteristics of the reservoir rock, the hydraulically fractured gas wells are very vulnerable to the damage by the fracturing fluids. However, the significance of the formation damage due to different causes in fractured wells is still not clear. It is highly desirable to have a simple method for predicting the degree of fracture face matrix damage and optimizing fracturing treatment. This paper fills the gap.
A new mathematical model was developed in this study to predict the effect of fracture face matrix damage on productivity of fractured gas wells in shale gas reservoirs. A unique feature of the new model is that it considers reservoir-fracture crossflow in finite conductivity fractures. Results of the model analyses were sensitized to reservoir properties and facture face matrix skin properties determined by the fracturing fluid properties and treatment conditions. Large ranges of possible leak-off coefficient and spurt-loss coefficient were investigated. We concluded that the significance of fracture face matrix damage to well productivity depends on reservoir properties (porosity and permeability) and fracturing fluid properties (leak-off coefficient, spurt loss coefficient, and viscosity). Reservoir properties determine the vulnerability of formation damage, while fluid properties controls the degree of formation damage. The productivity index ratio (PIR) for describing the significance of formation damage drops non-linearly with leak-off coefficient and damage permeability ratios. In the practical range of the leak-off coefficient, the PIR analysis reveals that well productivity should drop by less than 15% even the residual permeability is 5% of the virgin reservoir permeability in the damage zone. Accurate prediction of the effect of fracture face matrix damage on well productivity with this model requires conducting laboratory test to determine the residual permeability in the damaged zone for a given shale gas reservoir.