Coupled Damage Mechanisms Of Hydraulic Fracture In Tight Gas Reservoirs

Hydraulic fracturing is an effective approach to develop tight gas reservior.Due to the characteristics of low porosity,low permeability and complex pore structure,it is inevitable to cause formation sensitivity,water blocking damage and fracture conductivity damage during the reservior stimulation,which hinder the gas production.The formation damage include matrix damage and fracture secondary damage.This paper has studied the fracture-matrix damage mechanism and evaluation method of tight gas reservoir,based on numerical simulation and theory analysis,and the following conclusions are obtained:(1)Based on the laboratory experiment and geologic characteristic analysis,the lithological characteristic,reservoir properties and natural fracture development characteristic,the potential damage factors during stimulation process are studied,experimental research shows that strong water blocking damage exists in tight gas reservoir,the fracture has a great influence on the matrix-fracture system damage.The content of guar gum varies with the fracturing fluid systems,it has a great influence on hydraulic fracture conductivity.(2)The mechanism of water invasion when hydraulic fracture communicate with natural fracture is invegestigated by means of numerical simulation.The communication between natural fracture and hydraulic fracture enhance the gas flow capacity,but it can also cause the liquid phase flow into the natural fracture network from the communicated hudraulic fracture,which leads to severe water blocking damage,the degree of damage depend on the density,aperture,attitude of the natural fracture and the bottom hole pressure.(3)The hydraulic fracture secondary damage is the combined effect of multi-factors,including proppant packing embedment and compaction,fracturing fluids gelout and low recovery leads to residue damage and filter cake damage.An analytical model considering multiple damage factors is established on the basis of Hertz elastic deformation and Kozeny-Carman equation and numerical example was given,the influence factors are closure pressure,proppant diameter,elastic modulus,residue content and proppant concentration.(4)An evaluation model considering matrix Stress sensitivity damage,water blocking damage,proppant pack embedment,proppant pack compaction and residue damage is established,numerical simulation were carried out to study the combined effect of matrix damage and fracture secondary damage,the evaluation index is gas production rate.A simulation example is given based on a mini frac pressure-time curve,sensitivity analysis is carried out to analyse the influence of water blocking damage rate,critical invasion depth,residue content on gas production.The results indicate that,the hydraulic secondary damage has a greater impact on the gas production conpared to matrix water blocking and stress sensitivity.