Theoretical And Numerical Simulation Of The Rock Hydraulic Fracture Initiation And Propagation Behavior

Hydraulic fracturing is a key technology commonly used in oil and gas reservoirs,coal seam gas and enhanced geothermal systems production.During hydraulic fracturing,highly pressure water is injected into a rock formation,generating artificial fractures and communicating natural fractures to increase the conductivity of the target reservoir and thus enhanced production from oil and gas wells.The hydraulic fracture initiation behaviors,propagation path and fracture geometry are three important parameters involved in the hydraulic fracturing process and production from an oil and gas wells.Based on the minimum strain desity criterion and the maximum tengential strain criterion,this paper first investigates the effects of in situ stress,injection water pressure,Tx-stress,Biot's coefficient,Poisson's ratio,the ratio of the water pressures inside the fracture to that of the borehole and perforation parameters on both the hydraulic fracture initiation angle and the critical water pressure.Then a strength criterion is used to study the fracture initiation type and location during the hydraulic fracturing process.The hydraulic fracture propagation path and deflection angle are analyzed using the maximum tangential strain criterion and incremental crack growth method.Finally,effects of reservoir properties on the length and width of hydraulic fracture are studied by numerical simulation.The main work and results are as follow:(1)A minimum strain energy density criterion,which considers the effects of Tx-stress is proposed to analyze the hydraulic fracture initiation behaviros.The results of fracture initiation angle under different crack types indicate that its values not only related to the stress intensity factor but also affected by Tx-stress and Poisson's ratio.The critical water pressure and critical initiation angle during hydraulic fracturing process are investigated,and the theoretical results were consistent with the experimental results.Parameter analysis indicates that the critical initiation angle and critical watter pressure increases as the stress anisotropy coefficient increase,whereas Biot's coeciffent has an opposite effect on the critical water pressure and has no effect on the critical initiation angle.The results of Biot's coeciffient show that lower viscosity and injection rate of fracturing fluid can make the hydraulic fracture extend along the optimal direction under the condition of lower injeiction water pressure to obtain the bast fracturing effect.(2)For the model of a single radial perforation emanating from a borehole.The theoretical formulas of the mode-? and mode-?stress intensity factors for arbitrary perforation angle are proposed.The effects of stress anisotropy coefficient,rock mechanics properties,wellbore and perforation parameters on both the critical water pressure and the critical initiation angle are discussed using the maximum tangential strain criterion.The results indicate that there is a critical perfration angle,when perforation angle is greater than the critical value the critical water pressure decreases as the stress anisotropy coefficient increase;whereas this trend is reversed when perforation angle is less than the critica value.Moreover,the critical water pressure decreases as the perforation length and borehole radius increase,whereas the critical initiation angle increases as the perforation angle increase.The critical water pressure increases and the critical initiation angle decreases as the Poisson's ratio increases.During high-pressure water injection,the crack intiation angle is not affected by Poisson's ratio.(3)The theoretical formulas of the mode-? and mode-?stress intensity factors for a symmetrical radial perforation emanating from a wellbore are proposed.Then the hydraulic fracture propagation path under different factors are analyzed using the maximum tangential strain and incremental crack growth method.The results indicate that the greater the stress anisotropy coeciffent and Poisson's ratio,the sooner the fracture reorients to the direction of horizontal stress.A smaller perforation angle and a shorter perforation length is recommended to decrease the near-wellbore tortuosity and critical water pressure.The grey relative analysis indicates that the perforation angle is the most important controlling factors in complicating the hydraulic fracture propagation path near-the wellbore.In addition,Tx-stress has no effect on the open crack propagation path.However,under compressive loading conditions,the fracture criterion that considers the effect of T-stress simulates the fracture propagation path remarkably better than the fracture criterion that does not consider the effect of T-stress.(4)A strength criterion is proposed to determine the fracture initiation type and location during the hydraulic fracturing process.In addition,the stress path,which can accurately analyze the stress field around the crack tip,is given.The results indicate that the internal friction angle can prohibit the shear fracture initiation;it needs greater external loading to make the hydraulic fracture extension as the internal friction angle increases.With the increment of the in j ection water pressure,the tensile fracture primarily determines the fracture initiation type;whereas,the shear fracture will preferentially occur as the stress anisotropy coefficient increases.(5)The effects of reservoir properties on the length and width of hydraulic fracture are studied by numerical simulation.The results indicate that the low or high elastic modulus region on both sides of the model can decrease the fracture width and increase the fracture length.The high Poisson's ratio region on both sides of the model can also decrease the fracture width,while the effect on the fracture length and breakdown pressure is not significant.In addition,when the hydraulic fracture extends in the low-medium-high elastic modulus or Poisson's ration model,the hydraulic fracture extends faster in the high elastic modulus or Poisson's ratio region.The hydraulic fracture preferably extends to the lower tensile strength region.When the hydraulic fracture extends from low tensile strength region to the high tensile strength region,the hydraulic fracture presents the feature of "short and wide" with the increase of tensile strength in the two sides of the model.Higher reservoir permeability leads to increase of the pore pressure around the hydraulic fracture and the breakdown pressure for fracture initiations decreases,hence hydraulic fracture preferably extends to the higher permeability region.