Study On Fracture Propagation Mechanism Of Longmaxi Shale Hydraulic Fracturing By Considering The Acidizing Effect

The progress of shale gas developing technique is a significant key as the solution to the situations which conventional natural gas production shortage and highly dependent on the gas import in China.The high stress difference(15～20 MPa),low brittle index(≤50)and undeveloped natural fracture make it difficult to generate complex fracture network.The geographical environment of Sichuan Basin and the geological properties of Longmaxi shale formation cause the high cost of shale gas development in this area.However,the shale has micro pores and micro cracks in the matrix as other rock materials,especially when it has been acidized.The mechanism and characteristics of shale deformation and bursting under the influences of complicated stress state and acidizing are not clearly understood yet in hydraulic fracturing.It greatly restricts the fracturing design and application in shale gas treatment.In this study,Longnmaxi shale formation in South Sichuan Basin has been choosen as the research object.The evolution of mineralogy,microscopic pore structure,and mechanical properties of acidized sample of shale were studied by diverse experiments,respectively.Then the constitutive model applicable for Longmaxi shale was developed by combining the theories of plasticity and damage mechanics.The model was coded by Fortran and introduced into the hydraulic fracturing model to study the complex fractures propagation of acidized shale.The main research works are as follows:(1)Based on the literatures,the shale grains cemented strength and micro pore structure would be changed after the HCl dissolves carbonate,which leads to the changings of rock mechanical features,porosity and permeability of shale.Since the stress-leakoff coupling of shale formation,the complex fracrtues propagation process could be controlled by acidized shale and its properites.(2)The rock minerology,micro morphology and porosity/permeability of original shale and acidized shale were sysmatically studied by experimental methods.The carbonate was dissolved by HC1 and dissolution changes the shale matrix and weaken the cemented strength.In addition,the porosity and permeability of acidized shale increased which means the micro pores and cracks were generated after acidizing.(3)The triaxial compression test was conducted by triaxial servo system.The peak strength and Young’s Modulus decreased with increasing exposure time.The relationships of rock mechanical parameters and porosity/permeability were fitted to the linear regression.By analyzing the deformation and fracture features,the conclusion could be draw that the complex breakdown of acidized shale means it is beneficial to complex fracture propagation.(4)Based on plastic theory,the shale elasto-plastic constitutive model was presented according to Druker-Prager criterion.Considering that the porosity has increased after aicizing test,the damage factor and damage evolution function were developed based on damage mechanics.The acidized damage elastoplastic constitutive model(ADECM)was developed by introducing the damage factor into elastoplastic model.The model was coded by frotran.The simulation was carried out to study the rock deformation of acidized shale samples reacted with acid in various time,and the ADECM could simulate the aicidized shale deformation process.(5)The cohesive element method was introduced to simulate two fractures interacted based on plane strain condition.The results was verified by experimental method.The acidized damage elastoplastic model was introduced into hydraulic fracturing model to modify the current one.The simulations were carried out for elastic model and ADECM.The results showed that the induced stress field of damaged plastic model was much complex and the stress ratio was lower than them of elastic model,and it was the advantage for comples fractrure propagation.(6)Based on the geological features of X3 well,the 3D model was developed to simulate the multi-fracture propagation in acidized shale formation in various treatment parameters.The effects of treatment parameters on complex fracture propagation were studied.Then the 3D numerical simulation results were compared with the microseismic monitoring results of X-3 well.It indicates that the hydraulic fracturing model associated with ADECM for acidized Longmaxi shale could simulate the hydraulic fracturing in acidized shale formation.In this paper,the author combined experimental tests and theoretical moedel to study the rock mechanical behavior for pre-acidzing in Longmaxi shale reservoir,as well as hydraulic fracturing numerical simulation.The ADECM was developed to simulate the triaxial compression test for shale sample.Based on introducing the ADECM into hydraulic fracturing model,the fracture propagation in acidized shale formation was simulated.The results showed that the induced stress field of ADECM was much complex and the stress ratio was lower than them of elastic model,and it was the advantage for comples fractrure propagation.The ADECM based 3D hydraulic fracturing model reveals the mechanism of acidized shale fracture propagation,which could provide theoretical foundation for the shale formations which are similar to Longmaxi shale.