Mechanism And Effect Evaluation Of Multi-stage Liquid CO₂ Phase Transformation Fracturing And Permeability

China has a huge low permeability oil and gas reserves.As a new fracturing technology,multi-stage liquid carbon dioxide phase change fracturing and permeability increase technology is based on the release of huge energy when liquid CO₂ is converted into gas to increase reservoir permeability,which is different from the existing hydraulic fracturing and high-energy gas fracturing.Can remove the plugging near the well area;It causes damage to rocks,forms micro-cracks,improves rock permeability,and can effectively develop low permeability reservoirs.The key potential advantages of multi-stage liquid CO₂ phase change permeability include increased oil and gas production,reduced water consumption,and minimized environmental impact.In order to better meet the needs of the site,the mechanism and effect of permeability enhancement were studied.Firstly,a multi-stage carbon dioxide phase change cracking and increasing permeability device is designed by using three-dimensional software Solid Works.Then,the strength of the liquid storage pipe and shear sheet are checked by using ANSYS Workbench software based on finite element method.Next through the compressed gas and water vapor container blasting energy method to calculate the CO₂ phase change energy,carbon dioxide is established between the volume and the TNT equivalent relations and CO₂ volume and rock impact loading experiment of the relationship between height and weight,on this basis,the different impact speed,blockage of filling,crack number of rock cracking impact experiments.Finally,based on the HJC sandstone constitutive model and ALE calculation method,the finite element method was used to simulate the effects of carbon dioxide phase transformation on fracture morphology and permeability increase under different cracking times,different confining pressures,fracture presence,fracture distance and fracture micro-difference.The results show that in order to ensure the normal operation of the multi-stage carbon dioxide phase change cracking and increasing permeability device,the appropriate peak pressure of carbon dioxide and the wall thickness of the liquid storage tube should be selected.In the rock impact experiment,the increase of carbon dioxide loading represents the increase of loading rate.With the increase of loading rate,the degree of rock fragmentation increases.In the case of plugging material filling,compared with the core without plugging material filling,the rock fragmentation degree decreases at the same impact rate.With the increase of cracking times,the degree of rock fragmentation increases.Under multiple impacts,the cracks are easy to expand on the basis of the original cracks.The permeability of rock matrix increases obviously before and after rock impact.In the numerical simulation of carbon dioxide phase transition,with the increase of cracking stage,the fracture range and crack area of the rock increase,and the permeability enhancement effect becomes better.With the increase of confining pressure,the rock fracture range becomes smaller,the crack area decreases,and the permeability increasing effect becomes worse.The crack propagation in the direction of the maximum principal stress is inhibited by the phase transformation of carbon dioxide under different confining pressures,and the crack morphology perpendicular to the direction of the maximum principal stress is long and straight.If the distance between the crack and the central hole is too close,the crack propagation will be hindered.When the crack is long,it will be conducive to the development of crack.When the spacing between two holes is greater than 5m,the two holes are no longer connected,and when the spacing between two holes is between 3m and4m,the crack area is the largest.The existence of time difference is beneficial to the development of interpore cracks.