Flow Characteristic And Enhancement Mechanism Of Supercritical Carbon Dioxide Jet Fracturing

Chinese shale gas reservoir has characteristics of large area continuous accumulation,low porosity,extreme low permeability,high clay content and deep burial.Thus,it restricts the shale gas development and production by using water-based injection fracturing due to the water-sensitive,water consumption and simply fracture network.Currently,a novel technology of supercritical carbon dioxide(SC-CO₂)waterless fracturing is proposed and regarded as a alternative method to solve the above problems.Many advantages of SC-CO₂ fracturing for shale gas exploration are found by researches,such as ability of production enhancement,complex fracture network generation and low damage for shale reserve.On the other hand,much carbon dioxide could be used to storage in the deep reserve to reduce the carbon dioxide emission.As one of the advance SC-CO₂ fracturing,the SC-CO₂ jet fracturing could reduce the number of mechanical packers,prevent the damage for shale reserve and create complex fracture network under low fracturing pressure.However,the flow field structure and characteristics,pressurization mechanism,fracture generation along perforation and fracturing enhancement are still unclear,which restricts the development and application of SC-CO₂ fracturing technology.To above problems in SC-CO₂ jet fracturing research,the studies about flow field characteristics and jet fracturing mechanism of SC-CO₂ jet fracturing were comprehensively conducted,jet fracturing enhancement technology was proposed based on our studies,and many achievements have obtained as following:(1)Mechanical model of SC-CO₂ jet fracturing is established and the equation of total strain formula of the experimental sample is derivated.Based on the jet fracturing under shale wellbore,the experimental-scale model of SC-CO₂ jet fracturing is obtained by using similarity criterion.The testing strain curve could be used to monitor and display the process of SC-CO₂ jet fracturing,which indicated the fracturing initiation and propagation under the pressurization of perforation pressure.(2)The studying of flow field and strain response of SC-CO₂ jet fracturing is conducted in the laboratory.The flow field of SC-CO₂ jet fracturing is analyzed by using the self-developed visualized jet fracturing system and high-speed photography,and the induced strain variation is also studied.It is found that there are both jet impingement region and perforation pressurization region in the flow field of SC-CO₂jet fracturing,and dynamical perforation pressure is created due to the periodical variation of jet length and tail deflagration in the perforation.Similar evidences are also verified in the strain curve because of the existence of both impacted strain and pressurized strain.The frequency of jet fracturing under low confining pressure is mainly high frequency component,the frequency under high confining pressure is mainly low frequency component.The factors,including jet fracturing parameters,confining pressure,fracturing fluid and perforation size should be comprehensively considered before fracturing.It is found that cracks initiated at the root of perforation,at the wall of perforation and at the top of perforation,respectively.And the velocity of crack propagation is as short as 100ms⁷00ms.(3)The mechanism of SC-CO₂ jet fracturing and its enhancement mechanism are both revealed.The effects of jet fracturing parameter,rock degradation and shale bedding were comprehensively studied via the jet fracturing of artificial sandstone sample,shale sample and organic glass sample.Further analysis about jet fracturing mechanism in both macro and micro perspectives are conducted combining multi methods of strain response,rock CT scanning,fracture morphology,weight loss of rock sample,carbon dioxide absorption,shale micro-damage and composition variation.The enhancement mechanism of SC-CO₂ jet fracturing is strongly related to perforation pressurization,perforation cooling,dynamical pressurization process,perforation damage and shale bedding.(4)Based on the above analysis of fracturing enhancement,multi-times cycle SC-CO₂ jet fracturing technology is proposed and studied in laboratory.And the effects of multi-times cycle jet fracturing parameters on fracture network complexity and carbon dioxide absorption were studied.A new evaluation index for fracture tortuosity in SC-CO₂ jet fracturing is proposed and successfully verified.The flow field and enhancement mechanism of SC-CO₂ jet fracturing are comprehensively understood in this paper.It provides technical support for SC-CO₂fracturing development,and promotes the future studying for shale gas development foundation of SC-CO₂ fracturing.