Research On Mechanical Response And Permeability Stimulation Mechanism Of Rock Under Supercritical Carbon Dioxide Jet

Unconventional hydrocarbon resource is the new direction and strategic reassurance of energy,which has an increasing proportion of energy development in China.In the development progress of unconventional reservoir,reservoir reconstruction is indispensable because of poor reservoir property.With many excellent properties such as strong erosion ability and low formation damage,supercritical carbon dioxide jet has widely application prospect in reservoir construction.To promote this new technology,some key theory researches such as flow field analysis,jet impingement effect,swelling effect and the damage and permeability stimulation mechanism of supercritical carbon dioxide jet are indispensable and need to be study urgently.Effective studies of these problems are not only useful for the promotion of new reservoir construction technology,but also have significant influence on the expansion of supercritical carbon dioxide jet theory.Firstly,the mathematical model and algorithm of supercritical carbon dioxide impinging jet flow field are established,while using suitable thermophysical and friction factor equations for real carbon dioxide.Then,a calculation method for supercritical carbon dioxide impinging jet flow field is achieved.This method provides theoretical basis and technical support for the follow-up studies of impact dynamics,swelling characteristics and damage and permeability simulation mechanism of supercritical carbon dioxide jet.Secondly,the impact loads,damage mechanism and act degree of carbon dioxide jet in two periods of impinging progress are discussed.Then,the mian damage mechanism of jet impinging as stress wave is deduced.Furthermore,based on blast theory and spherical wave theory,this paper builds the calculation models of impact pressure and stress field of rock under supercritical carbon dioxide jet.Results show that,during the decay process of impact load,the stress of rock particle changes steeply from compressive stress to tensile stress;The peak value of compressive stress and tangential tensile stress decay at the rate of （r/R₀）^-1 or more;The peak value of radial tensile stress exsits in the 3 times nozzle radius.Thirdly,simulation experiment on CO₂ swelling-induced expansion of rock is carried out.Subsequently,the mathematical model of CO₂ swelling-induced strain field is deduced.Analysis indicate that the CO₂ swelling-induced expansion feature meets the supercritical adsorption theory.The swelling strain includes thermal strain（function of thermal expansion coefficient and temperature difference）and CO₂ adsorption-induced strain（temperature and density of permeating fluid）.With increasing permeating temperature,thermal strain increases while CO₂ adsorption-induced strain decreases,in this paper swelling strain increases because of strong thermal expansion effect.As permeating pressure increases,both CO₂ adsorption-induced strain and swelling strain increase,however,strain changes little when pressure is higher than 10 MPa.Fourthly,this paper builds a damaged model of rock under supercritical carbon dioxide jet and does corresponding experiment and test,then analyzes the damaged volume and permeability growth degree under different working condition.The results show that the variation law of permeability growth degree has highly consistency with damaged wolume.As nozzle pressure drop increases,the permeability growth degree increases,but the increment rate decreases.With increasing inject temperature,the permeability growth degree increases first then decreases,there is an optimal inject temperature in engineering application.When ambient pressure increases,the permeability growth degree severely decreases,so low ambient pressure should be maintain within safe limits in application site.Finally,the damage and permeability stimulation mechanism of supercritical carbon dioxide is studied.The essential mechanism is that during the impinging process of supercritical carbon dioxide jet,under the synergistic effect of jet impact,swelling and extrusion by ambient pressure,some damage cracks generate in partial area of rock,thus the rock permeability is improved.The dominant factor is the jet impact effect.However,the CO₂ swelling effect has positive advantage on tension failure of rock,which is a special feature of carbon dioxide and other adsorbent fluid.The extrusion effect of ambient pressure has very negative influence on damage and permeability stimulation result.Intergrating lab test and theoretical analysis,this paper establishes a damage mode of rock under supercritical carbon dioxide jet and improves experimental apparatus and method to test rock’s permeability variation under jet impinging,then derives the damage and permeability stimulation mechanism of supercritical carbon dioxide.This paper also develops a methodology to study supercritical carbon dioxide jet assisted eriosion and permeability stimulation technology.The results and methodology of this paper will make certain contribution to developing supercritical carbon dioxide jet technology and enriching jet theory.