Supercritical CO₂ Fracturing Of Shale And Its Permeability Variation Under Multi-Field Coupling Conditions

China has very large amount of shale gas resources,the development of shale gas has become a hot spot and focus on the field of unconventional oil and gas resources in our country,although the hydraulic fracturing can achieve large-scale commercial exploitation of shale gas,but the clay swelling,water resource waste and environmental pollution can not be underestimated,and using supercritical CO₂ fracturing shale and flooding shale gas which can improve the shale gas production,but also can realize the geological sequestration of greenhouse gas CO₂,so it is imperative to carry out research in this area.In recent years,domestic and foreign scholars on supercritical CO₂fracturing shale and improving shale permeability studies is still in the exploratory stage,fracturing mechanism has not been carried out in deepth and systematic study,in particular,the percolation mechanism of fractured shale is not clear.Based on a large number of domestic and foreign related literature research and shale shale samples study,The analyzed shale samples used in this study of supercritical CO₂ fracturing shale and improving shale permeability were collected from the lower Silurian Longmaxi formation in the Changning area of Sichuan basin.Adopting the research method of shale experimental data and mathematical model of the combination of supercritical fluid shale fracturing mechanism and percolation mechanism are as follows:?1?Based on the analysis of shale microstructure and mineral composition,the main conclusions are as follows:The measured TOC contents of shale samples are 4.14wt%to 4.26wt%.The main mineral of shale are quartz and clay,and most of them contain calcite,dolomite and other minerals.The main elements found in the shale core are Si,C,Ca,Al,Fe,K,Mg,and S,there are some minor components,such as Na,Rb,Ni and so on.?2?Deformation characteristic?stress-strain curve?of shale was combined with the mineral components of the shale to analyze the change rule of shale brittleness,and brittleness index obtained by three kinds of different indexes was compared.The shale deformation characteristic was obtained by uniaxial and conventional tri-axial compression tests,and the shale mineral components was determined by the XRD and XRF techniques.The brittleness index of shale decreases with confining pressure increasing,the fracture surface presents decreasing trend with confining pressure increasing,and the trend of brittle to ductile transition.;the brittleness index presents a linear increasing change rule with the mineral content of brittle.With the increasing of confining pressure,the failure laws include tensile failure,tensile-shear failure and shear failure,and show the trend of transition from brittle to ductile.Brittleness index B₃ and B₁ have a better correlation than B₂ and B₁,in comparation with North America which widely used content of quartz evaluating brittleness index,the complex of mineral components in southern China Paleozoic shale,quartz,calcite,dolomite used as brittle mineral content for brittleness evaluation is more suitable.Due to that the brittleness of shale was dual infected by mineral components and stress state in the reservoir,in the time of reservoir fracturing layer selection and the evaluation of the fraccability,the influence of in-situ stress state and mineral composition should be comprehensive considered,finally reasonable seletion reservoir of fracturing layer.?3?The experiment study of supercritical CO₂ fracturing shale was carried out by using the self-developed instrument for simulation of supercritical CO₂ fracturing and increasing the permeability of shale.The acoustic emission monitoring technology,combined with CT scanning technology,were employed to investigate the crack propagation law of the fracturing process,and the comparison of the permeability of the shale before and after fracturing was conducted to evaluate the fracturing efficiency.The main conclusions are as follows:1)The process of supercritical CO₂ fracturing shale can be divided into three stages,such as cavity filling stage,pore pressure accumulation stage and fracture propagation stage.The maximum counts of the acoustic emission is reached at the initiation pressure point,and the phase change occurred during the supercritical CO₂fracturing shale process.2)The observation of shale propagation form and the plane expansion form of crack through industrial CT technology,the crack extension direction on the whole with the direction of maximum principal stress tends to be uniform,damage in the form of comprehensive damage due to weak shear,the existence of crack extension will be changed,the heterogeneity of shale decided weak different properties of rock,cause the crack turning and micro fracture network development,supercritical fluid fracturing can form a complex network of crack,and the stress state is the important factor.Which effect the rock initiation pressure and crack propagation process.Different initial stress ratio has a effect on crack initiation pressure and crack propagation form by supercritical CO₂ fracturing.3)Shale has low permeability,the shale permeability ranges from 0.068×10^-18m²to 0.393×10^-18m² before fracturing,the shale permeability ranges from 0.732×10^-15m²to 1.520×10^-15m² after fracturing,the permeability of the shale increases 3-4 orders of magnitude compared to that before fracturing,so the permeability increase is noticeable by fracturing.?4?Through the experimental study on the law of permeability of fractured shale under three-dimensional stress,the main conclusions are as follows:1)In constant temperature and volume stress,shale permeability affected by gas Klinkenberg effect and effective stress,at low pore pressure range?1-3MPa?,Klinkenberg effect has significant influence on the permeability,which leading to the permeability decreased with pore pressure increasing,when pore pressure range form3Mpa to 5MPa,permeability increased with the pore pressure increasing as the effective stress is the dominant influencing factor.2)The permeability of fractured shale decreased due to the CO₂ adsorption induced swelling;the permeability of fractured shale has a negative exponential relationship with effective stress;At the temperature range of 32⁰C-48⁰C,the permeability decreased with the increasing of temperature,and tem-perature sensitivity of permeability is also decreased with the temperature increasing.3)The permeability of shale gas reservoir is commonly affected by the multiple factors such as geo-temperature,geo-stress and the pore structure of shale,based on the experimental study of shale permeability established in the temperature field,stress field coupling relationship between shale permeability?the fitting equation of gas pressure in 1MPa permeability:K=0.756exp?-0.011t-0.046?_e?;the fitting equation of gas pressure in 2MPa permeability:K=0.722exp?-0.021t-0.093?_e??,it can provide scientific basis for strengthening the CO₂ dynamic prediction of reservoir permeability in the process of shale gas development.4)The permeability of supercritical CO₂ in shale is closely related with fluid pressure and volume stress;When the injection pressure of supercritical CO₂ is constant,the permeability of shale decreases with the increase of volumetric stress.