Experimental Study On Initiation Pressure And Fractures Growth Of Hydraulic Fracturng With Supercritical CO₂ And Water

As a type of low-carbon,clean and green unconventional natural energy,CBM will certainly become an important direction for demand of social future energy.China's coalbed methane resources are very rich in reserves,and it has huge potential for development and application.It is a resource of great economic potential value in the future,and its development and utilization have gradually become the focus of attention.As we all know,fracturing is a key technology to improve the low permeability of reservoirs and improve the efficiency of gas recovery.However,China has complex coal deposits with high gas content,high heterogeneity,and low permeability;Coal fractures,bedding,joints and other natural fracture network development,low Young's modulus,high Poisson's ratio,easy deformation and other issues are inevitable.The number of artificial fractures generated by traditional hydraulic fracturing is limited associalted with the limited antireflection ability,and a large amount of water is wasted and even destroying the surface environment.Supercritical CO₂ is a kind of fluid between gas and liquid.Its density is close to liquid,the viscosity close to gas,the surface tension low,the diffusion coefficient small,the compression coefficient large,and the solubility to hydrocarbon compounds high,which is of great significance to facilitate the generation of complex fracture network and to use it as a fracturing fluid to perform fracturing tests on coal and rock.In this paper,for the cementing,coal seams and overlying strata encountered in the engineering fracturing process,combined the physical simulation tests of100 mm×100 mm×100 mm in size were performed,the theoretical analysis methods through experimental research.Comparing and comtrasting the fracturing characteristics of supercritical CO₂ and water.The physical and mechanical properties?tensile strength,compressive strength,elastic modulus,Poisson's ratio,cohesion,internal friction angle,brittleness,Type I fracture toughness etc.?of coal,overburden?sandstone mudstone?,and cement for cementing around this subject,supercritical CO₂ fluid characteristics,coal-rock supercritical CO₂ and water three-dimensional fracturing characteristics were systematically analyzed.The main results are as follows:?1?Compared with cement,coal and sandy mudstone exhibit anisotropic mechanical characteristics due to the presence of bedding,and there are significant differences in the mechanical parameters between the vertical bedding orientation and the parallel bedding orientation.The rock layers with larger elastic modulus,higher brittleness index,and higher tensile strength are conducive to the propagation of internal fractures and inhibit the extension and extension of cracks in adjacent rock layers.Coal cracks with smaller fracture toughness propagate easily and have a wide crack width.?2?When CO₂ reaches the critical condition?pressure 7.38 MPa,temperature31.4oC?,it is close to liquid CO₂ density,close to the viscosity of gaseous CO₂,zero surface tension,high compressibility,easily soluble to hydrocarbon compounds,so supercritical CO₂ fracturing is a process that accompanies with both physical and chemical processes.The physical process connects the original fissures while the chemical process reduces the strength of the fracturing media?3?For the cement specimens,the initiation pressure of supercritical CO₂fracturing is reduced by 13.7%under the?_h=8 MPa/?_V=10 MPa/?_H=12 MPa stress condition compared to water fracturing,reduced by 18.3%under the?_h=10MPa/?_V=12 MPa/?_H=14 MPa stress condition,and reduced by 23.4%under the?_h=8 MPa/?_V=12 MPa/?_H=16 MPa stress condition.Under the same stress conditions,the complex fracture network structure is more easily formed by supercritical CO₂ fracturing,and with the increase of the confining stress,the types of cracks increase.The second fracturing was performed under the same CO₂ gas media.The maximum pressure of specimens fractured by supercritical CO₂ fracturing was lower than that of water.It can be seen that the supercritical CO₂ fracturing can overcome the high initiation pressure of the cementing ring.The resulting fractures still have high conductivity when subjected to pressure,which is conducive to the expansion of the fractures to the reservoir and the formation to frame highly efficient oil and gas production aisle.?4?For the coal specimens,compared to water fracturing,under the same three kinds of triaxial stress conditions,supercritical CO₂ initiated the fractures more easily;the initiation pressure decreased by 28.3%,28.0%,and 27.4%,respectively,and the reduction gradually decreased with confining pressure increasing.Supercritical CO₂ fracturing could generate obvious bedding plane fractures and bifurcation fractures,and the more those fractures were generated,the smaller the growth range of main fractures was.The supercritical CO₂penetrated easily into the original bedding plane fissures and micro-fissures or pores in coal during the fracturing process,which caused the pressure to change gradually.Moreover,the fractured coal with supercritical CO₂ could not easily maintain the pressure so that the fractures width was narrow.The fractures width of water fracturing was mainly distributed in the interval of 0.450-0.650 mm and>1.250 mm,while the fractures width of supercritical CO₂ was concentrated in the interval of 0.050-0.250 mm.The gap between the two kinds of fractures width was approximately 3-6 times,for some local bifurcation and bedding plane fractures,even about 2 orders of magnitude.