Experimental Study On Characteristics And Permeability Enhancement Of Hydraulic/Sc-CO₂ Fracturing In Coal

The exploitation of coalbed methane?CBM?resources in China is difficult.In addition,the development of coalbed methane industry is relatively short and the exploitation technology used mainly refers to other countries,thus the technical system and related theories of coalbed methane exploitation are not deeply understood.The above reasons resulted in bad CBM well production.Even if the coalbed methane base developed by the foreign technical team has a large number of low-yield wells,this indicates that the existing technical means itself has a certain scope of application,and the effect of increasing production and infiltration is limited.It is urgent to develop new methods.Consequently,this paper firstly focuses on the theoretical research of China's CBM development technology system,finds out the cause of the low production of single-well coalbed methane,and proposes corresponding solutions based on this;Then,combined with the idea of non-aqueous fracturing,the Sc-CO₂ fracturing and hydraulic fracturing is used to perform fracturing and fracturing-seepage tests on cylindrical coal samples with?50×70 mm to study the fracture characteristics of fractured coal?curve of pump pressure-time,initiation pressure and fracture morphology?,the evolution law of the permeability of the coal after fracturing and the permeability enhancement.The main conclusions are as follows:?1?At present,the main problems faced in China's coalbed methane exploitation are:coal reservoirs have strong adsorption,and it is difficult to promote gas desorption in coal by drainage process;Heterogeneity and poor fractureability of coal make the hydraulic-fracturing stimulating scope of vertical well very limited;In conventional water-based fracturing,large water consumption,strong reservoir pollution and fracturing problems such as poor permeability enhancement and environmental pollution are not conducive to the development of coalbed methane.?2?Based on the above problems,the following solutions are proposed:It is recommended to adopt the idea of“expanding the extension of the well by the extension of the wellbore”for the coal seam with poor fractureability,and adopt horizontal drilling instead of the vertical well to prolong the fracture propagation distance.The problem of wearing laminated fracturing is a good way to avoid the problem of drilling horizontal holes in coal seams,and alleviating problems related to reservoir pollution?water lock effect,pulverized coal clogging?for broken soft coal reservoirs;Sc-CO₂ fracturing has outstanding advantages such as lower initiation pressure,better fracturing effect and less environmental pollution,and has good application prospects.?3?The hydraulic/Sc-CO₂ fracturing test was carried out on a cylindrical coal sample of?50 mm×70 mm.The test results show that under a certain confining pressure?10 MPa?,when the axial pressure was 12,14 and 16 MPa,the initiation pressure of Sc-CO₂ fracturing was 12.34%,13.15%,and 24.38%lower than that of hydraulic fracturing.Under the same conditions,Sc-CO₂ is more easily to promote fracture initiation and expansion.Similar to hydraulic fracturing,the fracture initiation and propagation direction in Sc-CO₂ fracturing is affected by three-dimensional stress state and weak surface such as bedding plane and primary fracture.Under the axial compression×confining pressure was 12 MPa×10 MPa and 14 MPa×10 MPa,transverse fracture was produced,while longitudinal fracture was produced at 16 MPa×10 MPa.?4?The maximum tensile stress theory can be used to predict the critical initiation pressure of longitudinal fractures when tensile failure occurs.However,for transverse fracture,the prediction results are quite different from the experimental results.Based on Mohr-Coulomb theory,the fracture initiation criterion under shear failure is derived and calculated,and the critical initiation pressure of the transverse fracture under hydraulic/Sc-CO₂ fracturing is calculated.The calculated results agree well with the experimental results.?5?As the temperature increases?40⁷0°C?,the viscosity of Sc-CO₂decreases gradually.Under the same stress condition,the initiation pressure decreases with the progressive of temperature:the average initiation pressure at40°C,55°C,70°C was 14.95 MPa,14.36 MPa,and 14.19 MPa,respectively.?6?Micro-CT results show that Sc-CO₂ fracturing can induce more secondary fractures while driving the main fracture propagation,which increases the pore and fracture rate after Sc-CO₂ fracturing?7.22%?,more than hydraulic fracturing?4.27%?,therefore the scope of transformation of Sc-CO₂ fracturing is better than hydraulic fracturing.The results of three-dimensional topography show that the values of S_a,S_z and RMS of the fracture surface of Sc-CO₂-fracturing coal are 31%,22%,and 58%higher than those of hydraulic-fracturing coal.Sc-CO₂ can enter the deep internal defects of the coal body and form a rough fracture surface.?7?Sc-CO₂ or hydraulic fracturing causes the gas seepage in the coal body to change from pore flow to pore and fracture flow.The permeability revolution with pore pressure increasing is as follows:first decrease and then increase slightly,and finally end with small fluctuation.In addition,the permeability of the coal after fracturing satisfies the negative exponential relationship with the effective stress.?8?After Sc-CO₂ and water fracturing under the axial pressure×confining pressure of 16 MPa×10 MPa,the permeability is increased by 472.0 times and34.0 times compared with the intact coal,respectively.While the permeability is increased by 46.5 times and 19.0 times after Sc-CO₂ and water fracturing when the axial pressure×confining pressure is 12 MPa×10 MPa,respectively.Under the same conditions,the permeability enhancement of Sc-CO₂ fracturing is better than hydraulic fracturing.