| As an unconventional natural gas resource,coal bed methane(CBM)is an important clean energy to improve the consumption structure of primary energy in China.However,because of the low permeability of CBM reservoirs,fracturing technology is usually used to increase the permeability of CBM reservoirs.At present,the development of CBM is mostly based on the fracturing technology and parameters in petroleum industry,but compared with the Brittleness characteristics of oil reservoir,the CBM reservoir usually presents the characteristics of“soft”,and its failure form is ductile failure,that is,there is obvious strain softening zone after the peak stress.The existing fracturing technology,whether vertical or horizontal well,has good fracturing effect in high brittle reservoir,but also faces the problems of low success rate,high development cost and low single well production.In addition,although China is rich in CBM resources,the proportion of middle and low rank CBM resources is as high as 78.9%,as a result,the fracture ductility is poor,the fracture is short and wide,the volume of reservoir reconstruction is limited,and the adaptability of mining technology area is very bad because of the difference of reservoir occurrence condition.Therefore,how to improve the permeability of low permeability CBM reservoir and realize industrial development is a key scientific and engineering problem to be solved.Based on the technology of hydraulic fracturing(Indirect fracturing)in horizontal well of coal seam roof,the low permeability coal measures strata are studied in this paper,the critical conditions for the propagation of hydraulic fractures across the interface,the optimal perforation spacing of multiple fractures and the optimal horizon of horizontal wells are revealed,the effects of stress,interface strength,fracturing fluid,fluid injection rate,roof lithology and horizontal well location on the propagation of fracture across interface are investigated.In addition,a model for predicting the propagation of hydraulic fracture across the interface under multi-factor coupling is established.The main research contents and results are as follows:(1)The hydraulic fracturing test of coal rock assemblage was carried out by TCHFSM-?large size tri-axial fracturing seepage simulation device.The influence of interfacial strength and stress on the across interfacial propagation of hydraulic fractures was investigated to reveal the evolution of injection pressure and the dynamic response characteristics of acoustic emission.The results show that:(1)Stress and interface strength significantly affect the propagation of hydraulic fracture across interfaces(rock?coal-rock interface?coal).There is a stress threshold for the hydraulic fracture to propagate across interfaces,and the greater the interface strength,the smaller the stress threshold for fracture across the interfaces.(2)Hydraulic fractures are prone to deflection in the abrupt region of interface strength,and the more significant the strength difference,the more obvious the deflection phenomenon.(3)When hydraulic fracture across penetrate the coal-rock interface,the injection pressure evolution curves show a secondary rise and the cumulative proportion of acoustic emission events increases as high as 51.4%.However,when the fracture fails across through the interface,no secondary fracturing phenomenon is found,and the acoustic emission events increase by only 6%.(2)The effects of stress difference coefficient,fracturing fluid,and injection rate on the propagation of hydraulic fracture cross-interface are studied through large-scale natural coal-rock specimens,and the mechanism is discussed.The results indicate that:(1)When the stress difference coefficient is greater than or equal to 2.00(???2.00),the fractures can penetrate across the coal-rock interface,otherwise,the fractures propagate along with the interface.(2)Comparing to the injection rate and fracturing fluid,the stress difference coefficient is the main impact factor affecting the fractures behavior penetrating across the coal-rock interface.In addition,when fracturing under low injection rate with freshwater,the complex fracture network was formed around the wellbore,and the far-field is still a single main fracture while fracturing by supercritical carbon dioxide,the complex fracture network was observed both around the wellbore and in far-field.(3)The higher the injection rate and the viscosity of the fracturing fluid,the more favorable it is for the fracture to expand penetrate across the interface.(4)When fracturing is freshwater,the higher rate of injected fluid,the shorter the fracturing time and the greater the initiation pressure of the specimen.When fracturing is supercritical carbon dioxide,the initiation pressure of the specimen is the lowest.Compared with the fresh water fracturing with the same rate of injected fluid,the initiation pressure is reduced by 5.79 MPa,and the sharp is nearly 39.3%.(3)Based on injection pressure,acoustic emission,digital image correlation(DIC),and 3D topography scanning techniques,the law of hydraulic fracture propagation under different fracture spaces is studied,the dynamic interaction propagation morphology between hydraulic fracture and the interface is intuitively revealed,and the fracture section topography is digitally characterized.The results indicate that:(1)The fracture propagation is significantly different in multi-cluster fracturing tests in specimens with different fracture spacing.Generally,there exists a critical spacing affecting the fracture development with the stress shadow effect.It is found that the stress shadow effect is obvious when the spacing is less than 50 mm in our experiment.(2)When perforation space is 10 mm,the ratio of perforation flow on the left and right sides is 49.86%and 41.63%respectively,while the ratio of perforation flow in the middle is only 8.51%.When the perforation space is larger(?50 mm),the stress shadowing effect of the middle hole gradually weakens,and the distribution of perforation flow in the three fractures is gradually balanced,accounting for 33%.(3)When the fracture interacts with the interface,the hydraulic fractures first pass through the pre-made artificial fracture,and then effectively open the artificial fracture till forming complex a“?”fracture network.(4)The hydraulic fracture presents an elliptical expansion style,and the elliptical area of the fracture does not present a double-wing symmetrical expansion form in the process of expanding to the interfaces on both sides of the test piece,but extends in a single-wing style and penetrates through the pre-made artificial fracture.(5)After fracturing with fresh water,the maximum displacement of the fracture tip is 4.2192×10-1 mm,the maximum strain is7.0317×10-3,and the JRC of the fractured red sandstone fractures can reach6?10 after being hydraulic fractured with large perforation spacing.(4)Based on linear elastic fracture mechanics,an interactive propagation model of hydraulic fracture and interface under multi-factor coupling is established,based on the elastic-plastic fracture mechanics,the effects of elastic modulus and the distance between horizontal wells and the interface on the propagation of fracture are studied.The results show that:(1)When the horizontal well is near or far from the coal-rock interface,the effect of hydraulic fracture across the interface is poor,and there is an optimum horizontal well spacing.At the same time,because of the different lithology of roof,the optimum spacing of horizontal wells is also different.(2)The numerical simulation results show that,compared with the linear elastic constitutive equation,the elastoplastic constitutive equation can accurately represent and predict the propagation law of hydraulic fracture across the interface.(3)A prediction model of hydraulic fracture propagation across interface under different intersection angles,interface friction,stress state and other factors is established and verified on the basis of physical tests.Indirect fracturing technology is recommended for the exploitation of CBM in low permeability reservoir.This technology can not only overcome the difficulties of drilling in the coal seam,but also effectively promote the extension and propagation of fractures.For indirect fracturing technology,it should be preferentially located in the geological area with large stress difference,especially suitable for deep reservoir.At the same time,the horizontal well should be optimized according to the occurrence and stress condition of roof rock,high viscosity fracturing fluid is used,and the large displacement sand-carrying fracturing technology is adopted to promote the cross-interface propagation of hydraulic fracture.In addition,when the horizontal well is pumped to the decay period,the supercritical carbon dioxide fracturing can be used for the second transformation to increase the far-field and near-field fracture morphology and extend the pumping-production period. |
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