| Tight gas,an unconventional natural gas,has extensively exploited and developed in China,due to its wide distribution and rich reserves.Tight reservoir has dense lithology,low porosity and low permeability.To obtain the ideal productivity,technological measures of increasing fractures are generally adopted to increase production.In addition,tight reservoirs are buried deeply,the in-situ stress is complex,and the rock mass generally contains a large number of discontinuities in different scales such as natural fractures,weak planes,joints and faults.These factors have a significant influence on hydraraulic fracture behaviors,e.g.crossing,offset,arrest,and bifurcation.The complexity of hydraulic fracture networks directly determines the effectiveness of stimulated reservoir volume and the production.Many studies have investigated the effect of the differential stresses,dip of natural fracture,fluid viscosity and injection rate on fracture propagation path and the interaction modes between hydraulic fracture and natural fracture.While there are few studies focusing on the effect of mechanical properties of natural fracture(such as strength,width,etc.)on the mechanism of fracture propagation.Furthermore,the failure nature of the hydraulic fracture and natural fractures should be determined further and studied in detail.This study uses bonded-particle model(BPM)approach,acoustic emission(AE)technique based on moment tensor inversion to investigate hydraulic fracture initiation,propagation and offset and interaction modes with weak planes,supported by the National Basic Research Program of China(973)"Seismogenic mechanism and wave propagation law of micro-seism induced by hydraulic fracturing in tight reservoirs"(Grant No.2015CB258500).Crack initiation,propagation and coalescence in limestone specimen under uniaxial compressive loading are studied by the BPM primarily,and the corresponding laboratory tests are conducted.The results show that the main characteristic levels of the stress-strain curves are comparable between the numerical results and experimental results(such as crack initiation stress,crack damage stress,etc.).The simulation of AE based on the moment tensor theory can realistically reproduce AE events:the AE parameters in the BPM including crack number and AE event are generally comparable with the AE count and AE hit in the experiments.Then,the moment tensor inversion is adopted to distinguish the failure nature of the micro-crack and the macro-fractured zone in the cracking processes.The numerical results are comparable with the experimental results.The results reveal that this method can reflect the forces acting on the particles and distinguish the nature of micro-cracks and macro-fractured zones.A series of fracturing tests are carried out in a model containing a single weak plane under different approaching angles and differential stresses at the lab scale.The model captures three interaction modes between hydraulic fracture and weak plane:(1)crossing the weak plane,(2)crossing and branching into the weak plane(3)slippage or arrested.The approaching angle is the main factor influencing the interaction modes.At ?=30°,the hydraulic fracture is slippage or arrested by the weak plane.At?=60°,the hydraulic fracture mainly crosses and branches into the weak plane.At?=90°,the hydraulic fracture crosses the weak plane perpendicularly.The nature of hydraulic fracture is tensile based on moment tensor inversion except that the hydraulic fracture propagates into the weak plane.For the weak plane,its nature is related with approaching angle of weak plane and differential stress:at low approaching angle(30°),the failure nature changes from 'Shear' to 'Tensile' as the differential stress decreases;at medium approaching angle(60°),failure nature is'Shear';at high approaching angle(90°),few cracks are formed in the weak plane that remains intact.To study the effect of strength and width of the weak plane and injection rate on the interaction mode and failure nature of weak plane,a series of fracturing tests are carried out in the single weak plane contained specimen.The results reveal that the strength of weak plane is too low or too large to arrest the main hydraulic fracture under low and medium differential stresses(?5 MPa);while at high differential stress(10 MPa),the strength of weak plane does not have an obvious effect on the interaction mode.The width of weak plane has a certain influence on interaction mode and failure nature of weak plane:under low approaching angle and differential stress(?=30°and ??=3 MPa),the failure nature changes from 'Tensile' to 'Shear'as the width increases;while at medium and high differential stresses(? 5 MPa),the failure nature is all along 'Shear' as the width increases.Under medium approaching angle and high differential stress(?=60°and ??=10 MPa),the interaction mode changes from 'Crossing' to 'Arrested' as the width increases;while under low and medium approaching angles and differential stresses(??60° and ??o?5 MPa),the interaction mode has not changed.When the weak plane has been under the influence of the stress field generated by hydraulic fracture,but the hydraulic fracture does not encounter with the weak plane.At this moment,the change of injection rate has no significant influence on failure nature of the weak plane,but it has a significant influence on the interaction mode of the low approaching angle(30°).Under the low differential stress(3 MPa),the lower the injection rate is,the more easily the branch fractures are formed and the main fracture arrested by the weak plane.Under the medium differential stress(5 MPa),the higher the injection rate is,the more micro-cracks are formed in the weak plane,and the hydraulic fracture crosses the weak plane and the complexity of the hydraulic fracture network is reduced,when the injection rate is 8e-9 m3/s.While under the high differential stress(10 MPa),the path of hydraulic fracture propagation is dominated by the differential stress,implying that the change of the injection rate has no effect on the interaction mode.When the specimen contains multi-weak planes,the interaction mode between hydraulic fracture and multi-weak planes are complex.At ?=30°,the hydraulic fracture will be arrested by the weak plane eventually,crossing from the tip of weak plane or weak points along the weak plane;when the differential stress is less than 5 MPa,the hydraulic fracture may cross the weak plane,in which few cracks are formed;while the differential stress is larger than 5 MPa,the hydraulic fracture may be arrested by the weak planes more than once and a large number of cracks are formed in the weak planes.At ?=60° and low differential stress(??=3 MPa),hydraulic fracture crosses the weak planes,in which a considerable number of cracks are formed;while at high differential stress(??? 5 MPa),hydraulic fracture is arrested by the weak planes and the propagation direction offsets in the triple or quadruple weak planes contained specimen.At ?=90°,the hydraulic fracture crosses the weak plane perpendicularly.As the differential stress and the number of weak plane increase,in the weak planes few cracks formed changes to a large number of cracks formed.Under the same approaching angle,the hydraulic fracture more easily arrested by the weak planes and complex fracture network is easily formed between weak planes as the differential stress increases.The failure nature of weak plane changes as the number of weak plane increases:at ?=30° and low differential stress(3 MPa),failure nature changes from 'Tensile' to 'Shear';when the differential stress is larger than 5 MPa,failure natures are mainly 'Shear' and 'Compaction'.When the approaching angle is larger than 60°,failure natures of the weak plane are mainly'Shear' and the few are 'Compaction',but the tensile micro-cracks are dominant in the weak planes.Finally,the initiation and propagation of hydraulic fracture in the tight sandstone reservoirs with different bedding plane angles are studied based on the BPM and AE technique under different differential stresses.The numerical results are in good agreement with the experimental results.The path of initiation and propagation of hydraulic fracture is summarized into four different modes:(a)complex fracture network(location at wellbore)+ multi-branch main fractures;(b)complex fracture network(location at the path of the main fracture)+ multi-branch main fractures;(c)multi-branch main fractures;(d)single main fracture.In the present study,the accuracy of numerical results of acoustic emission based on moment tensor is verified by the physical tests.Furthermore,the nature of micro-cracks and macro-fractured zones in the BPM is distinguished effectively by using the moment tensor.The effect of the initial contact force,FO,on permeability of the model and breakdown pressure in hydraulic fracturing is studied in detail.And the value of the F0 is suggested,which ensures that the numerical study of the path of hydraulic fracture and the pressure curve of borehole are comparable with those of field results and theory analysis.The physical and mechanical conditions of hydraulic fracture crossing,arrested,offset and formation of complex network fractures are obtained by studying the failure nature of weak planes and interaction modes between hydraulic fracture and weak planes under different differential stresses,which have different strengths,widths,approaching angles and number or is under different injection rates.The results clarify the main factor affecting the morphology of hydraulic fracture,analyze the influence of injection rate on the failure nature of weak plane and reveal the interaction mode between hydraulic fracture and weak planes/bedding planes and the formation mechanism of complex fracture network.This study has a great significance for improving the effectiveness of stimulated reservoir volume,reducing the risk,increasing the complexity of the fracture networks,optimizing construction technology in wells and conducting effective prediction of hydraulic fracture initiation and propagation. |
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