| The renovation of energy structure has been throughout the history human civilization,whose reform and replacement are accompanied by the innovation of economic and social structure.The energy security is an important component of national security and international competition.Oil shale resource reserves in China are huge and rank among the top in the world.As one of reliable alternative energy sources for oil and gas,it is necessary to make technical breakthroughs aiming at the development difficulties of oil shale,to provide technical reserves for China's energy supply security.Meanwhile,in order to implement the concept of"clear waters and green mountains are as good as mountains of gold and silver",the original oil shale exploitation technology dominated by ground retorting technology is changing to in-situ exploitation technology.Therefore,considering the extremely low oil shale porosity and thermal conductivity,hydraulic fracturing is needed to form a complex fracture network.The artificially formed fractures can be used as oil and gas flow channels to facilitate the output of pyrolysis oil and gas,thus improving the oil and gas recovery rate.Taking the Maquan oil shale as the research object,this paper aimed at its anisotropic mechanical properties and fracture initiation and expansion mechanism under the influence of weak bedding planes,by the way of theoretical calculations,laboratory tests and numerical simulations.The main contents are as follows.Firstly,X-ray diffraction measurements,X-ray fluorescence element determination experiments,mercury penetration experiments and scanning electron microscope were carried out to study Maquan oil shale's physical properties,such as its mineral composition,pore structure,porosity and density.The experimental results show that Maquan oil shale has low oil content,about 4.4%.Its density is 2.214 g·cm-3,and the porosity is 5.2660%.Its pore structure is mainly composed of mesopore,with developed laminar structure and randomly distributed darker blocks,whose mineral components are quite different from rock matrix of oil shale.The rock matrix is dominated by clay minerals(illite and mixed-layer illite/smectite with a small amount of chlorite and kaolinite)with a content of 59.2%,while the darker blocks are dominated by fluorapatite with a content of 42.1%and clay minerals with a content of21.1%.This phenomenon exacerbates the anisotropy of Maquan oil shale's mechanical properties.Subsequently,triaxial compression tests,Brazilian tests,semi-disk three-point bending tests and direct shear tests were carried out to analyse the anisotropy of Maquan oil shale's mechanical properties,with laminar dip angle as a variable.The experimental results show that the weak bedding planes change the failure mode of rock.In the triaxial compression tests,with the increase of laminar dip angle,the rock failure mode showed the trend of"splitting damage along and through the bedding planes--shear-tension composite damage through bedding planes--shear-slip damage along the bedding planes--vertical-splitting damage through the bedding planes";under the same confining pressure,the maximum measured compressive strength is always reached at the dip angle of 15°(the angle between the axial load and the bedding plane?'is 75°)and the minimum value is reached at?'=30°.That was to say,the maximum compressive strength of Ma?quan oil shale occurred when the loading angle with respect to the bedding planes was equal to 75°,whereas previous studies considered 0°or 90°.The elastic modulus at?'=0°is generally about 3 times higher than those at other dip angles.With the increasing?',the tensile strength of the oil shale gradually increases,and the overall trend of fracture toughness is increasing as well.The shear strength and internal friction angle of the bedding planes and rock matrix also differ greatly.By comparing the anisotropic ratio of triaxial compression strength,elastic modulus,Poisson's ratio,tensile strength,and fracture toughness of Maquan oil shale with those of other sedimentary rocks,its brittleness index and anisotropy degree are found more similar to Yeoncheon schist,showing good compressibility.At the same time,based on the triaxial compression test data,a modified rock failure criterion for oil shale,which can characterize its very strong anisotropy of mechanical properties,was established.Modified versions for Mohr-Coulomb criterion,Mogi-Coulomb criterion,Drucker-Prager criterion,and Hoek-Brown criterion were established,respectively.Mogi-Coulomb modified criterion was found to be the most applicable to Maquan oil shale.We used the self-developed true triaxial hydraulic fracturing experimental system to analyze the fracture initiation law in Maquan oil shale under the influence of parameters such as laminar dip angle and ground stress conditions.The following conclusions were obtained:generally,with the increasing difference coefficient between horizontal principal stress?and burial depth,the fracture initiation pressure increases;in which,the influence of burial depth on fracture morphology is small,while the influence of?on fracture initiation pressure is small.The influence of laminar dip angle and tendency on hydraulic fracture initiation pressure is greatly influenced by the change of wellbore direction.The fracture initiation pressure is the lowest when the dip angle is 0°and the wellbore axis is parallel to the maximum horizontal principal stress,but the fracture network complexity is low at this time;when the dip angle is the same,the fracture network complexity and fracture morphology are more similar.Natural fractures are beneficial to improve the fracture morphology and fracture complexity,but they are generally not directly related to the fracture initiation pressure.On the basis of hydraulic fracturing indoor tests,a fracture initiation and expansion model for Maquan oil shale considering the bedding planes and the induced multiple failure modes was established and validated.The fracture initiation modes in oil shale can be mainly divided into two types:failure across the intact rock matrix and tensile failure along bedding planes.On this basis,the optimization scheme of horizontal wellbore azimuth is proposed.It was found that for the failure along bedding planes,the magnitudes of the fracture initiation pressure and location changing with the wellbore azimuth were mainly controlled by laminar dip angle,while their change trends were mainly controlled by laminar dip direction.That is to say,the main factors affecting the fracture initiation pressure and location of fracture initiation along bedding planes are the laminar dip angle and the relative relationship between the wellbore axis and bedding planes.Besides,the greater the laminar dip angle,the stronger the tendency for the fracture initiation location to be away from the points of?=0?or 180?on the wellbore.Overall,fractures along bedding planes are susceptible to initiate in the Maquan oil shale,and the laminar dip angle is the main factor influencing the fracture initiation modes.The optimal design of wellbore azimuth for horizontal fractured wells in oil shale requires a comparative analysis of the fracture initiation pressures required for the different fracture initiation modes,and the wellbore azimuth corresponding to a smaller difference between the two fracture initiation pressures is more conducive to the formation of a complex fracture network.Finally,on the basis of the mechanical property tests and hydraulic fracturing indoor tests,a three-dimensional oil shale physical model considering rock non-uniformity and bedding planes was established using the RFPA numerical simulation software,to analyse the effects of bedding planes,wellbore azimuth and its relative relationship with the bedding planes,and fracturing fluid parameters on the fracture expansion behaviour.The simulation results show that weak bedding planes dominates the fracture expansion behaviour in oil shale to some extent.In essence,the influencing factors can be summarized in the strength of bedding planes(the magnitude of"interception force"),the encounter angle of the hydraulic fracture when encountering bedding planes,and the distance from bedding planes according to the influence degree.When the hydraulic fracture encounters bedding planes with relatively high strength,especially with the relatively high tensile strength,it is easier to pass through bedding planes;when the thickness of bedding planes is high,its"interception force"on the hydraulic fracture expansion will be enhanced accordingly,and the hydraulic fracture is less likely to pass through bedding planes;meanwhile,the dip angle of bedding planes will directly affect the encounter angle,and hydraulic fracture is more likely to pass through bedding planes when the dip angle belongs to 30°?60°.The influence of bedding plane occurrence is larger than the wellbore orientation,so the inclined well is to some extent more conducive to higher reconstruction volume and reservoir reconstruction effect.Meanwhile,increasing the fracturing fluid viscosity and flow rate could enhance the reservoir reconstruction effect,but there are threshold values for both of them.If any threshold value is over,the reservoir reformation effect cannot be effectively improved. |
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