| Since the 21 st century,the demand for energy resource is growing.With the deepening of the exploration,shallow buried conventional oil and gas resources might become exhausted in the next one or two centuries.Deeply buried unconventional oil and gas resources are becoming the new research hotspot in energy industry.Questing for the unconventional oil and gas resources is imminent,especially in the countries which lack of the conventional oil and gas resources.Shale gas has the characteristics of abundant resources and wide distribution.According to the report of U.S.Energy Information Administration(EIA),the world's proven recoverable shale gas resources reached 187.5 trillion cubic meters,of which 24.4 trillion cubic meters are in America and China's main basin recoverable shale gas is about 26 trillion cubic meters which is the first in the world.In 1821,with the first success in drilling of shale gas well in New York,from 1921 to 1975,industrialization production was gradually formed.Since the 1990 s,the U.S.shale gas production has been increasing year by year.By 2035,the U.S.shale gas production prediction will be accounted for 49% of total gas output.The success of American shale gas mining builds confidence for seeking unconventional resources in other countries.However,there is a big difference in reservoir geological conditions between China and the United States.The U.S.shale gas is mainly in marine sedimentary rocks and shale gas in China is concentrated in continental sedimentary rocks.In addition,marine shale in China has the characteristics of high thermal maturity and fully developed fault and continental sedimentary rocks are in low thermal maturity and deep buried depth.Shale gas exploration and production in China is in the initial stage,to achieve long-term stability of the shale gas production and the revitalization of China's energy industry,an exploitation plan should be established according to the characteristics of China geological resources.Shale gas stored in organic shale is in three states: free in fracture and pore,adsorbed in kerogen and clay surface and dissolved in asphaltene.Among them,free of shale gas proportion is 20% to 85%.Shale is a kind of dense rock and has low permeability characteristic which the porosity is usually less than 10%.Mineral composition of shale determines its mechanical properties.High brittleness mineral content of shale(such as calcite,quartz and feldspar,etc.)has stronger brittleness,easier to form complex fracture network when fracturing and natural fracture penetration is benefit to shale gas flow and improvement of hydraulic productivity.High clay minerals content of shale has strong plasticity and low fracturing production,because it is hard to produce complex fracture network.The average depth of the shale reservoir is about 6000 feet(1829 meters)hereinafter the surface.Reservoir under the influence of buried depth,in-situ stress and high temperature makes the mining technical more difficult and costs more.In-situ stress affects the initiation fracture pressure and crack propagation plane.Small horizontal stress difference is prone to produce fracture network which is benefit to shale gas transport and mining.Thus,understanding the influence of in-situ stress and mechanics properties of shale on shale rock crack initiation and propagation is of great significance in improvement of extraction efficiency of shale gas resource.Low permeability of shale reservoir influences the recovery efficiency of oil and gas resources.Practical engineering should rely on the fracturing of the reservoir to increase production.Hydraulic fracturing is one of the main approaches to increase production of low permeability reservoirs and it has been widely used in the exploitation of unconventional resources.90% of shale gas wells in the U.S.are using hydraulic fracturing to increase the production.Combined with horizontal well drilling,it greatly improves the oil recovery in single well.The principle of hydraulic fracturing stimulation produces a fracture plane through reservoir along the direction of maximum principal stress.In order to solve the theoretical and technical problems existed in the hydraulic fracturing operation,the hydraulic fracturing crack initiation and propagation issues are studied extensively by the domestic and foreign scholars.In theoretical research,based on plane strain theory,PKN and KGD models are established and quasi three dimensional models P3 D and PL3 D are applied to the fracturing problems.However,the above theoretical model assumes that the rock is a homogeneous,continuous and isotropic body and unable to analyze the fracture plane deflection.In fracturing experiments,people mainly study fracturing through the macro triaxial tests.Blair observed the fracturing fluid path by sandstone embedded in cement.Ito and Kayashi acquired the relationship between the crack width and the water pressure in rock's pore.Wu designed different perforating configuration and found that the crack propagation direction was affected by minimum principal stress.Athavale found that homogeneous rock fracture plane was like a wing and heterogeneity rock fracture plane had a complicated shape.Numerical simulation methods included the displacement discontinuity method(DDM),the extended finite element method(XFEM),the discrete element method(DEM)and the boundary element method(BEM),etc.The displacement discontinuity method assumes that the crack propagation is for plane strain problem,fracture toughness,young's modulus and poisson's ratio are the basic input parameters.The extended finite element method uses the integral method to calculate the stress intensity factor and its criterion is based on circumferential stress theory.Nagel analyzed the laminated shale fracture plane distribution using the discrete element method platforms(UDEC and 3DEC).Olson analyzed the influence of the natural fracture and the horizontal principal stress difference on fracturing through the boundary element method.Besides,the damage variable,no element method(EFM),discontinuous deformation analysis(DDA)and particle flow(PFC)also have a certain application in the numerical simulation of fracturing.Therefore,to investigate the physical and mechanical properties of shale,hydraulic fracturing crack initiation propagation,and characterization of solid material deformation and fracture features,we carried out the following research and some meaningful conclusions are obtained.Study of physical and mechanical properties of shale: 1)Aim at Shandong Shengli oilfield shale formation in the depth of 3300~3500 m,through 12 groups of porosity and permeability test,we acquired that the porosity is ranging from 1% to 6.2% and the permeability is ranging from 1.23?10-3 to 8.60?10-3 mD,which represents the typical low permeability reservoir.2)With the Application of X-ray diffraction technology,we analyzed the mineral composition of five shale cores in different depths.Results show that the quartz,calcite,dolomite and albite are the major parts of the brittle mineral of shale.The higher brittleness mineral content is,the easier it is to form complex fracture network.In early exploration stage,we should choose high brittleness shale for fracturing.3)Using uniaxial test,we measured the shale strength,elastic modulus and poisson's ratio and other parameters.The same horizon core has the anisotropy properties and the measured mechanical parameters provided the basis of construction of transverse isotropic constitutive model in shale.4)Through triaxial tests of shale cores in different depths,we obtained the compressive strength,internal friction angle and cohesion.To improve the efficiency of hydraulic fracturing,in practical engineering,we should choose the reservoir has large internal friction angle and small cohesion which is easy to form complex fracture network.5)We measured the fracture toughness of shale using three-point bending of Cracked Chevron Notched Brazilian Disc(CCNBD).The determination of the fracture toughness provides the fracture criterion in analysis of crack initiation.Based on measured rock mechanics parameters and transverse isotropic constitutive equation,using the finite element method combined with the fracture mechanics theory,this section introduces the effects of perforating parameters and in-situ stress conditions on the hydraulic fracturing crack initiation and propagation.The results indicate that: 1)the increase of the angle between perforating direction and horizontal maximum principal stress(from 0? to 90?)leads to the increase of the crack initiation pressure.The optimal perforation direction should be parallel to the maximum horizontal principal stress.2)The crack plane gradually turns toward the direction of the maximum horizontal principal stress when they are not in parallel and mode I and mode II stress intensity factors increase at the same time,which shows that the crack surface is in tensile and shear state.3)Compared with the linear and symmetric pattern,the staggered perforation is the optimal one.4)The proper perforation density is four to six holes per meter.5)The optimal perforation diameter in this model is 30 mm.6)The influence of the perforation depth on the fracture initiation pressure is low.7)In different in-situ stresses,the crack initiation position rises with the increase of the horizontal stress ratio while the crack initiation pressure decreases with the increase horizontal stress ratio.Fracture plane propagates along the horizontal maximum principal stress.Stress concentration area moves with the crack tip and has an increasing trend.Stress intensity factor first increases,then decreases,which reflects the energy accumulation and release in fracturing.Numerical analysis of hydraulic fracturing shows that the crack tip region is stress concentration area and fracture occurs due to the stress intensity factor at crack tip is over the fracture toughness.We report a physical method to quantitatively visualize the evolution of the stress field during crack initiation and propagation in a thick disc under diametrical compressive loads.Three-dimensional printing technology and a photopolymer resin were used to produce a seamless transparent disc containing a penetrating fusiform crack.The results show: 1)the disc before and after loading in a circularly-polarized field has no isochromatic fringe,which exhibits no residual processing stress in the manufacturing process and failure behavior is similar to brittle material with no residual plastic stress.Its elastic failure behavior ensures the application of the photoelastic test and theory to observe the dynamic isochromatic fringe and quantitatively analyze the stress field evolution.2)High-quality dynamic isochromatic patterns can be clearly captured by the high-speed photoelastic system consisting of a pulsed laser-light source(FWHM of 220 ns),optical instruments,a high-speed camera and a portable motorized loading machine.3)In crack propagation process,the stress concentration area moves with the crack tip which is in line with hydraulic fracturing numerical results.The big stress concentration at crack tip leads to the crack of the material.4)Crack propagation velocity variation is consistent with the maximum stress of crack tip,which reflects the crack tip stress field controls the superficial phenomenon of crack extension.5)The increase of the stress intensity factor in compression reflects the energy accumulation and the decrease of the stress intensity factor reflects the energy release in fracture process.6)The calculated stress fields in numerical analysis and photoelastic test are consistent with each other,which mutually verify the accuracy of the two approaches.Study of stress field of 3D solid structure during fracturing: 1)3D printing technology was applied to manufacture a cuboid model with a fusiform crack.Through stress frozen tests,two fractured models in crack initiation and propagation stages were acquired,which met the experimental requirement of the stress field visualization in different fracture stages.2)An effective phase unwrapping algorithm is adopted in self-edited MATLAB program to automatically and quantitatively analyze stress field of solid structure in different crack growth stages.3)The stress field at crack initiation stage was quantitatively characterized and the stress concentration region was at the middle of the model around the initial fusiform crack which represented the crack initiation was induced by the local concentrated stress.The less crack-tip stress field of cracked model was consistent with crack arrest.The crack-tip stress field at crack initiation and crack arrest provided judgement for complex structure's crack initiation,propagation and crack arrest.4)The numerical stress fields at crack initiation and propagation stages were in line with the experimental test.The first increase then decrease of the stress intensity factors(SIFs)at crack tip in fracturing represented the energy accumulation and release. |
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