The Effect Of Bedding On Fracture Propagation Of Hydraulic Fracturing In Oil Shale

In order to meet the increasing energy demand of human beings,it has become a key issue to find alternative energy sources of oil.According to the data,the oil shale resource reserves are huge.The world's oil shale resources can be converted into 411 billion tons of oil shale oil,and China has about 47.644 billion tons,which is a good alternative energy for oil.In order to improve the production efficiency of in-situ transformation of oil shale,it is necessary to use hydraulic fracturing technology to improve the permeability of oil shale reservoir.For this reason,we have carried out in-depth study on the law of fracture propagation in hydraulic fracturing.Based on the characteristics of the development of oil shale bedding,this paper studies the influence of bedding on the fracture propagation of oil shale hydraulic fracturing.The calculation formula of fracture propagation based on stress intensity factor is established by theoretical formula derivation;the fracture toughness of oil shale with different angle between bedding angle and loading direction is measured by semicircle plate three-point bending experiment method(NSCB),and the relationship between fracture toughness and different angle between bedding angle and loading direction is obtained by fitting the fracture toughness value obtained by experiment.The relationship between fracture toughness and different angle between bedding angle and loading direction is obtained by electric horizontal testing machine.Finally,we use the quasi side of hydraulic fracturing expansion to get the calculation formula of oil shale expansion.The key parameters of friction coefficient between oil shale are measured and the influencing factors are analyzed.Based on the hydraulic fracturing test,the fracture propagation law of oil shale with different angle between bedding angle and loading direction is studied,and the correctness of the theoretical model is verified.The research results will provide important guidance for the study of in-situ fracturenetwork of oil shale.Firstly,based on the theory of fracture mechanics and the principle of mechanical superposition,the fracture propagation model of hydraulic fracturing in oil shale horizontal well under different formation stress conditions is deduced.Through the transformation of stress coordinate,the in-situ stress field is transformed into the oil shale formation stress on the horizontal well bore.According to the theory of maximum circumferential stress,the theoretical models of stress intensity factor and vertical principal stress,horizontal maximum principal stress,horizontal minimum principal stress,hydraulic fracturing and other key parameters are established.The NSCB is used to measure the fracture toughness of oil shale,and the influence of its parameters(span,fracture length,etc.)and loading rate on the fracture toughness of oil shale is explored.In this paper,the values of fracture toughness are measured when the angle between bedding and wellbore direction(b)is 0 °,15 °,30 °,45 °,60 °,75 °,90 °.The functional relationship between fracture toughness and is obtained by fitting Origin software.When it is 0 ° and 90 °,most of the cracks in the fracture toughness fracture specimen propagate along the parallel loading direction;when it is 15 °,30 °,45 °,60 ° and 75 °,the fracture of the fracture toughness fracture specimen will deviate from the loading direction and produce the phenomenon of torsional extension of the crack.It is found that the fracture toughness of oil shale decreases with the increase of loading rate and length of prefabricated fracture,and increases with the increase of loading span,but the influence of prefabricated fracture length on fracture toughness is relatively small.The friction coefficient between oil shale and the influencing factors such as the pulling speed,the surface roughness,the quality of dragging oil shale and the filling viscosity of drilling fluid between oil shale are analyzed by using the electric horizontal testing machine.The oil shale samples were analyzed by field emission scanning electron microscopy.The friction coefficient of oil shale is 0.55-0.75,mostly0.6-0.7.It is found that the friction coefficient between oil shale increases with theincrease of oil shale mass,decreases with the increase of oil shale roughness,decreases with the increase of apparent viscosity of low-pressure fracture fluid,and increases with the increase of apparent viscosity of high-pressure fracture fluid.When the pulling speed is large,the friction coefficient between oil shale is relatively stable,so the pulling speed between 120 mm \ min-140 mm \ min is selected,and the quality is larger For the amount of oil shale samples,the factors of low apparent viscosity and low roughness of fracturing fluid were tested.In the fracture toughness test,samples were taken from the fracture surface of oil shale,and field emission scanning electron microscope test was carried out.It was found that the particle size of oil shale was about 30um-150 um,which led to the zigzag fracture in the fracture toughness test.The fracture generally extends along the gap between particles.When the oil shale bedding or soft part is encountered,the rock particles will be split to extend the fracture.The oil shale on the surface of oil shale friction coefficient test sample was analyzed by field emission scanning electron microscope.It is found that the micro surface of the sample is uneven,which results in different roughness between oil shale.Using the self-developed true triaxial hydraulic fracturing equipment,the oil shale samples with angle between different bedding and wellbore(b)were tested,and the correctness of the theoretical derivation was verified according to the experimental value of the fracturing pressure.The fracture pressure of oil shale with different b is obtained when hydraulic fracturing is expanding.It is observed that oil shale with different b mainly expands along bedding,but secondary fractures will also occur.In the second chapter,based on fracture mechanics,the fracture propagation model of oil shale horizontal well under different formation stress conditions is derived.In the third chapter,the functional relationship between fracture toughness and b is obtained by fitting test values,and the relationship between fracture initiation pressure and b is obtained.The theoretical value of fracture initiation pressure of b is calculated by the selected appropriate test parameters.Compared with the results of hydraulic fracturing test,it is found that the error is less than 15%,the theoretical value of fracture initiation pressure of b is less than the test value,and the change trend is the same,so this formula is more suitable for oil shale.This result will provide theoretical guidance and technical support for fracture propagation and field test of oil shale in horizontal wells.