Study On Mechanical Behavior And Hydraulic Fracturing Performance Of Tight Sandstone Reservoirs

Sandstone gas is an unconventional natural gas resource with clean and high efficiency.With the exploration and development of conventional oil and gas resources gradually entering the bottleneck stage,the exploitation of sandstone gas has become an important strategy for solving the energy crisis,mitigating the greenhouse effect and achieving"carbon peaking"and"carbon neutrality".However,the geological conditions of China’s sandstone gas reservoirs are more complex,with poor physical properties,strong non-homogeneity,low reservoir permeability,and large differences in burial depth,leading to high construction costs and difficulty in extraction,etc.Therefore,in view of the characteristics of China’s sandstone gas reservoir,it is urgent to explore a set of theories suitable for sandstone gas fracturing in China.In this paper,triaxial compression tests,permeability tests and hydraulic fracturing indoor simulations tests at real time temperature under different confining pressures conditions were simulated for tight sandstones in the Bohai Bay Basin.The permeability variation law,mechanical behavior,brittle characteristics and failure mechanism of tight sandstone under different confining pressures conditions were studied.The variation law of fracturing curve with formation conditions,fracture morphological characteristics and fracture propagation law were analyzed.The relationship between fracture pressure and permeability and leak-off coefficient were revealed.In addition,the fracture surface after hydraulic fracturing was scanned in 3D and the roughness calculation program was written with the help of Matlab software.The variation pattern of temperature and confining pressure on roughness was investigated,and the effect of roughness variation on production capacity was explained.Finally,based on the strength criterion and crack extension criterion,an axisymmetric mechanical model was introduced to analyze the stress changes in the elastic and plastic zones during the fracturing process.The relationship between fracture pressure and brittleness index,plasticity radius and wellbore size were explored.The mechanism of sandstone crack initiation and propagation were revealed.The results aim to provide a certain technical reference for the hydraulic fracturing of sandstone gas in China.The main contents are as follows:（1）The evolution law of sandstone permeability with different permeability pressure and surrounding pressure is studied.The test results show that the permeability increases with the increase of surrounding pressure,and the tight sandstone will undergo elastic and plastic deformation under the action of surrounding pressure.The elastic deformation is recoverable,which leads to the increase of the permeability when unloading,while the non-recoverable plastic deformation leads to the permeability does not return to the original initial value,and different degrees of damage occur under different permeability pressures.（2）The damage modes of sandstone under different temperature-pressure coupling conditions are mainly in the form of tensile failure,conjugate shear failure,Y-shaped failure,tension-shear composite failure,and single-shear face failure.Overall,with the enhancement of the confining pressure level,the brittleness of sandstone gradually decreases,and the rock damage mode gradually transitions from tension failure to tension-shear composite or single shear surface failure form.The effect of temperature leads to a decrease in cohesion within the rock particles and makes them more susceptible to deformation under load.（3）In hydraulic fracturing experiments,With the increase of confining pressure,the rupture pressure increases and decreases with the increase of temperature.The first fracture pressure is the largest,then decreases sequentially,and the third fracture pressure is close to the confining pressure value.The confining pressure control function is strong at low temperature,and it will be attenuated at high temperature.The stress drop in the sandstone is independent of the fracturing sequence and the number of fractures,and is similar to the fracture pressure law,with the first stress drop η₁>the second stress drop η₂>the third stress drop ₃η.（4）Under the condition of low temperature and low pressure,the pore pressure in sandstone is not enough to open the weak bedding surface,and sandstone mainly presents a single crack perpendicular to the direction of minimum principal stress on the side and a symmetrical double-wing crack on the end face.However,the pore pressure applied internally is enough to open the weak bedding plane under the condition of high temperature and high pressure.During the initiation and extension of hydraulic fractures,the crack tip will turn after encountering the bedding plane,and then expand along the bedding plane after turning,or continue to extend along the original direction while expanding along the bedding plane.（5）Based on the unified strength theory,the hydraulic fracture initiation model is established,and the brittleness index of rock is introduced into the initiation model,and the theoretical formulas of hydraulic fracture cracking pressure and brittleness index are derived,and the factors affecting fracture pressure and brittleness are analyzed.The results show that the theoretical values of fracture pressure and brittleness index are basically consistent with the experimental values.（6）Under the condition of low temperature and low pressure,the fracture surface of rock is smooth and has high flatness,while under the condition of high temperature and high pressure,the fracture surface has obvious sawtooth shape.The roughness coefficient R_s,dip angle θ_s,gradient modulus Z2_s and elevation difference SD all show a positive correlation trend with rock sample temperature and confining pressure.Temperature will cause more micro-cracks and pores in induced sandstone,so it is easier to produce complex fracture network for sandstone gas reservoirs with high temperature.The confining pressure makes the fracture pressure of rock samples greater,the energy accumulated before sandstone failure is higher,and the instantaneous release of energy during failure causes the damage degree to be aggravated,resulting in a rougher fracture surface.