Study On Fracturing Mechanics Responses Of Shale Reservoirs Stimulated By Liquid Nitrogen

Unconventional shale gas has the characteristics with wide distribution,long-term stable production,cleanliness and high efficiency.Therefore,accelerating the development of shale gas resources will help ensure China’s natural gas supply and optimize China’s energy consumption structure.Horizontal drilling well combined with large-scale hydraulic fracturing technology is essential to achieving the economic and efficient exploitation of shale gas.The purpose is to create a three-dimensional complex fracture network system to promote gas desorption from pore surfaces and gas flow in pore channels.However,field practice has shown that some shale gas-rich reservoirs cannot inudce a sufficient and complex stimmlated rservoir volume（SRV）when adopting conventional water-based fracturing fluids.In addition,the use of water-based fracturing fluid is accompanied by a series of serious problems such as formation damage,backflow difficulty,and water resource waste.In order to solve or mitigate the negative problems associated with conventional hydraulic fracturing,it is imperative to develop alternative fracturing techniques that not only improve the SRV but also avoid ecological problems.Considering that liquid nitrogen（LN₂）contacting rocks has the characteristics with low temperature enhancing brittleness,frost heave and thermal shock inducing crakcs,it may have a promising application in low temperature waterless fracturing.Taking shale reservoirs as the research object,from the perspective of micro-meso-macro-multiscale,this paper adopts physical experiments,theoretical analysis and numerical simulation methods to investigate the feasibility of LN₂ fracturing to enhance the SRV of shale reservoirs.Firstly,the uniaxial compression experiment for shales under cryogenic nitrogen freezing with different temperatures and the triaxial compression experiment for shales before/after LN₂freezing-thawing treatment under different confining pressures are carried out.The stress-strain curve characteristics,energy change characteristics,strength change characteristics and fragmentation distribution characteristics of shales under cryogenic nitrogen fluid（liquid and gas）action are analyzed.The research finds that shale stress-strain curves at low temperatures demonstrate greater Young’s modulus,enhanced yield modulus,higher peak strength,faster stress drop and lower residual strength.Shale stress-strain curves after LN₂ freezing-thawing cycle have the obvious micro-crack closure stage,and the elastic modulus and peak strength are reduced.Shale stress-strain curves under high confining pressures have the greater elastic modulus,peak strength and residual strength,and the stress drop rate is slower.Based on the evolution law of the energy,a universal shale brittleness evaluation model suitable for different confining pressures and different temperatures is proposed by considering the characteristics of the full stress-strain curve.The results show that shale brittleness increases with the decrease of the freezing temperature,but decreases with the increase of the confining pressure,and shale brittleness decreases slightly after the LN₂ freezing-thawing treatment.Secondly,from the perspective of fracturing mechanism inducing fracture networks,comprehensively considering the engineering sweet spot factors such as shale brittleness index,horizontal stress difference coefficient,micro-fracture development degree,fracture propagation ability and discontinuity surface slippage ability.A quantitative evaluation model for shale fracability suitable for different confining pressures and different temperatures is established based on the analytic hierarchy process theory.A series of physical and mechanical experiments including SEM,NMR,ultrasonic and mechanical testing are performed to investigate these fracability-related factors affected by cryogenic nitrogen treatment.The research finds that as the freezing temperature decreases,the shale brittleness index and horizontal stress coefficient gradually increase,but the fracture propagation ability,micro-crack development degree and discontinuity surface shear-slip ability gradually decrease.Regardless of shales with or without LN₂ freezing-thawing treatment,the increase of the confining pressure will result in a significant decrease in the shale brittleness index,micro-crack development degree and fracture propagation ability.LN₂ freezing-thawing treatment will lead to an increase in micro-crack development degree,fracture propagation ability,and discontinuity surface shear-slip ability.Considering all influencing factors,it is found that the shale fracability index gradually increases with freezing temperature decreasing,but significantly decreases with confining pressure increasing,and the shale fracability index increases after LN₂ freezing-thawing treatment.Then,from the perspective of shale meso-heterogeneity,based on the thermo-poroelastic theory,two-phase seepage theory,local thermal non-equilibrium theory,water-ice phase change theory,and elastic brittle damage theory,a fully coupled thermal（T）-hydro（H）-mechanical（M）-water-ice phase change-damage theoretical model is developed to simulate the process of LN₂ interaction with shale reservoirs.This model can reasonably capture the following phenomena:（1）LN₂ driving unfrozen water flow in porous media;（2）pore water freezing into ice below the freezing temperature;（3）rock’s hydro-thermal-mechanical properties associated with phase saturation-damage-temperature;（4）temperature and pressure-dependent fluid thermodynamic properties;（5）triple fracturing mechanisms including pore pressure,frost heave pressure and thermal stress.The Galerkin method is used to discretize the temperature,pressure,saturation and displacement fields,and the solving method for the fully coupled model is introduced in detail.The developed model is verified against a classical thermo-poroelastic coupling model,a coupled THM-water-ice phase change model for freezing rocks and a HM-damage model for fracturing experiments.Subsequently,based on the developed THM-water-ice phase change model,the response characteristics of the reservoir temperature,pressure,stress,strain,fluid saturation and fluid thermodynamic properties under LN₂ different action conditions are analyzed.The pros and cons of LN₂ fracturing in saturated water-bearing and dry gas-rich shale reservoirs are initially discussed from different perspectives.Finally,based on the developed THM-water-ice phase change-damage coupling model for LN₂ interaction with shale reservoirs,the fracture initiation characteristics and damage evolution of shale reservoirs during LN₂ fracturing are investigated for different influencing factors.The multiple damage mechanisms of the reservoir are revealed during LN₂ fracturing.The study finds that for fully dry shale reservoirs,the fracture of the shale reservoir is dominated by tensile damage under pore pressure and thermal stress.As the horizontal stress difference,injection fluid temperature,or reservoir initial permeability increases,the number of induced macroscopic fractures and the complexity of the fracture networks decrease,but the initial reservoir temperature,homogeneity coefficient or elastic modulus has the opposite influence.Except for the homogeneity coefficient and injection fluid temperature,the fracture initiation pressure decreases with the increase of above influencing factors.For saturated water-bearing shale reservoirs,the fracture of the reservoir is still dominated by tensile damage,but there are isolated shear damage elements under pore pressure,thermal stress and frost heave pressure.Compared with dry shale reservoirs,the number of induced macroscopic fractures is more and the fracture network is more complex for saturated water-bearing shale reservoirs,but the fracture length and the fracture aperture are significantly reduced,and the fracture initiation pressure is also relatively lower.Compared with conventional hydraulic fracturing and supercritical CO₂ fracturing,LN₂ fracturing can generature a more complex fracture network,and the fracture initiation pressure is also lower.The above results reveal the feasibility of LN₂fracturing to increase the SRV of the shale reservoir,and can provide some theoretical support for LN₂ waterless fracturing.