Study Of Mechanical Properties And Initiation Pressure Of Gas Shale Under The Effect Of Liquid Nitrogen Cooling

As the production rate in conventional oil and gas reservoirs decreases day by day,unconventional oil and gas reservoirs such as gas shale play an increasingly important role in the worldwide oil and gas exploration and development.Due to the typical characteristics of low porosity and ultra-low permeability in shale gas reservoirs,corresponding stimulation techniques are required to effectively develop such kind of reservoirs.Applying cryogenic liquid nitrogen to cool the rock or even using liquid nitrogen as a fracturing fluid in fracturing operation is a relatively new concept and method in recent years to develop shale gas resources underground.Its main idea is to utilize the great temperature difference between the liquid nitrogen and formation and thermal cracks can be induced on the wellbore surface after liquid nitrogen contacting the wellbore wall,thus increasing the communication area between the wellbore and reservoir and finally achieving the goal of increasing production rate.In order to obtain the evolution law of both physical and mechanical properties of shale rock samples before and after liquid nitrogen freezing and thawing treatment,acoustic wave experiment,nuclear magnetic resonance experiment,gas permeability experiment,Brazil splitting experiment and tri-axial compression experiment were conducted.At the same time,based on the finite element software ABAQUS,fluid-solid-heat coupled numerical model is established.And the distribution of stress field,temperature field,displacement field around the wellbore wall after liquid nitrogen freezing and thawing is studied.Meanwhile,the magnitude of initiation pressure and propagation pressure after liquid nitrogen treatment is calculated and the effect of operational parameters and formation properties on initiation pressure and propagation pressure is analyzed.In addition,utilizing extended finite element method,the fracture propagation behavior was investigated after liquid nitrogen freezing-thawing treatment.And the fracture propagation behaviors under different fracture propagation sequences and different fracture spacing were analyzed.The obtained main conclusions are shown below:(1)The impact of liquid nitrogen will lead to the emergence of new micro-pores or micro-cracks in the shale rock samples,which decreases the ultrasonic longitudinal wave velocity by 16%,increases the magnitude of T₂curve by 46%,increases the permeability of the rock sample by 57%,and decreases the tensile strength,compressive strength and elastic modulus of the rock samples by 40%,25%and 27%,respectively.(2)Freezing pretreatment with liquid nitrogen can generate cold shock to the formation rock and cause the changes in stress field,temperature field and displacement field.When freezing for 2 days,the distance between the frontier of 0?and the wellbore surface is 0.65 m and the distance between the frontier of-70?and the wellbore surface is 0.22 m.Meanwhile,the original compressive stress area around the wellbore gradually turns into a tensile stress area,and the shallower the formation is located,the larger the formed tensile stress area is.(3)After freezing and thawing by liquid nitrogen,both the magnitude of initiation pressure and propagation pressure decreases by 12%.In addition,the variation rule of initiation and propagation pressure of shale formation against operational and geomechanical parameters are calculated.More specifically,the larger tensile strength leads to the larger initiation pressure and propagation pressure.The larger elastic modulus yields the greater initiation pressure and propagation pressure.The larger permeability leads to the smaller initiation pressure and greater propagation pressure.The larger leak off coefficient is,the smaller the initiation pressure is and the higher the propagation pressure is.The larger the ratio of the maximum and minimum horizontal stresses results in the smaller initiation pressure and the smaller propagation pressure.The larger fracturing fluid injection rate leads to the greater initiation pressure,but the propagation pressure remains constant.The influence of fracturing fluid viscosity on fracture initiation pressure can be ignored.However,the larger the viscosity of fracturing fluid is,the greater the propagation pressure will be.(4)After freezing and thawing by liquid nitrogen,the effects of fracture propagation sequence and fracture spacing on fracture geometry are investigated.When fractures are created in sequence,the deviation angle of fracture F3increases from 5°to 15°and the fracture mouth width of fracture F3 decreases from 6.9 mm to 4.7 mm with the fracture spacing decreasing from 50 m to 20 m.When the Texas two-step fracturing method is simulated,the deviation angle of fracture F2 increases from 0°to 3°and the fracture mouth width of fracture F2 decreases from 6.4 mm to 3 mm with the fracture spacing decreasing from 50 m to 20 m.Furthermore,the area of fracture network created by Texas two-step fracturing method is 62%higher than that of sequential fracturing method.