Study Of Crack Initiation And Propagation Mechanism In Shale Under Cryogenic Nitrogen Fracturing

Shale gas is trapped in the tiny nanoscale holes in the shale rock,the diameter is about 50 000 th the size of a human hair,where it is fractured and pumped out.But conventional fracking has several adverse effects and has made it impossible to develop shale gas in areas where water is scarce efficiently.The technique of liquid nitrogen and other low-temperature fluid injection is one of the solutions.It minimizes water consumption and minimizes formation damage,offering great promise as the oil industry improves recovery speed and efficiency.The degree of crack initiation and propagation determines the controlled volume of SRV fracturing in shale gas reservoirs.Therefore,it is of great significance to study the fracture initiation and propagation mechanism of shale under low-temperature liquid nitrogen fracturing to improve reservoir reconstruction volume and realize its industrial development.However,crack formation and propagation is a fast and highly complex process,and it is not easy to control the real-time data of low-temperature fracturing in liquid nitrogen.Therefore,this article combined with visualization,laboratory test,and numerical simulation method,such as using RFP software,3 d printing,high-speed cameras and other research methods,considering factors such as heat transfer coefficient,stress conditions and interference of bedding during fracturing,as well as the original non-uniformity and defects of shale reservoir,analysis of different homogeneous degree,in situ stress,heat transfer coefficient under a different environmental condition such as bedding,shale internal stress field,temperature field and the change rule of crack,crack extension rule,can to a certain extent,relevant research results of the liquid nitrogen of low-temperature fracturing complex formation mechanism,further understanding.And it will be helpful to select the most effective application mode and technology of liquid nitrogen low-temperature fracturing in the production site.The main research conclusions are as follows:(1)Two different crack forms were observed in the field test: horizontal plane crack and vertical tensile crack.There are two different modes of crack development:one is the horizontal plane's radial propagation caused by longitudinal thermal shrinkage;the other is the vertical propagation caused by circumferential shrinkage.The crack overgrows quickly,and there is always a specific length interval between the horizontal cracks.(2)Different homogeneous samples of its primary distribution along the vertical direction of liquid nitrogen interface crack propagation.When m=2and5,crack surface roughness,microfracture disorderly,dotted around the crack tip.When m?10,near the crack tip point of raw no damage,crack extension with homogeneous material results is closer.The Angle between the fundus of the marker well(line AF)and the bottom crack propagation direction is Angle,when m?5,the larger m is,the smaller its Angle is;when m ? 10,the included Angle is almost zero,that is,the bottom crack growth is parallel.(3)Other conditions in the same situation,when a liquid nitrogen cold shock after a certain period,the heat transfer coefficient,the greater the contact boundary.So the faster the temperature drop rate,the temperature,and the temperature gradient,the greater the lead to tensile stress value and the larger the contact boundaries,which make the rock crack is easier to crack,has better effect of cracking(crack more,faster,greater damage range).Increasing liquid nitrogen flow rate,increasing liquid nitrogen pressure,or using nitrogen fixation and fracturing proppant as a stable suspension in liquid nitrogen are useful measures to reduce the Leidenfrost effect and increase the heat transfer coefficient.(4)The lateral stress coefficient affects the direction of crack propagation.The low-temperature crack propagates to a deeper depth along the direction of more major principal stress.In contrast,the crack is concentrated in the direction of major principal stress under different lateral stress coefficients.In the field application,vertical stress can be changed by ground loading or loading to control the direction of crack propagation.(5)In the damaged fracture zone,the tensile stress concentration occurs at the fracture tip of the bedded shale rock,which causes the vertical contact surface of shale fractures to expand,and the fracture spacing and length show certain similarity or classification phenomenon.At the same time,the stress concentration is concentrated at the cementation of the surface.When the fracture is spreading in the direction of parallel bedding,the weak cementation effect of the bedding plane makes it have a weak ability to prevent the fracture from spreading.Except for a small part turning towards the weak bedding plane,the primary fracture extends vertically to the contact surface.When the fracture is spreading vertically to the bedding plane,the bedding plane has a strong ability to prevent the fracture from spreading,and the fracture is prone to turning and bifurcation.This property enables the fractures to form a complex network of fractures that can improve shale gas recovery.