Fracturing Mechanism And Permeability Enhancements Effect Induced By Gas Fracturing In Low Permeability Reservoir Rocks

China holds the largest amount of unconventional natural gas reserves in the world.However,the exploitation of unconventional natural gas in China is faced with a series of problems,such as complex reservoir conditions,low permeability of reservoir and lack of water resources in the accumulation area of unconventional natural gas reservoirs.It is imperative to renovate the methods and techniques for stimulation of the unconventional natural gas formations and to establish theory and technology for achieving high-efficiency exploitation and utilization of unconventional natural gas in China.As a waterless fracturing technology,gas fracturing is a research hotspot in the fracturing and reforming of the low permeability reservoirs currently,and it may be the key to solve the bottleneck of unconventional natural gas resource exploitation in China.Compared with the traditional hydraulic fracturing technique,gas fracturing can not only save a large amount of water resources,but also avoid the pollution caused by hydraulic fracturing to the surface and underground water.Therefore,to achieve high efficiency fracturing of low permeability reservoir,the mechanism of gas fracturing in low permeability reservoir is studied,and the method of high quality stimulation of low permeability reservoir rocks in gas fracturing is explored,the theory and technology for quantitative evaluation of the permeability improvement effect of low permeability reservoir after fracturing are established.Laboratory test,field industrial test,theoretical analysis and numerical simulation methods are involved in this study.The main achievements are as follows:?1?The differences between hydraulic fracturing and gas fracturing in crack initiation and propagation of different low permeability reservoirs are systematically analyzed.Compared with hydraulic fracturing,in gas fracturing the crack initiation pressure of low permeability reservoir rock decreases obviously,the initiation and propagation of micro-cracks are more active and more intense,more complex and tortuous fractures are easily formed.The feasibility of gas fracturing in low permeability reservoirs in China has been verified.?2?The crack initiation criterion of reservoir rocks in gas fracturing is established,and the mechanism of crack initiation and crack propagation is revealed.Based on the stress intensity factor of type I crack and characteristic length,the crack initiation criterion of gas fracturing is established,which avoids the selection of initial crack length,and is verified by experimental data.The differences of crack initiation and extension between hydraulic fracturing and gas fracturing are explained by the stress intensity factor distribution in crack tip of reservoir rocks and the established crack initiation criterion.The influences of reservoir rock permeability,fracturing fluid viscosity and fracturing fluid loading rate on the crack initiation are analyzed,which provided a specific criterion for fracturing of low permeability reservoirs using gas fracturing.?3?The methods of producing serious volume fracturing for different low permeability reservoirs in gas fracturing are proposed.For reservoirs with relatively well-developed pore-fracture structures,a gas fracturing method of special graded particles is proposed to plug natural pores and fractures in fracturing process to improve pore pressure distribution.Then,by converting fracturing with external load into fracturing with gas pressure in pores and fracture,volume fracturing of low permeability reservoirs will become more serious.For reservoirs with extremely poor pore structure,a pulsed gas fracturing method is proposed to promote efficient fracturing of reservoirs by changing the loading pattern because that the pulsed fatigue load can generate multiple cracks and create a high surface area.?4?3D pore-fracture structures of low permeability reservoir rocks generated by gas fracturing are constructed.Firstly,3D pore reconstruction of reservoir rock is performed using a high-resolution three-dimensional X-ray microscopic imaging system,argon ion polisher and electron probe microanalyzer.The representative elementary volume of reservoir rock is obtained by quantitative analysis of structure characteristics in the restructured core.Then,considering the fracture morphology of reservoir rocks induced by gas fracturing,3D fractures with different fractal dimensions and 3D fractures networks with different number of fractures are generated in the representative elementary volume of reservoir rock using the Weierstrass-Mandelbrot fractal function.?5?The quantitative evaluation of permeability for different complex pore-fracture structures generated by gas fracturing is achieved.Using the established micro-scale lattice Boltzmann model,the micro-nanoscale migration mechanism of gas is revealed.Based on the mechanism of gas migration at microscale,the three-dimensional pore scale and the REV scale lattice Boltzmann model considering the Klinkenberg effect are established,respectively,then CH₄ seepage simulation in the three-dimensional pore-fracture structure is carried out.The cross-scale problem of fluid migration is solved.The gas migration mechanism in the complex pore-fracture is visually described,and the effect of fracture complexity and gas pressure on permeability of reservoir rocks are quantitatively evaluated.