Research On Crack Penetrating Propagation Mechanism Of High-energy Gas Impact Fracturing In Coal Measure Composite Reservoirs

In view of the problems that the hydraulic fractures are difficult to penetrate the interface resulting in fractures passivation,"T" or "工" type expansion,during the fracturing process of coal measure composite reservoirs with complex lithology and structure,high content of clay minerals,and strong plasticity.A high-energy gas impact fracturing method with advantages of pressure strength and transmission speed was proposed to cross layer fracturing in coal measures composite reservoirs.Emphasis on the combination of theoretical analysis and numerical simulation,the geological conditions of coal measure composite reservoirs,high-energy gas impact parameters and their influence on the penetrating propagation of fractures were systematically studied.The research results will provides the basic theory,fracturing method and engineering parameters optimization basis for realizing coal measure composite reservoirs combined transformation and coal measure gas efficient combined production.The main work and conclusions are as follows.(1)The geo-engineering model of high-energy gas impact fracturing coal measure composite reservoir was established.Taking the coal bearing strata in Linxing block of Ordos Basin as the research background,based on the analysis of the structural characteristics,mechanical characteristics,mineral composition,gas bearing property of the reservoir and the construction scheme of high-energy gas impact fracturing,a geo-engineering model of high-energy gas impact fracturing coal measures composite reservoir was established,which consider the geological characteristics of coal seam and its roof and bottom lithology combination,physical and mechanical properties of reservoirs and interfaces,in-situ stress,as well as engineering characteristics such as horizontal well layout,initial guided perforation position,blasting position and charge quantity.It lays a foundation for the study of mathematical model and numerical simulation of high-energy gas impact fractures penetrating propagation.(2)The mathematical model of dynamic fracture propagation in coal measure composite reservoirs by high-energy gas impact fracturing was established and solved.Based on the theory of propellant combustion,fluid mechanics,and fracture mechanics,combined with basic assumptions such as propellant geometric combustion,steady gas flow,and stable crack propagation,the propagation process of high-energy gas impact crack in composite reservoirs was divided into five sub modules: propellant deflagration loading,gas flow in the crack,gas filtration,crack propagation,and crack penetration discrimination,and the corresponding mathematical model was established.Taking the pressure variable as the bridge,the sub modules were coupled.The micro element method was used to take the increment of time step as the main variable.Combined with the mass and energy conservation equation and the gas state equation to complete the cyclic iterative calculation between gas state-fracture pressure-solid deformation,so as to realize the rapid and accurate solution of the mathematical model.This achievement realizes the quantitative description of crack morphology at any time in the impact fracturing process,judge whether the crack penetrates the interface or not.It makes up for the defect that the traditional model can only calculate the crack propagation in single layers.(3)The influence of geological conditions and high-energy gas impact parameters on the penetrating propagation of fractures was revealed.Based on the mathematical model of dynamic fracture propagation under high-energy gas impacting and its solution method,taking the geological conditions of No.9 coal seam and its roof and floor in Linxing block as the research background,the influence of geological conditions and high-energy gas impact parameters on the penetrating propagation of fractures was simulated and analyzed.The results show that when the impact parameters are constant,the cracks are more likely to penetrate from high shear modulus,low Poisson’s ratio,and high density layers into low shear modulus,high Poisson’s ratio,and low-density layers.The smaller the interface dip angle,the greater the interface bond strength,the more conducive to crack penetration.Under specific geological conditions,the greater the loading rate,the more conducive to crack penetration.With the increase of elastic modulus difference between layers,the height,width and volume of the high-energy gas impact crack increase linearly.With the increase of the Poisson’s ratio difference between layers,the height and width of the high-energy gas impact crack decrease exponentially,and the volume of the crack decreases linearly.With the increase of loading rates,the height,width and volume of the high-energy gas impact crack increase exponentially.With the increase of the peak pressure,the height and width of the high-energy gas impact crack increase linearly,and the volume of the crack increases exponentially.(4)The energy mechanism of the high-energy gas impact crack penetrating propagation was revealed.Based on the qualitative analysis of the energy transfer law at the fracture tip,ABAQUS simulation software was used to simulate the change law of the energy release rate at the fracture tip when it meets the interface under different loading rates.The exponential cohesive constitutive model and Newmark-β display dynamic time integration scheme were used in the simulation.The results show that after the crack meets the interface,the ratio of growth rate between the penetration energy release rate and the deflection energy release rate increases approximately in a logarithmic manner with the increase of the loading rate.High loading rate is more conducive to the energy transfer and accumulation at the crack tip in the direction of penetration.When the crack reaches the interface of composite reservoirs,its expansion behavior depends on which energy release rate of the two directions at the crack tip reaches their respective critical values first.The faster the loading rate,the faster the increase rate of the energy release rate at the tip of the interlayer,the easier it is for the energy release rate to reach the critical fracture energy of the matrix layer first,and the easier it is for the fracture to penetrate the layer.Compared with hydraulic fracturing,the loading rate of high-energy gas impact fracturing is 5～6 orders of magnitude higher.The energy at the crack tip is transferred to the penetration direction more,so that the penetration energy will first exceed the fracture energy of the matrix layer.This is the reason why the hydraulic fractures are easy to expand along the interface and the high-energy gas impact fractures propagate through the layers under the same geological conditions.(5)The optimum fracture initiation position of high-energy gas impact fracturing in coal measure composite reservoir was optimized.Based on the geological conditions of No.9 coal seam and its roof and floor in Linxing block,the propagation height of the high-energy gas impact crack under different fracture initiation layers was simulated.Taking the propagation height of cracks as the evaluation index,the best fracture initiation position was optimized.The results show that when the roof sandstone layer cracks at 1/4 of its thickness,the propagation height of the crack is the largest,which is 13.5 m.The 1/4 of the roof sandstone layer thickness is the best initiation zone for high-energy gas impact fracturing.