Research On The Mechanism Of Fracture Crossing Layer Expansion Of High Energy Gas Fracturing Coal Reservoirs

With the breakthrough in basic geological theory and exploration and development technology,the exploration and development of coal-based gas,mainly characterized by the symbiosis of tight sandstone gas(TSG),shale gas(TG)and coal bed methane(CBM),has become a research hot spot in the field of unconventional gas exploration and development.At present,the main means of coal system gas mining is hydraulic fracturing,but due to the differences in the physical and chemical mechanical characteristics of different coal system reservoirs,hydraulic fracturing in the interface between the adjacent layers of the composite reservoir expansion is prone to passivation,"T"type expansion and deflection expansion of the phenomenon,can not achieve the purpose of multi reservoir co-pressure combined mining,while high-energy gas fracturing can easily compress open the stratum,but also can use the resulting pulse high pressure to expand the fracture extension scale,and has achieved good technical results in the conventional oil and gas reservoir transformation.In view of this,this thesis presents the technical problems of poor permeability of coal system reservoirs in China,the geological characteristics of multi-layered system and complex reservoir formation,and the difficulty of hydraulic fracturing in the process of stratified fracturing,and proposes to implement stratified fracturing of coal system complex reservoirs by high-energy gas fracturing in order to realize the joint mining of coal system complex reservoirs.Theoretical analysis was used to study the pyrotechnic burst loading,intra-seam gas flow,seam wall gas infiltration,and high-energy gas-driven fracture initiation and layer penetration expansion processes involved in the composite reservoir high-energy gas fracturing process,and the corresponding mathematical models and their solutions were established.Based on the mathematical model of high-energy gas fracturing in the complex reservoir and its solving method,the simulation software was compiled and the correctness of the simulation software was verified from the angle of gunpowder bursting pressure and so on.The main work of the thesis and the main conclusions reached are as follows:(1)The mathematical model of high-energy gas fracturing in the composite reservoir was established by adopting the method of rock fracture mechanics and considering the influence of gunpowder parameters,injection hole parameters,well structure parameters,reservoir thickness,stratigraphic interface properties and the stress distribution of the reservoir septum and rock mechanical properties on the fracture penetration expansion.The model consists of four parts,namely,a gunpowder burst loading model,an in-seam gas flow model,a seam wall gas infiltration model,and a fracture penetration expansion model,which can realize the dynamic simulation of the fracture penetration expansion process of high-energy gas fracturing in the composite reservoir and quantitatively describe the fracture morphology at any time during the high-energy gas process in the composite reservoir.This model makes up for the fact that the past high-energy gas fracturing models can only calculate the expansion pattern of fractures in a single reservoir,which makes it difficult to accurately evaluate the difference between the fracture pattern before and after the fracture penetration,and makes the calculation results have a big bias.(2)Based on the solution of the pyrotechnic burst loading model and the fracture penetration extension model respectively,the method of solving the composite reservoir high-energy gas fracturing model was established using the temperature and pressure transfer relationship between the modules as the interface conditions,and the simulation software of the composite reservoir high-energy gas fracturing was compiled accordingly.The simulation software consists of a user interface management module,a basic data input module and a system model calculation module.The iterative calculation of time step and pressure,temperature,and fracture width and height is performed through the iterative calculation program in Visual Basic language to simulate the fracture penetration and expansion process of the high-energy gas fracturing complex reservoir.(3)The expansion distances of high-energy gas fractures are higher than those of hydraulic fractures under a certain interlayer ground stress difference.When the interlayer ground stress difference is small(Δσ≤7 MPa),both high-energy gas fractures and hydraulic fractures cross the interlayer interface into the mudstone layer;when the interlayer ground stress difference is large(Δσ≥8 MPa),high-energy gas fractures cross the interlayer interface into the mudstone layer,while hydraulic fractures expand along the interlayer interface.With the increase of the interlayer ground stress difference,high-energy gas fracturing fracture expansion distance decreases exponentially and hydraulic fracturing fracture expansion distance first decreases and then remains unchanged.(4)High-energy gas fracturing fractures extend at higher distances than hydraulic fracturing fractures at a certain elastic modulus of the mudstone formation.When the elastic modulus of the mudstone layer is small(E≤10GPa),the high-energy gas fractures enter the mudstone layer through the interlayer interface,while the hydraulic fractures extend along the interlayer interface;when the elastic modulus of the mudstone layer is large(E≥20 GPa),both high-energy gas fractures and hydraulic fractures enter the mudstone layer through the interlayer interface.As the elastic modulus of the mudstone layer increases,the expansion distance of high-energy gas fractures increases linearly and the expansion distance of hydraulic fractures increases logarithmically.(5)At different Poisson ratios of mudstone layers,both high-energy gas fractures and hydraulic fractures entered the mudstone layer through the interlayer interface,and at certain Poisson ratios of mudstone layers,both high-energy gas fractures extended longer than hydraulic fractures.As the Poisson ratio of the mudstone layer increases,the expansion distance of high-energy gas fracturing fractures increases linearly and the expansion distance of hydraulic fracturing fractures increases linearly.(6)At different pressurization rates,the high-energy gas fractures all cross the interlayer interface into the mudstone layer;when the injection rate is small(q≤1 m~3/min),the hydraulic fractures have not yet reached the interlayer interface,and as the injection rate increases,the hydraulic fractures all cross the interlayer interface into the mudstone layer.With the increase in the rate of pressure,high-energy gas fracturing fracture expansion distance is a second polynomial law increases and then decreases;with the increase in the rate of liquid injection,hydraulic fracturing fracture expansion distance is linear law increases.(7)Based on the study of the control factors of high-energy gas fracturing in composite reservoirs,combined with the advantages of the high-energy gas fracturing technique to achieve fracture penetration and the transformative effect of the guide hole with cutting trough on the local ground stress of the reservoir,the method of directional injection hole high-energy gas fracturing based on the guide hole with cutting trough circular guide is proposed.The method takes advantage of the modification of the stress distribution of the reservoir ground with slotted guide holes and the technical advantages of high-energy gas fracturing to make the high-energy gas fracturing fracture and the expansion of the penetration seam according to the pre-determined or engineering requirements through the reasonable steps of injection holes,distribution holes,slotting and fracturing,so as to achieve the purpose of directional permeability of the coal system reservoir.The method extends the fracture through the layer farther than conventional directional injection hole high energy gas fracturing.