Study On Composite Permeability Increment Technology By Acid Fracturing In Low Permeability Coal Seam

Gas drainage is the main measure to prevent the gas disaster in coal mine,and the coal seam permeability directly determines the effect of gas drainage.Because of China's coalbed generally poor permeability,and with the depth of coal mining gradually increased,mining conditions is more complicated,with characteristics of high stress,high gas,high heterogeneity,low permeability.Some of the pores in coal seams are blocked by a large amount of minerals.These characteristics lead to a small range of gas drainage,a large amount of drilling,gas extraction rate is low.Conventional gas extraction method is difficult to play a role.Increasing the permeability of coal seam has become the key to improve the efficiency of coal seam gas drainage.In this paper,a new method is proposed to increase permeability by acid fracturing in coal seam.The theoretical analysis,experimental test,numerical simulation,industrial test and other methods are used to study the mechanism and the effect of enhancing the permeability by acid fracturing.By using SEM,mercury porosimetry,XRD,CT scanning etc.experiment equipments,the coal pore structure characteristics,composition and content of minerals and mineral distribution characteristics were tested.The pore size and pore volume of coal are regionally distributed,and the micro pores of coal are the most developed.The secondary minerals such as calcite and dolomite are mainly filled in the fissures and the larger pores.The theory of elastic mechanics,damage mechanics and chemical kinetics was used to reveal the mechanism of acid fracturing.The main role of fracturing is to make coal seam cracks,the main role of acidification is to dissolve the minerals in the coal seam,the two functions promote each other,and further increase the connectivity of the network of coal fracture porosity.Based on the experiments of nuclear magnetic resonance?NMR?,the displacement effect of CO₂ on CH₄ in coal seam was discussed.According to the application conditions of acid fracturing in coal seam and the requirement of acidizing fluid system,CAFAS acid system was developed.The surface characteristics,coal structure,mineral composition and permeability of coal were tested and compared before and after acidification.The hydraulic fracturing process was simulated by using the self-developed coal and rock frac-seepage experiment system,and the permeability of coal samples before and after hydraulic?acid?fracturing were measured.The permeability of coal samples after acid fracturing is the best,and the bigest permeability increases 28.275times.On the basis of defining the mechanism of coal acid fracturing,the THMC coupling model was constructed according to the law of conservation of mass and energy conservation.Then,the numerical simulation was carried out by using the solid mechanics module and PDE module of COMSOL Mutiphysics.The process and effect of increasing permeability by diffrent schemes were analyzed.The simulation results show that the effect of acid fracturing is the best among hydraulic fracturing and acidizing.Sihe coal mine is the field test base of acidizing fracturing technology.The experiment proves that the technology is feasible.In this paper,a set of theoretical system has been established to enhance the permeability of coal seam by acidizing fracturing,the antireflection effect of acid fracturing in coal seam is verified.The research results provide an important scientific basis for increasing the permeability of coal seam,increasing the gas drainage rate and ensuring the safe mining of coal mine.