Influence Of Hydraulic Fracturing On Reservoir Characteristics Of High-rank Coal And Its Productivity Of Coalbed Methane Wells

As an important part of unconventional natural gas,the development and utilization of coalbed methane is not only conducive to coal mine gas disaster,but also can reduce greenhouse gas emissions and enhance energy efficiency.China has abundant coal and coalbed methane resources.Therefore,obtaining the characteristics of coalbed methane reservoir and its development and utilization conditions is an indispensable means of increasing production of coalbed methane.Based on Fanzhuang mining area of southern Qinshui Basin,this paper systematically analyzed the influence of hydraulic fracturing on the characteristics and productivity of high rank coal reservoirs in Fanzhuang mining area by using the methods of statistical analysis,experimental research,theoretical analysis and numerical simulation,studied the hydraulic fracturing characteristics and permeability variation law of coal body,and analyzed hydraulic fracturing experinent and nuclear magnetic resonance experiment.Through adsorption-desorption experiments,the change rule of pore permeability in coal body before and after fracturing is obtained.The controlling effect of different fracturing fluids on desorption of high rank coal during fracturing is further discussed.The relationship between the characteristics of coal reservoir and productivity in Fanzhuang block is revealed.Based on the analysis above,the evaluation method of reservoir productivity is established,which can provide a reasonable theoretical basis for improving the productivity of CBM wells.The main achievements and understandings are as follows:(1)By studying the basic physical data of Fanzhuang 3#coal reservoir in southern Qinshui Basin,combining with field sampling and laboratory microscopic observation,the development characteristics of pore and fissure and its evolution law of coal reservoir in the study area are analyzed,and the reasons for poor permeability of high rank coal reservoir are revealed,that is,exogenous fissures are not developed,endogenous fissures are developed,but most of them are filled with calcite,and micro-fissures are formed.The permeability decreases exponentially with the increase of burial depth,the pressure of coal reservoir increases with the increase of burial depth,and further reveals the distribution law and controlling factors of gas-bearing property.The theoretical desorption rate of coal is over 80%,which shows that the high rank coal in the study area has better physical properties.(2)Through hydraulic fracturing experiments,the physical characteristics of coal reservoirs are obtained.During hydraulic fracturing,the degree of coal and rock damage and the change of permeability are mainly divided into three stages:1.The initial compaction stage of coal and rock,the permeability changes slowly or even declines;2.The permeability increases slowly during the expansion stage of rock;3.During the coal and rock fracture stage,the permeability increases rapidly.The overall permeability of coal reservoir varies negatively exponentially with the change of effective stress;fracturing has obvious improvement on permeability,the lower the initial permeability of coal and rock,the better the effect after fracturing;during the process of coal and rock fracturing,clay mineral particles and pulverized coal particles gradually infiltrate into coal and rock micro-pore with long time pressure diffusion,which strengthens clay expansion and micro-pore in cleats.The smaller the micro-fissures,the longer the holding time,the greater the irreversible wetting expansion degree of particles,and the lower the permeability of coal and rock macroscopically.(3)Based on nuclear magnetic resonance(NMR)experiments,the variation law of coal pore before and after fracturing is studied.After fracturing,the changes of porosity of different coal samples are different,but overall increase.The pore size of macropore,mesopore and micropore in coal and rock is increasing.After decompression,the pore size of micropore is easy to restore to the pre-fracturing state,while before and after macropore fracturing,the increase of porosity is larger,and it is difficult to restore after decompression.Experiments show that proppant should be added in time to prevent pore and fracture closure due to pressure relief in the actual fracturing process.(4)The adsorption-desorption experiments of coal show that the larger the coal particle,the more cracks with larger opening,which is beneficial to the desorption and transport of gas,and the larger the amount of methane desorption.The smaller the coal particle is,the larger the specific surface area of the coal sample is,and the easier it is to form a strong interaction with the fracturing fluid with high concentration.Water-insoluble solid particles in fracturing fluid intrude into micro-cracks and matrix pore in coal,which can easily cause pore blockage and lead to low methane desorption.(5)Reservoir control factors affecting CBM well productivity are divided into four categories:reservoir material basis,difficulty of CBM desorption,CBM migration and production capacity and reservoir reformability.Reservoir evaluation methods are established to distribute weights among control factors,and to re-predict high CBM production areas in Fanzhuang block.Hydraulic fracturing is obtained by numerical simulation method.Controlling effect of fracturing fracture length and fracturing fluid type on productivity of CBM wells.