| V CBM(Coal Bed Methane) development needs reservoir renovation is decided by the characteristics of low permeability of coal reservoirs China.Hydraulic fracturing is one of the major reservoir remolding method at present,because of its wider source,cheaper price,convenient construction and the better transformation effect.Renovation area is small and side effect is small when uses vertical wells of hydraulic fracturing the point-line-plane-body comprehensive volume reconstruction can be realized,fracturing effect is extended and fracture diverting capacity is greatly improved when uses horizontal well staged fracturing technology.With the coal bed methane development theory,rock mass mechanics and seepage theory,this paper optimizes the technological parameter of the CBM horizontal well staged fracturing.The main research contents include:(1)According to the theory of CBM geology,with the application of regression analysis and structure curvature method,shows of CBM enrichment characteristics and permeability distribution rule of the study area.Basing on the data of CBM and coal mine exploration and development in Hebi No.6 coal mine,with the help of coal bed methane geology,rock mechanics and the seepage theory,the influence factors of gas content is analyzed systematically.The results show that the root cause of the high CBM content depends on the coal metamorphism.Under the metamorphism of coal,the content of CBM determined by the original coal forming material types, to be called the rock mechanics characteristics. With the regression analysis method to obtain the relationship between the gas content and the burial depth of coal seam,the gas content distribution in study area:the lower bound to within 750 m depth of coal seam gas weathered zone,the gradient of CBM content is 3.42 m3 /100 m.t;the depth of range 750~1100 m,the gradient of CBM content is 1.42 m3 /100 m.t,the increase of deep CBM content is small,shallow coal seam gas content is relatively large;With the analysis of low temperature nitrogen adsorption test and mercury injection analysis,Hebi coal reservoir pore specific surface area and V total is larger;pressure mercury porosity is higher,suction hole dominated of pore structure,accounted for about 78% of total void content.According to the constructional curvature method,permeability is predicted in the study area.Hebi mining area coal permeability in 0.03~0.05 m D.After the formation,coal seam in damage by later structural,making reservoir permeability become lower,permeability is generally low.The TWO1 coal,in the vertical direction,can be divided into three natural stratification.The Native structure of coal and coal particles,0.5~1.0 m of thickness, mylonite at the top;more primary structure coal and fragmentation coal in the central;0.5 m particles coal developed at the bottom and distribution is not stable.The destruction of Hebi six mine coal body structure is more serious,??? are primary in the whole block,there mainly exist local area with ?coal in southwest of the block of Hebi Six mine.Based on the isothermal adsorption experiment,at the beginning of coal sample methane in the Hebi coal mine, desorption rate is high,attenuation fast, desorption fully,residual stock less,80% gas from coal sample can be desorbed, adsorption time only hours.The time of desorption is short,can quickly reach peak of production forecast.(2) The relationship among stress,strain and permeability of different coal structure is obtained through the experiment.The factor of fracture propagation form of hydraulic fracturing horizontal well is analyzed systematically,and the horizontal section hydraulic fracturing fracture morphology under different conditions is obtained.The results show that,when the difference of Young modulus between by fractured layer and upper and lower layers,the Young's modulus is one of the most important factors that control the longitudinal crack propagation;when Young's modulus of fractured layer is more than 5 times smaller than the upper and lower layer,the fracture height may be limited to the fracturing layer,forming "T" type joint;Crustal stress difference is the main controlling factors of fracturing fracture form single or multiple cracks,the bigger the crustal stress difference,cracks along the direction of maximum principal stress,not easy to form cracks,the smaller the crustal stress difference,the crack extension direction is random,likely to extend in all directions,fracture morphology mainly radial mesh expansion;proper increase in the scale of construction,improve the construction displacement,sand ratio,which means increase influence sphere of fracturing,fracture length and flow conductivity,improved the fracturing effect,to benefit achieving stable production,high yield of coal bed gas well.The relationship among stress,strain and permeability of different coal structure is obtained through the experiment,the result is that the relationship between permeability and effective stress accords with negative exponential equation in the process of type?and?coal loading.Type ? and IV coal shows the strain softening properties after acoustic emission peak mainly,axial stress reduces slowly,but raw coal stress drops,and axial pressure changes with the instantaneity or rapid characteristics of coal and gas outburst are identical with each other.For type?coal,internal pore fissure are not developmental,because almost not affected by large tectonic movement,the internal structure of coal is relatively complete,internal pore fissure connectivity is poor.At the stage of densification,coal-rock overall deformation mainly in particle migration;in the stage of elastic deformation,coal-rock overall deformation mainly in particle deformation;in the stage of plastic deformation,Coal-rock overall deformation mainly in Particle damage;failure stage after the peak,coal-rock overall damage.This stage is that the whole macro fracture of coal-rock,pore fissure among cracks are connectively, and macroscopic fracture increases gradually as the stress load.After completely destroyed,as the load for further,because of the squeeze between coal-rock particles,pore fissure began to close,permeability decreases.For type II coal,influenced by tectonic activity,pore fissure development in the internal coal,connectivity is better among pore fissures at the same time,the scale of Pore fissure size overall larger.In the stage of densification, coal-rock overall deformation mainly in particle migration.In the stage of elastic deformation,since the particle migration towards ordering and stabilization,migration difficulty gradually increase.In the stage of plastic deformation,because of accumulated a large amount of elastic energy in early deformation stage,as the elastic energy gradually achieve the coal-rock intensity,the area of particle damage become more and more large,new cracks generated in large quantities,permeability increase gradually.After the peak failure,as the stress continues to increase,when the coal itself is difficult to resist the deformation,the whole failure of coal-rock along the fracture zone.After the complete coal-rock destruction,with further loading,because of compaction,lead pore fissure among coal-rock particles to close,permeability decreases.For type ? and IV coal, the cracking and extending of crack in the coal and rock deformation process is crack evolution between particles. Because coal particles are broken, there are stress distribution differences in the process of loading, making particles migration from the small stress area to the large. As the particle migration, the new crack will be formed. Because of poor connectivity between the particles, the regional gap will be filled with other regional grain fast, and the fracture closes again after stress loading and particles migration. As the fluid flow in the process of hydraulic fracturing, extrusion particles form crack, but fracture closes again as the fluid flow back, so type ?, IV coal area is forbidden area of the conventional hydraulic fracturing and development.(3) According to the theory of seepage and rock mechanics theory, the mathematical model of fracturing fracture morphology and productivity prediction model are constructed, the perforating parameters, sand fracturing fluid volume and ratio of pump injection parameters are optimized.According to the theory of seepage and rock mechanics theory, the mathematical model of fracturing fracture morphology and productivity prediction model are built, the perforating parameters, sand fracturing fluid volume and ratio of pump injection parameters are optimized.According to the reservoir parameters and productivity prediction model established, gas production of HB02-H2 Wells group is forecasted, Calculating the area pressure effective influence radius is 20.4m, effective desorption radius is15.5m, and calculate the largest gas production in theory of each fracturing section calculation, the results show that gas production of section 1 is 930000m3, section 2 is 873000m3, section 3 is 806000m3, section 4 is 1132000m3, section 5 is 1058000m3, the accumulative total volume is 4799000m3.According to the effective desorption radius is 15.5m and each section affect length is about 31 m calculation, the cumulative affect length of 5 sections is 155 m, which is far less than coal seam length(583m). The results show that influence scope of U well drainage mining is limited, and fracturing is divided into five sections, leading to accumulative effective influence radius of pressure is less than coal seam length in the region and eventually accumulated effective desorption radius is lesser also. The influence radius of pressure and effective desorption radius increases by increasing the fracture section number, and pressure interference between fracturing section is formed easily, which can improve the gas production rate of coalbed methane wells greatly.Because the influence length of HB01-H1 well less than coal seam section, 11 period of staged fracturing is chosen when design HB02-H2 Wells, and gas production of HB02-H2 Wells group is forecasted according to the reservoir parameters and productivity prediction model established. And the fracturing effect is simulated using the Mayer numerical simulation software for, and shows the half seam length is 58.27 m.Calculating the largest gas production in theory of each fracturing section calculation, the results show that gas production of section 1 is 930100m3, section 2 is 873100m3, section 3 is 860100m3, section 4 is 1132100m3, section 5 is 1057600m3, section 6 is 974200m3, section 7 is 916900m3, section 8 is 927400m3, section 9 is 959000m3, section 10 is 853200m3, section 11 is 858900m3, the accumulative total volume is 10288600m3.Comparing the fracturing effect of HB01-H1 well which fractures 5 sections with HB02-H2 well which fractures 11 sections, the accumulative gas production of HB02-H2 well productivity prediction is 10.2886 million m3, which is more than HB01-H1 w ell of the accumulative gas production 4.799 million m3 by 2.1 times, the actual highest daily production of HB02-H2 well is 3580 m3, which is more than HB01-H1 well of t he highest daily production 1340 m3 by 2.6 times, the fracturing effect had significantl y improved. |
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