| Coalbed methane(CBM)drainage of broken soft and low permeable coal seam from ground surface has always been a technical bottleneck that prevents CBM industrialization and coal mine gas disaster prevention and control work for a long time.It is also a worldwide technical problem.Technical breakthrough for CBM drainage of broken soft and low permeable coal seam from ground surface is a major scientific and technological proposition.The paper is based on CBM drainage demonstration project under the condition of coal seams in Huainan and Huaibei mining areas of the National Science and Technology Major Project in 12th five-year-plan.The mechanism of fracture propagation and feasibility of CBM mode of multi-stage hydraulic fracturing and horizontal wells in roof strata of broken soft and low permeable coal seam,high efficiency hydraulic fracturing and optimization of process parameters were studied by the means of theoretical research,analysis and test,physical simulation analysis,numerical simulation and engineering application,applying the theories and methods of coal geology,petroleum and natural gas geology,seepage mechanics,fracture mechanics,oil and gas reservoir development and reservoir simulation.,and apply the research results to CBM horizontal well drainage demonstration project of broken soft and low permeable coal seam in Luling coal mine of Huaibei coal field.The conclusions and results of the paper are as follows:(1)The engineering and geological model is constructed,which shows mechanical properties of roof-coal-floor,stress characteristics and horizontal well position.It is prepared for the research on fracture propagation law of multi-stage hydraulic fracturing horizontal well in roof of broken soft and low permeable coal seam.(2)The multiple physical simulation tests for characterizing directional fracturing horizontal well in broken soft and low permeable coal seam roof is designed and carried out under confining pressure condition.It is revealed that fracture initiated from horizontal well in coal seam roof can cross-strata and grow through artificial coal seam under some conditions such as reasonable placement of horizontal well in coal seam roof and a certain pumping rate.The simulation experiment research provides a laboratory demonstration for practicability of the technology of multi-stage hydraulic fracturing and horizontal wells in roof strata of broken soft and low permeable coal seam.(3)Hydraulic fractures propagation rules of horizontal well in in roof strata of broken soft and low permeable coal seam under the condition of downward directional perforation has been revealed,and the corresponding fracture propagation model is established.Hydraulic fracturing in adjacent to the coal seam roof strata,the hydraulic fractures not only can grow from the high stress value of roof strata towards and throughout lower stress value of broken soft and low permeability coal seam,and also the length of fractures formed in underlying broken soft and low permeability coal seam are longer than that of directly fractured in broken soft and low permeability coal seam due to fractures propagation in the direction of length in relatively friable roof strata produce tear effect on broken soft and low permeability coal seam at the same time of fractures propagation.In a given geological and reservoir condition and under the condition of fracturing parameters the former length is 6.7 times as long as the latter.Obviously,hydraulic fracturing initiated in broken and soft low permeability coal seam roof strata where horizontal well is placed,and thus the strengthen the result of production of broken and soft low permeability coal seam is greatly increased.(4)Under the condition of pumping rate of 8 m3/min,the principle of a smaller effect on subsequent fracturing by earlier fracturing according to induced stress by fracturing and the range of steering angle of five degrees of maximum horizontal principal stress range,Determine the optimal order fracturing fracture spacing is 100 m and optimal spacing of alternately fracturing is 70 m.(5)Applying the research results to CBM horizontal well drainage demonstration project of broken soft and low permeable coal seam in Luling coal mine of Huaibei coal field,the feasibility and efficiency of drainage mode was verified.The length of horizontal section of the well is 586m;the fracturing stages were 7;cumulative fracturing fluid was 6627 m;adding sand 541 m3 and sand scale was 77 m3 per fracturing interval;the highest CBM production rate was 10760m3/d;cumulative CMB production was 4.01 million m~3.The engineering test has created a new record of adding sand amount and CBM production of the horizontal well in the broken soft and low permeability coal seam of our country,and has made a major breakthrough of CBM drainage technology of broken soft coal seamThe distribution of broken soft and low permeability coal seams in late Paleozoic in China is widely distributed,which are mostly high gas outburst coal mine,The research results of this paper can be used for the CBM drainage in this kind of coal seam,in particular,which can be used as a reference and guidance for the rapid coal gas pre-pumping in the heading of coal seam strip of outburst coal seams,regional gas outburst control and gas disaster prevention.The drainage model has a good prospect of application. |
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