Research On Gas Extraction Parameters And Application Of Directional Long Boreholes In Mining-induced Overburden Fractures

Coal mining breaks the original state of occurrence of coal and rock,resulting in deformation and failure of overlying strata and fracture evolution.A large amount of gas existing in coal seam and rock mass is released.Under the action of air leakage and concentration,the pressure-relief gas flows and converges in the mining-induced fracture network,forming the gas accumulation in goaf and the gas overrun in working face.This not only brings serious potential safety hazards to the production of the working face,but also has become an urgent problem to be solved to realize safe and efficient production in coal mine.In recent years,it has become a trend that adopting horizontal directional long boreholes to extract the pressure-relief gas,which has the advantages of cost effectiveness,long extraction time,and high construction efficiency.In this paper,the layout layer and extraction and layout parameters of directional long boreholes were studied by means of numerical simulation calculation,physical simulation test,theoretical analysis and field test.The main research results obtained in this paper are as follows:1.The distribution characteristics of horizontal“four areas”of mining-induced fractures were put forward,and the optimum area for directional long borehole layout was theoretically defined.Based on the evolution and distribution characteristics of mining-induced fractures,the fractures in the fractured zone was divided into horizontal“four areas”,which were initial fissured area,tensile fractured area,structural fractured area(SFA),and compaction fractured area.The SFA was the optimum area of the borehole layout based on the analysis of mining-induced fractures distribution and borehole stability,and spatial location boundaries of the SFA in overlying strata were theoretical defined.2.It was proposed that gas accumulation rate,fracture permeability rate and borehole stability rate were the three factors affecting the borehole layout,and then the layout layer and its priority of the borehole in the SFA was determined.Through nine-grid blocks division of the SFA,the above three factor values of each block were obtained,respectively,and then the layout suitability rate of the borehole in each block was obtained.Based on 10%and 20%,the SFA was divided into three small areas(areaⅠ-Ⅲ).Thus,the borehole layout layer and its priority were determined.3.The mathematical models of gas extraction by directional long boreholes were established,and the distribution characteristics of gas flow and the interaction law of boreholes extraction were discussed.Based on the complex variable function and mirror image principle,the mathematical models of gas extraction by single row parallel boreholes arrangement,double row parallel boreholes arrangement(alignment and staggered arrangement)and non-linear boreholes arrangement were established.The analysis of the models shows that the layout layer and spacing of boreholes have a great impact on boreholes extraction efficiency.Taking three or four boreholes can obtain better boreholes extraction efficiency and boreholes equivalent radius when non-linear boreholes arrangement mode is adopted.There is little difference between the two borehole arrangement modes of double row parallel arrangement in extraction volume and borehole shielding effect.However,due to the large shielding coefficient of double row parallel arrangement of boreholes(0.7379),when a single row of boreholes cannot meet the extraction demand,double row boreholes arrangement can be used to strengthen extraction.4.The extraction and layout parameters of directional long boreholes were determined,and the extraction scheme was put forward.The borehole extraction efficiency was positively proportional to the four factors of borehole diameter,borehole extraction pressure,fracture permeability and borehole effective extraction length.Considering the economy and borehole extraction capacity,the extraction parameters of the borehole were determined.The confluence velocity of gas to the borehole was proposed as the definition index of the influence radius of directional long boreholes extraction.Combined with the mathematical model of boreholes gas extraction,the borehole layout parameters such as boreholes layout spacing,boreholes layout quantity and mode were determined.Based on the determination of boreholes layout layer and boreholes extraction and layout parameters,four extraction schemes of three and six boreholes layout were put forward.5.Numerical simulation was used to analyze and verify the directional long borehole layout layer and extraction scheme,and the best extraction scheme was determined.The numerical simulation results show that the borehole in the SFA should be arranged in the following order:outer-middle-inner block in horizontal direction,and upper-middle-lower block in vertical direction,which is consistent with the previous analysis of the borehole layout sequence of each block in the SFA.The arrangement of three boreholes has remarkable extraction effect,which can realize gas control in goaf and working face.Among them,the non-linear arrangement scheme has better extraction effect.The borehole shall be arranged in area I,followed by area II in the SFA,so as to obtain better extraction effect.The extraction capacity of six boreholes double row staggered arrangement is stronger than that of alignment arrangement.On the premise of a certain number of boreholes,the extraction capacity of boreholes increases slowly with the increase of gas emission in goaf,and with the increase of the number of boreholes,the single borehole extraction efficiency decreases gradually.6.The gas extraction of working face 16031 was carried out by using non-linear arrangement scheme of three boreholes.According to the theoretical formula,the boundaries of the SFA were defined,the layout layer of boreholes was determined.The gas extraction was carried out through the non-linear arrangement scheme of three boreholes.The application results show that the corresponding boundary of the SFA determined from boreholes extraction data is consistent with the theoretical formula.During the extraction period,the borehole stability is good and the extraction capacity of the borehole is strong.The pure amount of gas extraction of a single borehole is up to5.52m~3/min(borehole 3#),and the total average pure amount of gas extraction of the three boreholes is 3.45m~3/min.The gas concentration in the upper corner of the working face is less than 1%and in the return airway is in the range of 0.18～0.46%.The directional long borehole has effectively extracted the pressure-relief gas in the goaf.It verifies the practicability and rationality of borehole layout layer and borehole extraction and layout parameters.The paper contains 114 figures,25 tables and 144 references.