Research on primary parameters of high pressure water jet during rock cross-cut coal uncovering

According to the problems of gas drainage in rock cross-cut coal uncovering on high gassy coal seam with low gas permeability coal seam, a novel technology which uses high pressure water jet to slot coal seam is presented to raise the methane drainage radius and permeability. Based on the distribution of water jet velocity, the relationship of impact force of water jet and characters of coal is analyzed. Besides, the velocity of coal partial basing on the relationship of force of partial and dip of drilling is studied. Then, the effective diameters of slotting are calculated. Finally, the novel technology is applied to the Pingdingshan mine. The results show that quantities is decreased 38.2%,gas drainage rate is increased 2.86 times and the period of gas drainage is reduced 57.1% and the area of the gas drainage is widened 2.05 times compared to original technology.     

Experimental analysis of matrix shrinkage’s influence on mechanical permeability of gas filled coal

Based on the coal seam of Zhao zhuang coal mine and Si he coal mine belong to Jin cheng anthracite Mining Group in Shanxi province, by using the self developed three axial servo controlled seepage equipment for hot fluid solid coupling of coal containing methane, parallel experiments for permeability of helium and methane under different gas pressures are carried out to quantitatively study the influence of Klinkenberg effects on the permeability of coal. Meanwhile, the problem of permeability decreasing caused by fluid pressure decreasing and gas desorption is eliminated by controlling the effective stress. The results show thatthe Klinkenberg effect is obvious under low gas pressure. With the increase of gas pressure, the permeability of coal decreases at first and then increases. The inflection points of gas pressure for Klinkenberg effect are about 0.9 MPa both for coal samples in Zhao zhuang coal mine and Si he coal mine. When the effective stress is constant, with the increase of gas pressure, the permeability increment caused by Klinkenberg effect decreases, while permeability increment caused by coal matrix shrinkage increases. Coal matrix shrinkage has a close relationship with the mechanical properties of coal rock mass. For the coal sample with a poor mechanical property, coal matrix shrinkage is more obvious.     

Analysis on rock cross-cut coal uncovering using high pressure water jet slotting in close distance seam group

Safe production and procedures at the driving face are restricted in mines of Guizhou province by the long periods required for rock cross-cut coal uncovering in high gas coal seam group. A new method is proposed to solve this problem using high pressure water jet slotting in penetration boreholes. This is proposed to increase the gas desorption rate and reduce drilling hole, which will shorten the cycle of rock cross-cut coal uncovering. Numerical simulation is used to compare the pressure relief effects of different slotting methods in penetration boreholes. The results show that the pressure relief effect of slotting all coal seams is more ideal. Field application indicates that high pressure water jet slotting has good effect of the pressure relief in coal seam group. The drilling number is reduced by 30, the drilling length is shortened by 610 m and the standard time of gas pre-pumping decreases by 39 days.     

Indexes optimization of outburst forecast while a rock cross-cut coal is uncovered by pulsed water jet

To obtain the best outburst prediction index of rock cross-cut coal uncovering slotted by pulsed water jet,the effects of the rigidity coefficient’s measured value and the initial velocity of gas diffusion to the integrative indexes D and K are analyzed based on the dynamic evolution of coal crake field under water jet. Thus the integrative indexes D and K are excluded. A grey relation model of outburst prediction indexes(K1,S and P)from 7 coal mine’ rock cross-cut coal uncovering in Chongqing,Sichuan and Henan is set up. And the grey relation degrees of the 3 prediction indexes to outburst hazard are calculated by using grey relational analysis method. The results show that the outburst hazard can be more accurately reflected by the cuttings desorption index K1 when a cross-cut coal uncovering slotted by pulsed water jet,and it should be considered as the best outburst prediction index.     

Experiments on the relationship between permeability of gas bearing coal and coal body structure

Permeability tests of type I to V coal from Jiaozuo coalfield are carried out to study the gas permeability difference of coals with different structures. The experimental results show that the relation curve of permeability to different coal structures show normal distribution, and the value of permeability first increases quickly and then decreases sharply. Type II coal’s permeability is the maximum and that of type V is the minimum. There is little difference of perm abilities of type I, type IV and type V. The concepts of variation coefficient of permeability and permeability ratio are brought in to evaluate the heterogeneity and anisotropy of No.21 coal seam. It found both of them are high, however, the homogeneity of a single type structure coal is good. Mechanisms of gas sub region and banding, gas emission, gas outburst of gas bearing coal are explained by the permeability differences. Permeability changes are the direct result of pore fracture development and the major controlling factor of gas distribution, gas emission and coal and gas outburst.     

Experimental analysis of the evolution law of the coal seam gas permeability under long-term load

Gas-filled coal permeability evolution tests are carried out before and after the long-term load with temperature and effective stress changing by using the triaxial permeability equipment. Based on the experiments,temperature sensitivity coefficient and effective stress sensitivity coefficient of the permeability of the coal sample are analyzed before and after the long-term load. The experimental results are shown as follows：1)Before and after the long-term load,with the increase of effective stress,the permeability and effective stress sensitivity coefficient both decrease when the temperature keeps constant. At the same time,with the increase of temperature,the permeability and temperature sensitivity coefficient also decrease when the effective stress keeps constant;2)The difference of the permeability between before and after long-term load is different with temperature and effective stress changing,and it reflects thermal-fluid-solid coupling effect;3) Before and after the long-term load,the difference of temperature sensitivity coefficient first increases and then decreases with effective stress increasing,and it becomes negative when the effective stress is about 4.3 MPa. What’s more,the difference of effective stress sensitivity coefficient will decrease with the temperature increasing,and become negative after 26 ℃.     

The effect of original moisture content in coal beds on coal and gas outburst risk level

By using self-made “Coal Containing Gas Thermal-Hydrological-Mechanical (THM) Coupling Experimental Equipment”, an experimental study is carried out to investigate the gas seepage law of containing-gas coal during the complete stress-strain process with different original water contents. Under the conditions of constant gas pressure and confining pressure, and the experimental results are compared with the field test of the gas emission variation of coal seams before and after being infused. The research results show that the triaxial compressive strength and the elastic ratio of coal decrease with the increase of original water content in coal, but the axial strain of coal increases and the absolute value of transverse strain and volume strain of coal increases at the triaxial compressive strength; the methane effective permeability of coal decreases throughout the complete stress-strain process. Outburst prevention by coal seam infusion attributed is analyzed from two aspects: the mechanics of coal and the gas flow in the coal seams. The higher original water content in coal seams is, the smaller the risk of coal and gas outburst becomes. So the original water content in coal seam can be treated as a significant criterion for the judgments of coal and gas outburst criticality.     

Experiments on triaxial mechanical properties of soft coal containing gas

Taking briquette specimens composed by soft coal with outbursting trends as research object, systematic research of the influence on triaxial mechanical properties of soft coal containing gas caused by confining pressure and gas pressure is done. The results show that confining pressure can significantly affect mechanical properties of soft coal containing gas, the compression phase of soft coal containing gas is obvious and the triaxial strength is increased gradually as confining pressure is increased; the influence on elastic modulus and shape of stress-strain curve is not obvious. The triaxial strength of soft coal containing gas shows decrease trend as gas pressure is increased; the influence on strength of soft coal containing gas caused by gas pressure is obvious when the confining pressure is smaller, but it is not obvious when the confining pressure is bigger. the influence caused by gas pressure on elastic modulus and deformation of yield step is not obvious; gas pressure has not only obvious mechanical effects,but also have chemical action on soft coal containing gas; stress-strain curve of soft coal containing gas can be expressed by conic, the influence caused by gas pressure can be considered as equivalent confining pressure effects.     

Elasto-plastic analysis on surrounding rock of roadways based on seepage, strain softening and dilatancy

According to seepage body force, strain softening and rupture expansion of rock mass, elastic-plastic mechanics theory, distribution laws of stress and displacement of surrounding rock in seepage field are derived, and relationships between different zonation range of the surrounding rocks, pore water pressure, strain softening and rupture expansion are established. The results show that the pore water pressure and rupture expansion rock mass have a more obvious impact on the surrounding rock rupture area than the plastic zone. Both the plastic zone and the rupture area are larger when the seepage field is considered. The impact of strain softening on both the plastic zone and the rupture zone of surrounding rock is very significant, and both seepage and strain softening rupture expansion affect deformation of surrounding rock of roadways obviously. It provides certain reference values for the roadway support engineering in seepage field.     

Experimental analysis of coal bed methane desorption and seepage under sonic vibrating of cavitation water jets

A way to enhance coal bed methane desorption and seepage by sonic vibrating of cavitataion water jets is proposed because the gas coal seams have the intrinsic properties of micro-porosity, low permeability and high adsorption. The mechanisms of how the sonic vibrating effects promote methane desorption and seepage are analyzed, and the contrast experiments of methane desorption and seepage under sonic vibrating effects are also provided. The results show: when the cavitation number is 0.020 0, under sonic vibrating effects, the volume of coal bed methane desorption increases by 36.9%, and the time for coal bed methane desorption reduces by 19.6%. The speed of coal seams methane seepage is significantly increased by 35.3% and arrives at 0.383 3 ml/s.     

An analysis on the energy of coal and gas outburst process

Based on the hypothesis of the synthetic coal and gas outburst operating, we study the work done by elastic energy of coal mass and methane expansion energy in the coal and gas outburst process through theory and experiment. By using MTS815 rock mechanics test system, the triaxial compression test of the coal sample is carried out under different confining pressures and the power function relation between coal bodys elastic energy per unit volume and body stress is worked out. It shows there is a linear relation between methane expansion energy and methane pressure obtained by testing methane expansion energy under different moisture contents, different methane pressures and different compactness of coal mass, with the help of self-developed methane expansion energy testing device. The temperature change in the system is about 1 ℃ when methane does work in expansion. The result also shows that it could be regard as an isothermal process during coal and gas outburst. The results from experiment do great help in researching the mechanism of coal and gas outburst and establishing energy criterion.     

Mechanical control effect of hard tectonic structure on coal seam gas flow

To analyze the controlling effect of hard tectonic structure to gas flow at the coal seam heading face, two physical mechanical models are established, normal coal seam driving and hard body embedding coal seam driving. Then according to the above two numerical models, it numerically analyzes the stress distribution characteristics, the coal seam permeability change and the gas flow rule at the coal seam heading face by using rock fracture finite element software RFPA 2D-GAS with solid-gas coupling analysis module, and carries on comprehensive comparative analysis. Finally, combining with two ordinary cases from product field, it verifies the conclusion from the numerical simulation that the concentrated stress influences the coal seam permeability and controls the gas flow. The results show that: the concentrated stress at the coal heading face with hard tectonic structure is like a “closed gate”, as it makes the coal seam permeability obviously lower and blocks the normal migration of gas in the vicinity to the freedom space. In order to eliminate the gas abnormal emission threat at the high gassy coal seam heading face, stress and gas relief measures must be taken to rip the “closed gate” and effectively remove the concentrated stress to provide safe releasing channels for the blocked gas.     

Test and numerical simulation on effect of enhanced coalbed methane drawing by injecting gas under coal mine

In order to improve the extraction effect of ultra-low permeability coalbed methane, injecting and drawing industry test of enhanced coalbed methane recovery is carried out by injecting concentration 78% N2 under 850 m depth coal mine of Tianfu Mining Co. Ltd.,in Chongqing. The experiments of free methane emission, vacuum pumping, side injecting and side emission, side injecting and side drawing and intermittent injecting are tested under the condition of single hole and multi holes respectively. The testing results indicate that time-concentration curve of methane drawing is a single peak wave curve which increases and decreases during injecting N2 after reaching the limits of ordinary extraction. The methane concentration of intermittent injecting gas is greater than that of side injecting and side drawing. However, the former declines rapidly and has a shorter migration distance than the latter. The injecting effect of long time side injecting and side drawing is better than that of intermittent injecting. When hole space is 4 m, permeability 1.112×10-4mD, the methane recovery ratio increases to 58.6％ from conventional recovery ratio 52.4% after injecting 5 day. Meanwhile, the seepage flow equation of injecting air is provided and the parameters, such as rational hole mesh space, time and pressure of injecting gas, are simulated. At the same time, application condition and assistant technology about injecting gas are discussed, which provides engineering and theoretical reference to promote the injecting gas technology. Also, the problem of variable definite conditions seepage flow equation is put forward.     

Analysis on AE characteristic and its confining pressure effect of outburst coal containing gas

Briquette samples made of outburst coal are taken as study objects to study AE characteristic and its confining pressure effect by using self-developed equipment, Shimazu servo test machine and AE monitor system, and the characteristic of AE parameters are analyzed, too. The results show that in the process of outburst coal containing gas triaxial compression, there is no serious dense or sparse area of AE events, but three relative dense appear in compression phase, middle area of linear elastic stage and near peaking phase area so that the characteristic of AE events look like three peaks. But the amplitude of AE events is small and the good consistency between energy change and amplitude change is showed. Microstructure change caused by external stress field change, fracture and fragmentation of particle of samples, movement besides particle of samples and microstructure change caused by dissipation of gas are the sources of AE events. The number of AE events is reduced as the confining pressure is increased, but the influence on total number of AE events caused by confining pressure show nonlinear, expressed by quadratic function. The number of larger amplitude of AE events show decreasing tendency as the confining pressure is increased, the relation between numbers of AE events whose amplitude is more than certain value and confining pressure is expressed by quadratic function.     

饱和渗透系数空间变异性对边坡稳定性的影响

研究了降雨雨型、雨强和持时对边坡孔隙水压力分布的影响以及降雨入渗条件下饱和渗透系数的空间变异性对孔隙水压力、含水率、抗剪强度参数、局部安全系数等参数空间分布的影响。研究结果表明,降雨特性对边坡孔隙水压力的影响较大;受渗流主方向影响,降雨条件下饱和渗透系数的水平波动尺度对孔隙水压力及含水率的变异性影响小于竖向波动尺度,降雨致滑坡的临界滑面基本与坡面平行且深度较浅;随着饱和渗透系数水平波动尺度的增加,滑坡深度随之增大,从总体上坡体上部局部安全系数随饱和渗透系数竖向波动尺度的增加而减小。     

Experimental study on adaptability of well pattern using sandstone plate model with ultra-low permeability

Because the porous flow state is difficult to be simulated by one-dimensional cores and two-dimensional sand filled models,this paper proposes a method of making large-scale natural outcrop flat model and defines two parameters to evaluate adaptability of different well pattern on the ultra-low permeability sandstone flat model. With natural sandstone plate model,production of different well patterns in homogeneous oil reservoirs is simulated. A concept of pressure sweep efficiency and deliverability index for researches on adaptability of well patterns are put forward. Through experiments,plane pressure gradient field of different well patterns under different drawdown pressures are measured,and then seepage areas of the models are divided into immobilized seepage area,nonlinear seepage area,and quasi-linear seepage area,with the help of small core nonlinear seepage curve. Experiments show that adaptability of inverted nine-spot equilateral well pattern is better than those of inverted nine-spot rhombus well pattern and rectangular well pattern. With the increase of drawdown pressure,deliverability index of all well patterns increases,immobilized seepage area reduces,and quasi-linear seepage area expands. At the same time,nonlinear seepage area of inverted nine-spot equilateral well pattern reduces,while those of inverted nine-spot rhombus well pattern and rectangular well pattern fluctuate as expanding first and then reducing.     

Experiments analysis on the influence induced by parameters of nature gas huff and puff in foamy oil reservoir

Foamy oil phenomenon of heavy oil reservoirs gradually disappears in the latter period of solution gas drive, which causes the decrease of oil development. Based on the formation of secondary foamy oil by gas injection, foamy oil unconventional experiments and pressure depletion tests are conducted to reveal the characteristics of the foamy oil and analyze the applicability of various types of foamy oil reservoir simulation models. Evaluation of improving oil recovery by nature gas huff and puff is conducted and the influence of injection and production parameters is studied. The study shows that foamy oil exists quasi bubble point which decreases with the decrease of time. The foamy oil compression coefficient is in the range of 10～0.012 MPa -1, which is higher than that of the conventional crude oil. Foamy oil model of six components is the most suitable model for the foamy oil reservoir. There is optimal gas injection and soak time for nature gas huff and puff. Increasing the gas injection speed, gas injection pressure and fluid production rate will improve the performance of nature gas huff and puff. Increasing the speed that dissolved and injected gas change into disperse gas is very important during the solution gas drive and gas injection process.