| Xin’an Coalmine, a mine in the western Henan Province, is one of the high-gas bursts mine. However, the underground drainage technique is hard to succeed in extracting gas within the reservoir, both because the depth of mining increases and quality of excavation expands. And it is urgent to research new drainage method, together with new outline, in order to promote gas extraction rate and efficiency. Accordingly, combined with CBM geology, simulation theory and surface drainage experience, this thesis screened out the most optimum development program for deep tectonic coal within mining area, based on analysis of permeability, adsorbability, gas content and reservoir pressure, theory of CBM diffusion, migration and production, and also the mechanism of differences between all the cases designed. The main targets and relative recognitions concluded in this research are as below:1) Determines reservoir parameters, on the base of exploration drilling wells data, lab test results, investigating reports, and earlier relative literatures:a) thickness:in Xin’an mine, floor of coalbed fluctuates dramatically within different areas, as a result of the deposition process and tectonic movements; the minimum thickness of the coal bench is Om, while the maximum is14.2m; at the Southeast of11and13panel, the thin bench band, thinner than2m, developed prevalently, and on the other hand, the thicker parts of the coal seam distribute nearby12and14panels, in which the coalbed developed stably with small thin coal bench area, and thickness ranges from2m and4m; b) burial depth:depth of the coal bench ranges from96.58m to644.61m, with the average of340.24m and it increases from the north to south generally; c) coal rank:as a result of the burial depth and reservoir temperature, coal rank (Ro) runs ranging from1.96%to2.35%, with the average of2.16%, belonging to the interval between PM and SM. The Ro tends to increase from Northwest to Southeast, and abnormal values emerge in the panel area of11,12and14in the mid-west of the mine, in which coal can be heated during tectonic movements and the Ro could reach even2.30%; d) roof lithology:at the Southeast, Middle and some East of the mine, the roof strata are mudstone or carbon mudstone, which could seal the gas effectively and thus contribute to the high gas content area in part of12panel, majority of13and14panels; e) hydrodynamics:the high salinity value of Ⅱ1coalbed appears at the Middle and North of the mine, which illustrates that the underground water flows from the Northwest to Northeast, however, the area with high salinity value is corresponding with low gas content, which proves that the hydrodynamic trapping is in low effective; f) reservoir pressure:with an average of2.86Mpa, the reservoir pressure ranges from0.47Mpa to7.33Mpa, and the high value center locates in the12&14panel; g) Langmuir parameters:The range of Langmuir volume is from12.03m3/t to39.05m3/t (averaging in22.24m3/t), whereas the range of Langmuir pressure is from0.46Mpa to1.04Mpa (average0.66Mpa). In addition, the critical desorption pressure tends to be slightly low in the whole block, ranging from0.2Mpa to0.4Mpa. It is recognized that drainage of the water is required to decrease the pressure leading to the gas desorption during the gas production period, and because of the low critical desorption pressure, it is difficult to exploit the gas, e.g. the panel14, with the maximum of critical desorption pressure (0.47Mpa), the desorption will not happen only until the reservoir pressure (4.7Mpa) decreasing by the critical desorption pressure (0.47Mpa), which is, of course, time and money consuming, h) gas content: The gas content in the study block ranges from0m3/t to14m3/t, and it increases with the increasing of the depth of the coal, i.e. on the occasion when it is shallower than200m, the gas content is generally lower than4.0m3/t, while the gas content reaches as high as14m3/t with the burial depth deeper than600m; i) gas saturation:The regulations of gas saturation distribution in the whole block is more or less the same as that of gas content. The maximum of gas saturation emerges in No.14panel (46.4%), while the minimum appears in No.13(32%), which indicates that the14panel is with the greater production potential.j) permeability:The permeability of the whole block appears to be slightly low, and it is defined as0.5mD by integrating the estimated results of contrasting analysis, the test interpretation results of adjacent area and the lab calculating results.2) probes into causes of high gas outbursts and difficult gas extraction:burial depth tends to be relevant deeper in the Southwest; the gas content increases with increasing burial depth starting from the crops of Ⅱ1coal seam; specific surface tests indicate that in the reservoir there are both open and enclosed type of pores, with the dominant enclosed and intergrading type, which provide extensive spaces for gas adsorption; the cleats were impaired, to some different extent, due to the impact of tectonic movement, and some pores were filled up by the minerals, which both contribute to making the Southwest panel as the severe gas outburst area; meanwhile, the disadvantageous reservoir properties including high reservoir pressure, low critical desorption pressure, low gas saturation, poor permeability dramatically limit the gas extraction effect and the implement of surface wells drainage before exploitation.3) demonstrates the mechanism of how mining affecting the gas exploitation, combined with the typical CBM transporting theory with the impaction of the mining on the reservoir:the mining decreases the adsorption pressure which could undermine the adsorption balance and promote the gas desorption; furthermore, the mining could produce fractures as the gas transporting tunnels. Therefore, gas extraction at the mining areas could be an effective method to reduce and even eliminate the gas outburst.4) optimizes well pattern based on the simulation results:plum-blossom pattern with a well spacing of250m and fractured half length of50m is optimized well pattern for gas drainage. The simulation results revealed that the well pattern with well spacing of200m*250m could depress fast and the production could maintain plateau with long time. The vertical well with half fracture length of50m could achieve high cumulative production but with low-difficulty process;5) inquiries mechanization of the production differences among variable mining area drainage schemes:pressure depletion rate and drainage area are affected by coal permeability. The permeability of a coal bed methane reservoir will be changed by two effects which are effective stress and matrix shrinkage. The methane desorption rate and volume will be affected by effective stress and matrix shrinkage will affect on gas production rate thus the production differences which among variable drainage schemes are generated by the above two effects;6) proposes optimum gas drainage technique for tectonic coal according to the formation property:a) it’s vertical well not horizontal well is recommended for the mellow coalbed and its roof, and deviation control drilling tools be equipped on the bit; b) to better formation permeability and production, completion with perforation is strongly suggested; c) to extract gas naturally desorbed while excavating, slotted casing should be run into the hole; d) to control coal fines, process-depressure and screw pump aree also recommended;... |
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