Establishment Of Gas Emission Model For Deepâ…¡₁Coal In Xin’an Mine Based On Reservoir Properties

After nearly30years’ mining, seam in shallower areas(belonging to the first level), within Xin’an Coalmine, which is of high gas/coal-outburst mine for the principle coalseam-Ⅱ1Coal is tectonic coal, has been mined out. During mining down, gas emission increases dramatically, while the high heterogeneity in reservoir properties makes it more difficult to design new panel reasonably. However, available geology reports lack comprehensive analysis or researches on factors that control gas emission. Accordingly, it is more important to characterize and predict gas emission in deeper areas (belonging to the second level) on the base of reservoir heterogeneity for reasonable design.This thesis studied the Ⅱ1Coal in Xin’an mine, clarified relationship between gas-geology parameters(i.e. coal seam depth (D), seam thickness (T),20m-sandstone ratio (20m-SR),40m-sandstone ratio (40m-SR), ash content (A), moisture content (M), total sulfur (S) and tectonic indicator (TI) et al.) and reservoir parameters (including gas pressure (GP), ⅤL, gas content (GC) and fracture in seam), based on available drilling reports, field observation results and in-situ test data; constructed numerical model for gas emission prediction, combined with theories both Numerical Theory I and gas geology; and concluded how reservoir properties influence gas emission. The steps are as belows:1)screen out control factors for qualitative and quantitative research, including depth, thickness, floor lithology, ash content, moisture content, total sulfur, coal rank, maceral groups and tectonic indicator et al.;2) characterize each parameter for further analysis:observe fracture in coal under mine and microscope, measure fracture length and width, research micropores type and there connectivity, describe reservoir isothermal constants, permeability, gas content, specific surface and gas pressure by experiment and in-situ test;3) divide recovered area into smaller blocks, distribute every parameter selected to small grid, compare and screen the main control factors and establish multivariate function on gas emission and its influential factors;4) divide un-mined area into smaller blocks, distribute every parameter selected in step3to small grid, predict gas emission combined with function established in step3and factors characterized in step4. After analysis about reservoir properties influence on gas emission, it’s concluded that:1) coal in this area has been strongly destroyed so the Protodyakonov’s coefficient of coal in studied area is too low, between0.17to0.22, indicating low permeability of reservoir because coal fine is easy to block fracture which is the migration pathway;2) specific surface test results depict that there are both open and blind hole, while mainly blind hole; and observations under microscope confirm that micropore, which methane is desorbed on, is well developed;3) The values of isotherm constant a, b are a little bit large, which indicates coal has strong adsorption capacity. The desorption gas volume is low when the gas pressure declines from5MPa to1MPa, which depicts that gas cannot release from coal easily. When the gas pressure is lower than IMPa, the gas desorption rate ascends dramatically, and the coal and gas emission is more likely to happen, thus this pressure interval becomes the pretty significant part of gas prediction.4) The average permeability of target seam is0.42md. Coal in15061face and12201face individually have highest and lowest permeability which is0.57md and0.29md respectively.5) there is negative relationship between specific surface and fault displacement, coalseam depth and tectonic indicator, while positive relationship with coal rank (Ro), vitrinite content (VC), ash content (A), moisture content (M) and inertinite content (IC); significance test result indicate that the T-statistic of VC, D, M and TI is greater than the critical value (2.660) at0.01level; that of Ro, A, IC and fault displacement is greater than the critical value (1.671) at0.1level; This indicates that those parameters including VC, D, M and TI have significant influence on specific area, while Ro, A, IC and fault displacement have little effect on specific area. Specific area in the whole mine is lower in the middle part, while the higher value locates at13and16mining district, in which the specific value is greater than0.8m2/g, and the lower value lies in12district, where the specific value is0.4m2/g.6) The isothermal adsorption constant is proportional to the Ro, specific surface, vitrinite content, throw of the fault, burial depth, inertinite content and the tectonic complexity, while it is inversely proportional to ash content as well as water content, significance test result indicates that the T-statistic of D is greater than the critical value (2.660) at0.01level; that of VC and M is greater than the critical value (1.671) at0.1level; The T-statistics test illustrates that the burial depth, vitrinite content and water content are the3main independent variables that have profound effect to isothermal adsorption content(VL), whilst the influence of other6independent variables including Ro, specific surface, ash content, fault throw, inertinite content and the complexity of fault is quite inferior. In this paper, it has been predicted that the value of VL diffuses variably in the whole field with high heterogeneity and the maximum value might be35m3/t, while the minimum is only23m3/t. The value of VL in most area of the11panel could be as high as32m3/t, whereas in other panels, the VL is not constant.7) The gas content is proportional to coalbed thickness, gas pressure, vitrinite content, burial depth, isothermal adsorption constant(VL), water content and inertinite content, while the relationship between the gas content with the20m sandstone ratio,40m sandstone ratio, specific surface, the throw of fault, ash content, Ro, Dazhan sandstone thickness, the tectonic complexity is inversely proportional, significance test result indicate that the T-statistic of GP, specific surface, and Dazhan Sandstone (DS) is greater than the critical value (2.660) at0.01level; that of20m-SR, Ro, A, D, M and VL is greater than the critical value (1.671) at0.1level; that of40m-SR, fault displacement, T, A, D, IC and TI is greater than the critical value (1.671) at0.1level; The T-statistics test indicates that the8main factors (gas pressure, specific surface, the thickness of Dazhan sandstone,20m sandstone ratio, Ro, vitrinite content, water content, VL) are more likely to affect gas content than other7independent variables(40m sand stone ratio, fault throw, coalbed thickness, ash content, burial depth, inertinite content and tectonic complexity).Gas content in the whole field changes dramatically, ranging from3m3/t to16m3/t. From the northwest to the southeast of the field, gas content increase gradually, with a high gradient. Gas content is high in the southwest of the unmined area, while relatively low in northeast as well as partial area in the middle, with gas content less than10m3/t.8) Gas pressure is proportional to the coalbed thickness, specific surface, gas content, burial depth, Dazhan sandstone thickness and tectonic complexity, while is inversely proportional to the throw of faults, significance test result indicate that the T-statistic of D, DS, and specific surface is greater than the critical value (2.660) at0.01level; that of T, GC and TI is greater than the critical value (1.671) at0.1level; that of fault displacement is greater than the critical value (1.671) at0.1level; The T-statistics test demonstrates that the coalbed thickness, specific surface, burial depth, Dazhan sandstone thickness and tectonic complexity influence the gas pressure to some great extent, while the effect of fault throw on the gas pressure is not that apparent.9) The gas emission is proportional to gas content as well as gas pressure, which is inverse with the relationship between the gas emission and specific surface and VL. significance test result indicate that the T-statistic of GP and GC is greater than the critical value (2.660) at0.01level; that of VL is greater than the critical value (1.671) at0.1level; The T-statistics test shows that the gas pressure, gas content and specific surface have large effect on gas emission while the effect of VL is small. Gas emission shows an upward trend from northeast to southwest, and distributes heterogeneously in some areas. Gas emission tends to be quite higher in12panel,14panel,16panel than in others.10) Gas emission is primarily controlled by gas pressure which is dominantly influenced by burial depth and Dazhan sandstone thickness. Therefore, coalbed burial depth is the most critical factor which controls the gas emission. Due to the low level of tectonic degree, the gas was preserved quite well in the coalbed, and the fragile coal contributes to the lack of gas flowing path, In this study area, Dazhan sandstone prefer to make the gas pressure increase rather than make the gas escape.There is an unconspicuous negative relationship between gas emission and specific surface&Langmuir volume which indicates gas pressure and gas content weaken the effect of specific surface and Langmuir volume on gas emission. The above analysis provides evidence from the other side that the characters of gas pressure and gas content should be the two key parameter for gas prevention in non-mining areas. In order to give effective gas prevention rules in non-mining areas, gas pressure analysis for non-mining areas should be put on the first place. At present, there are only minor structures found in extracting face in known areas, however it does not mean there is no fault in unknown areas. Detecting minor faults, investigating the effect of faults on gas pressure and predicting gas emission based on relationship between faults and gas pressure should be the first job for further gas emission prevention.