Study Of Coal-bed Methane Occurrence Regularity At Taiyuan Group Of Northeastern Basin Based On Gas Pressure Build-up Curves

After analyzed the geological occurrence background of coalbed methane(CBM)in Qinshui Basin,we researched the CBM enrichment status at the basin level,according to the macroscopic characteristics of CBM occurrence and distribution in the basin.Combined with all aspects,we will do some research on the enrichment mechanism of CBM in slope.The CBM occurrence parameters was the basic of mastering the coal seam gas emission law,the productivity prediction,the hazard controlling and the clean utilization,so the CBM pressure and the permeability coefficient seepage parameters exactly determination had become essential foundation work in the coal seam disaster prevention and the resource mining.The application technology of the pressure build-up curves was based on the oilfield groundwater dynamics,and the method had become a common and essential method in the oil and gas field development.The oil well analysis and deduction method applied to the gas well was consistent in the guidance production safety.When a set of analytical methods and inferences of oil wells applied to CBM wells,the basic principle is consistent.Based on this we find a method about CBM pressure recovery curve used for determining the seepage parameters of CBM in coal mine.In this paper,we measured seepage parameters of CBM in pressure-measuring borehole for coal mines by the pressure recovery curve measurement method.It is aimed at CBM occurrence data about coal reservoir permeability,production capacity and other characteristics.The results show that the method for determining seepage parameters of CBM is suitable for the testing of CBM occurrence parameter.The determination of seepage parameters by pressure recovery curve of CBM is consistent with that measured by conventional method.However,the pressure recovery curve method measures the seepage parameters of CBM quickly and automatically it also shortens the determination time to 1/4.As a new measurement technology in underground coal mine,it proved that the well testing theory can be used under the coal mine,and the CBM reservoir pressure and permeability can be accurately measured.We researched the occurrence law of CBM No.15 in the northeastern part of Qinshui Basin by combining with the new method for determining the seepage parameters of CBM,and also tested a large number of coal seam samples about ab value,pore crack rate and coal reservoir parameters(coal seam thickness,industrial analysis of coal,etc.).Besides 62 test holes were constructed under the coal mine to determine the CBM seepage parameters for No.15 coal seam of 31 coal mines by using pressure recovery curve method.At last summarized the geological distribution curve of seepage parameter in the research area,the conclusions of research is consistent with the measured permeability of the gas wells in the northeastern part of the Qinshui Basin.The gas permeability of coal sample has a negative exponential function relationship with the effective stress,and also has a negative exponential function relationship with the buried depth of the coal seam.With the increase of effective stress and buried depth,the permeability of CBM reservoir decreases according to the law of negative exponential function.The permeability of the system has a semi-logarithmic linear relationship with the effective stress and the burial depth,indicating that there is same between the influence mechanisms.It is the compressive action of the gravity of the overlying stratum on the fissure.The in-situ stress increases with the increase of depth,the influence of coal seam burial depth on permeability is the influence of in-situ stress on permeability.While the coal reservoir pressure increases linearly with the burial depth of the coal seam,the permeability of the coal reservoir decreases rapidly according to the exponential function law.The influence of reservoir pressure on the permeability is to affect the permeability of the coal reservoir as part of the effective stress and the effect is much less than the control of the penetration depth.The theoretical evaluation of CBM mining in the research area is carried out through the occurrence regularity of CBM and the preservation conditions,such as burial depth,hydrogeology and geological structure.The comprehensive evaluation of CBM development in the study area was carried out by using multi-level analytical method and fuzzy evaluation method,combined with the factor of CBM seepage parameters and the geological structure.The basic geological factors of the evaluation of CBM development mainly refer to the geological conditions of coal reservoirs,the physical properties of coal reservoirs,and the conditions for CBM mining.The comprehensive evaluation of CBM development in the study area and the prediction of favorable blocks are realized by ?multi-level fuzzy comprehensive evaluation?.The evaluation results are divided into the No.15 CBM in the northeast of Qinshui Basin into four categories.The first category in the study area is a favorable block,and the comprehensive evaluation coefficient ?0.65.Most of them are distributed in the middle and deep mining areas of Xiyang,Pingding city and Yangquan mines.The second category is a medium-favorable block with a comprehensive evaluation coefficient between 0.55 and 0.65.The shallower mines in Xiyang-Pingding and the deep mining mines in the left area are distributed in this category.The third category is unfavorable blocks,and the comprehensive evaluation coefficient is between 0.45 and 0.55.Most of the mines are in Shouyang,Jinzhong city,and some of the left mining areas also belong to this category.The fourth is mainly for mines with a higher elevation in the No.15 coal seam in the study area and the category <0.45.The coal seam has a shallow depth and generally located at the edge of the Qinshui Basin,and the coal seam has the outcrop in the mine or a methane weathering zone.In the evolution process of CBM,the thickness of the caprock is relatively thin,and the generated CBM is dissipated by the tectonic action,these areas are not suitable for CBM development.According to the above research rules,the qualitative and quantitative results of the difficulty of CBM development in the study area are obtained,and the optimal area of No.15 CBM exploitation in the northeast of Qinshui Basin is proposed.