Microstructure characteristics of Daji area in east Ordos Basin and its control over the high yield dessert of CBM

There are abundant deep coalbed methane (CBM) resources over 2 000 m deep in China,but the overall exploration and development level is low,and there is no clear understanding of development laws and systematic development geological theory.Based on the comparative analysis of 20 geological parameters between the deep coal seam with a buried depth of more than 2 000 m and the middle deep coal seam with a buried depth of 1 000-1 500 m in Daji area,the detailed description and classification of microstructural areas on monoclinic structure of deep coal seam are carried out.From the study of production characteristics,logging,fracturing parameters and fracturing curve characteristics of deep CBM wells,it is found that the difference of coal seam microstructure has obvious control effect on the high yield of deep CBM wells.From the phenomenon,the occurrence mechanism,development mechanism and main control factors of high yield of deep CBM are analyzed in depth,and the ideal production curve of deep CBM is predicted.Finally,the similarities and differences between deep and shallow CBM drainage curves are put forward.Research shows that ① the deep coal seam shows the advantages of “high gas content and high saturation” and the characteristics of “good coal structure and ultra-low permeability”.Deep CBM wells show the characteristics of “short time before gas breakthrough,high gas pressure at gas breakthrough,and little liquid production after gas breakthrough”.② Five types of micro structural areas on the slope structure are finely depicted.Deep CBM high-yield wells are mainly distributed in the positive microstructure and gentle zone of coal seam under good roof sealing,with relatively good permeability,easy sand addition,relatively low fracturing pressure and good stable production capacity.A stable gas production is significantly positively correlated with construction displacement and total sand addition.On the premise of medium high sand adding scale,the daily stable production of vertical wells in positive microstructure is 5 000 m3/d,and that of wells in gentle structure is 3 500 m3/d.The wells located in other parts are tested with different technologies,construction displacement,acid amount and sand adding scale,and the stable production level is 1 500 m3/d.③ Different from shallow CBM development mechanism “drainage induced desorption”,deep CBM development is “gas production induced desorption”.At a certain depth,when high temperature plays a dominant role in desorption,part of the desorbed CBM will exist in the coal seam as “free state”,and the “free gas” is the main form in the initial stage of development,in which the production characteristics are similar to shale gas.④ It is considered that the compactness of deep coal seam is the most critical factor to restrict the exploitation of deep resources.Deep coal seam has the advantages of primary structure coal and the disadvantages of ultra-low permeability.Under the condition of deep high pressure,the microstructure has “amplification” effect on the permeability of coal seam,or improves or has great influence.Therefore,in addition to the limited range of fracturing,the positive or gentle microstructure areas with relatively good permeability and more developed natural fractures should be targeted,which can not only help the fracturing effect,but also contribute to high production.⑤ Due to the low proportion of anticline structure,it is suggested that the fine microstructure research of deep coal seam be considered as an important direction to achieve a breakthrough in the exploration and development of deep CBM,and the positive and gentle structural parts can be regarded as the preferred “geological engineering” double dessert location.To tackle the key problems of fracturing technology in the negative microstructure and horizontal compression position will be a difficult problem that must be solved,which is related to whether the deep CBM can achieve a larger strategic development in the future.This study provides a strong theoretical basis for the exploration and development direction of deep CBM,the deep CBM desserts evaluation,the engineering and technical countermeasures and the reasonable production system establishment of deep CBM wells.