Abstract:
The deep coalbed methane (CBM) resources in the Ordos Basin are abundant and have great development potential, making it the main battlefield for exploration and development of deep CBM in China. Among them, clarifying the enrichment law of deep CBM and delineating enrichment sections are important foundation for CBM exploration and development. Based on the analysis of data from a number of deep evaluation CBM wells recentlyimplemented by China petroleum Changqing Oilfield company in Ordos Basin, the geological control factors and enrichment rules of deep coalbed methane differential enrichment in the basin are studied by means of mathematical statistics, model calculation and geological analysis. It has been proposed that the effects of material composition, depth, and preservation are the "three controlling effects" that affect the differential enrichment of deep CBM.The material component is the main factor controlling the enrichment of CBM, among which ash yield and coal metamorphism degree are the main parameters, low ash content, high metamorphic coal with high gas content. The depth effect is a comprehensive effect of superimposed porosity, temperature, and pressure. The porosity decays with the increase of burial depth, resulting in the free gas will tend to slow down with the increase of burial depth, and the adsorbed gas exists obvious critical conversion depth, and the critical conversion depth of total CBM content is deeper than the critical conversion depth of adsorbed CBM, which is generally in the range of
2000~
3000 m, and in this depth range, the total CBM content is higher. For depths deeper than
3000 m, the depth effect intensifies, and the total CBM content gradually decreases, often below 20 m
3/t. The preservation effect is an important controlling factor for the enrichment of deep adsorbed CBM and free CBM, especially free CBM, among which the type of cap rock is a key factor in the preservation effect, while groundwater total dissolved solids and pressure coefficients are important indications of the preservation effect.The coal-rock combination of coal-mud、coal-ash and coal-ash mud in the study area has good capping property, corresponding to its high groundwater salinity. Within the buried depth of
2000~
3000 m, the total gas content is often higher than 20 m
3/t, and the coal-sand combination capping property is relatively weak, corresponding to its relatively low groundwater salinity, gas content is often lower than 20 m
3/t. At last, a deep CBM enrichment model in the Ordos Basin was proposed by coupling depth effect and preservation effect.