Geological characteristics and resource potential of deep coalbed methane in Guizhou

Accelerating the exploration and development of deep ( > 1 000 m)coalbed methane (CBM) resources is an important way for the large-scale development of CBM and an important development direction in recent years. Guizhou Province is rich in CBM resources, with a large proportion of deep CBM resources. However, up to now, the geological research and exploration of deep CBM resources have not been carried out. Based on the geological background of CBM in Guizhou, this paper analyzes the geological characteristics and resource potential of deep CBM in Guizhou Province using a large number of CBM well test data. The research results show that the deep CBM resources in Guizhou are mainly distributed in many syncline units of the Liupanshui, North Guizhou and Zhina coalfields. Sedimentary differentiation and structural zoning lead to regional differences in the geological conditions of deep CBM reservoirs. The Liupanshui coalfield is characterized by “obvious overpressure, low temperature and high stress”, the Zhina coalfield is characterized by “overpressure, high temperature and low stress”, and the northern Guizhou coalfield is characterized by “overpressure, high temperature and high stress”. The permeability of deep CBM reservoir is low, and the permeability of 1 000 m deep will be generally lower than 0.1×10⁻¹⁵ m². The CBM content of the coal reservoir is relatively high. The average CBM content of the middle-rank coal reservoir represented by the Liupanshui coalfield is 10.03 m³/t, and the average CBM content of the high-rank coal reservoir represented by the Zhina and Qianbei coalfields is 14.51 m³/t, and the deep supersaturation phenomenon is more obvious at 400 m. Based on the prediction model of free gas and adsorbed gas, considering the change of coal rack, it is predicted that the overall gas content in the deep part will further increase, and the supersaturation phenomenon will be more obvious, and the proportion of free gas will increase. At about 2000 m, the proportion of free gas will generally reach 10%−25%, and the proportion of free gas in medium-rank coal is higher than that in high-rank coal. On this basis, the geological resources of CBM in the key synclines of Guizhou Province with a depth of more than 2000 m are calculated. Based on the calculation results, the geological resources of CBM in Guizhou Province is 3.85 trillion m³, of which the geological resources of CBM in the depth of 1000 m reach 1.90 trillion m³, and the deep CBM resources in the Liupanshui and northern Guizhou coalfields account for a large proportion. Considering the scale and abundance of resources comprehensively, the development sequence is proposed. The Panguan-Tucheng-Zhaozihe-Jiupu'an syncline and the Gemudi syncline are the preferred development units. The above synclines are large gas fields with high abundance.