Sweet spot prediction and technology application of coalbed methane development in Tiaohu-Malang depression of Santanghu Basin

Unconventional oil and gas resources are characterized by low abundance and strong heterogeneity. Effective sweet spot prediction and mining technology are important for their efficient development. At present, the exploration and development of coalbed methane in the Santanghu Basin is still in the initial stage. Many problems in coalbed methane sweet spot prediction and mining technology seriously restrict the efficient development of coalbed methane resources. In order to study the most suitable evaluation indexes and methods for sweet spot prediction of coalbed methane development in Tiaohu-Malang Depression of Santanghu Basin, various technical means such as high-pressure mercury injection experiment, liquid nitrogen adsorption, and nuclear magnetic resonance are jointly used to analyze the occurrence state and mining technical conditions of coalbed methane in this area, establish a coalbed methane resource evaluation system, and accurately predict the sweet spot area of coalbed methane development. The results show that the main buried depth of the No.9 coal seam of Xishanyao Formation in Tiaohu-Malang Depression is 600–2600 m, and the thickness of coal seam is 5.0–60.9 m. The proportion of pore size volume of No.9 coal seam in the T1 well is mainly large pores and micropores, the proportion of mesopores is slightly smaller, and the specific surface area is opposite. The open pores of coal samples are more developed and the connectivity is better. The smaller pore size section of the coal sample is larger. When the P/P₀ value of the coal sample is about 0.5, there are sudden drop points and more ink bottle-shaped pores are developed. The lithology of the roof and floor is mainly mudstone, and the occurrence condition of coalbed methane is superior. Based on the geological conditions of coalbed methane in this area and the experience of coalbed methane exploration and development at home and abroad, 14 index evaluation indexes such as coal seam thickness, burial depth, lithology of coal seam roof and floor, structure, and so on are optimized and optimized. The sweet spot area of coalbed methane (type I area) is predicted to be mainly distributed in the northeast of Tiaohu Depression and the west of Malang Depression. Taking T1-5 well as an example, the five-stage dual-pressure control method is adopted in the drainage system. The bottom well flow pressure drop is controlled to be ≤ 0.02 MPa/d in the pressure holding and production release stages, and the bottom well flow pressure drop is ≤ 0.01 MPa/d in the stable production stage. The above methods and technologies have achieved initial results in the development of well T1-5, with the highest gas production of 2297 m³/d and the average gas production of 1300 m³/d, showing good gas production effect, which has important guiding significance for the subsequent efficient development of low-rank coalbed methane in Santanghu Basin.