Stress state of deep seam and its influence on development performance of CBM wells in South Yanchuan Block, Odors Basin

The buried depth of the coal seams of South Yanchuan block is between 700 m and 1500 m. And for the most area, the depth is deeper than 900 m. In order to clarify the in-situ stress state of deep coal-bed methane reservoir and its influence on the development of CBM wells, the maximum and minimum horizontal principal stress and the vertical stress are calculated based on the pressure drop data of hydraulic fracturing. The results show that the whole area is in the state of tension. The maximum horizontal principal stress, the minimum horizontal principal stress and the vertical stress are positively correlated with the buried depth. The type of the ground stress is mainly type Ia, followed by Ⅲ type and Ⅱ type. The reservoir with type Ia ground stress is dominated by vertical fractures with the lowest fracture index. However, the stress sensitivity is weak, and the stress sensitivity coefficient is only 0.479. The main and secondary fractures of type Ⅰ stress reservoir are 16μm and 11μm respectively, with the largest width and the strongest reservoir stress sensitivity. Type Ⅰ fractures are mainly horizontal fractures, with the highest fracture index, the longest compression fracture, the best reconstruction effect and the highest gas production. Reservoirs with type Ⅲ in-situ stress develop complex fractures, with medium permeability, stress sensitivity and hydraulic fracture length. However, the production for type Ⅲ is relatively low, and cannot achieve economic development.