Energy Geoscience (Apr 2023)
In-situ stress of coal reservoirs in the Zhengzhuang area of the southern Qinshui Basin and its effects on coalbed methane development
Abstract
In-situ stress is a critical factor influencing the permeability of coal reservoirs and the production capacity of coalbed methane (CBM) wells. Accurate prediction of in-situ stress and investigation of its influence on coal reservoir permeability and production capacity are significant for CBM development. This study investigated the CBM development zone in the Zhengzhuang area of the Qinshui Basin. According to the low mechanical strength of coal reservoirs, this study derived a calculation model of the in-situ stress of coal reservoirs based on the multi-loop hydraulic fracturing method and analyzed the impacts of initial fractures on the calculated results. Moreover, by combining the data such as the in-situ stress, permeability, and drainage and recovery data of CBM wells, this study revealed the spatial distribution patterns of the current in-situ stress of the coal reservoirs and discussed the impacts of the in-situ stress on the permeability and production capacity. The results are as follows. (1) Under given fracturing pressure, longer initial fractures are associated with higher calculated maximum horizontal principal stress values. Therefore, ignoring the effects of the initial fractures will cause the calculated values of the in-situ stress to be less than the actual values. (2) As the burial depth increases, the fracturing pressure, closure pressure, and the maximum and minimum horizontal principal stress of the coal reservoirs in the Zhengzhuang area constantly increase. The average gradients of the maximum and minimum horizontal principal stress are 3.17 MPa/100 m and 2.05 MPa/100 m, respectively. (3) Coal reservoir permeability is significantly controlled by the magnitude and state of the current in-situ stress. The coal reservoir permeability decreases exponentially with an increase in the effective principal stress. Moreover, a low lateral pressure coefficient (less than 1) is associated with minor horizontal compressive effects and high coal reservoir permeability. (4) Under similar conditions, such as resource endowments, CBM well capacity is higher in primary structural coal regions with moderate paleotectonic stress modification, low current in-situ stress, and lateral pressure coefficient of less than 1.
