Background Deep coalbed methane (CBM) reservoirs are characterized by well-developed microstructures and natural fractures, along with highly heterogeneous rock mechanical properties and in-situ stress field distribution. The CBM production of the reservoirs tends to cause constantly evolving formation pressure and in-situ stress within the control ranges of CBM wells, further enhancing the complexity of stress distribution. Additionally, severe fracturing channeling can be observed between new and old wells. This phenomenon is primarily triggered by in-situ stress variations induced by large-scale fracturing and CBM production. Therefore, accurately assessing the dynamic evolutionary patterns of in-situ stresses in reservoirs after fracturing and during CBM production holds great significance for new well placement, fracturing design optimization, and anti-channeling during fracturing.
Methods This study investigated the Daning-Jixian block in the Ordos Basin as an example. Based on the geological characteristics of the study area, this study established a model for predicting the evolution of four-dimensional in-situ stresses in deep CBM reservoirs during long-term CBM production. Specifically, models for initial three-dimensional geomechanical characteristics, post-fracturing stress fields, and production-induced stress fields were established first, collectively enabling the dynamic, fine-scale characterization of stress fields from the initial stage to long-term CBEM production. The accuracy of the proposed four-dimensional model was verified using data from field microseismic and production monitoring.
Results and Conclusions In the target well block, the difference between the maximum and minimum principal stresses decreased after fracturing and then gradually increased as CBM production proceeded. Over 24 months of CBM production, the minimum horizontal in situ stress in the vicinity of the well group decreased by 2.10−5.75 MPa, accompanied by significant variation in the in-situ stress direction (average: 4.12°). A greater stress drop corresponded to higher connectivity of the fracture network, a larger range of formation energy supply, and greater production potential. The simulation results indicate that the optimal reservoir stimulation performance can be obtained under angles between the horizontal well trajectory direction and the maximum principal stress direction measuring from 70° to 110°, combined with a well spacing of 400 m. This helps maximize the estimated ultimate recovery (EUR). The construction approaches for zipper fracturing are determined. For zipper fracturing of two adjacent wells in zones with imbalanced stresses, the well in the low-stress zone should be fractured first. After the post-fracturing stress balance, the well in the high-stress zone should be fractured. This approach facilitates the adequate coalescence of the fracture network. For zipper fracturing in three wells, for instance, in zones with similar stress levels, two lateral wells should be fractured first to allow the stress differences on both sides of the middle well to vary consistently, contributing to stress balance. Then, the fracturing parameters of the middle well should be optimized to achieve adequate coalescence of the fracture network. Additionally, a method for fine-scale division of fracturing stages and perforation clusters is proposed based on stress magnitudes, providing robust technical support for the fracturing and development of deep CBM wells.
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