Characteristics of in-situ stress field of coalbed methane reservoir in the eastern margin of Ordos basin

Abstract

The eastern margin of the Ordos Basin in China holds substantial coalbed methane resources. Nevertheless, coalbed methane production in this region is significantly constrained due to the unclear characteristics of the in-situ stress field within the coal reservoirs. Drawing on extensive coal seam test data, including well testing and hydraulic fracturing, and uses hydraulic fracturing calculation methods, this study conducts a comparative analysis of the in-situ stress characteristics across various CBM blocks in the region. Results indicate that the in-situ stress of the coal reservoirs along the eastern margin of the Ordos Basin predominantly exhibits medium-high stress, with stress levels higher in the south and lower in the north. The stress field is dominated by the inverse fault stress field (${\sigma }_H>{\sigma }_V>{\sigma }_h$) in the north while in the south, it is characterized by a normal fault stress field (${\sigma }_V>{\sigma }_H>{\sigma }_h$). The in-situ stress characteristics of the coal seams are primarily governed by burial depth and local structural factors, with the three principal stresses showing a strong linear positive correlation with burial depth. The vertical stress field distinctly exhibits a “three-zoned” distribution. At a depth of 750 m, the in-situ stress relationship is ${\sigma }_H>{\sigma }_V>{\sigma }_h$, indicating that the coal reservoir is predominantly under compressive stress. At depths between 750 and 1500 m, the coal seam stress state follows the relationship ${\sigma }_H\approx {\sigma }_V>{\sigma }_h$, transitioning into a stress regime characteristic of a transition zone. At a depth of 1500 m, the in-situ stress relationship is ${\sigma }_V>{\sigma }_H>{\sigma }_h$, and the coal reservoir is predominantly characterized by tensile stress. Despite the relatively deep burial depth of the coal seams in the Hancheng block, its location within the Tongchuan-Hancheng fault zone, where north-south faulting is significantly developed, causes substantial local structural effects. As a result, the coal seam stress field in this area remains dominated by reverse faulting.