In situ stress of coal reservoir in southwestern Qinshui Basin, China, and its influences on hydraulic fracturing

In situ stress (ISS) is a key factor influencing the hydraulic fracturing (HF) of coalbed methane (CBM) wells. The study of coal reservoir stress and its impact on HF can provide a theoretical basis for CBM development and optimizing fracture design. Based on HF test results of the No. 3 coal reservoir in the southwestern Qinshui Basin, China, theoretical analysis and statistical modeling methods were used to analyze the ISS and its influence on the HF of CBM wells. A model to represent the relationship between the HF of CBM wells and ISS and its stress state is established; a method of statistical modeling was proposed, and the control mechanism of ISS on HF of a coal reservoir is revealed. It is shown that the coal seam in the southwest of Qinshui Basin is deeply buried and the ISS is relatively high; the ISS and pressure of the No. 3 coal reservoir ascend linearly with increase in depth. On the plane, the lateral pressure coefficient (λ) descends from the south to the middle and north of the study area; the λ value of the Zhengzhuang block is high, that of the Mabi block is relatively low, and that of the Anze block is between them. The fracture pressure (Pf) of the coal reservoir is positively related to minimum horizontal principal stress (σhmin) and coal tensile strength and negatively related to maximum horizontal principal stress (σhmax) and coal reservoir pressure. Additionally, the hydraulic fracturing of a CBM well is controlled by the ISS of the coal reservoir and its stress state. Fracturing in coal seams with low σhmin and/or low tensile strength, where existing fracturing is low, and where fracturing fractures more easily develop. Under the stress state dominated by vertical stress (σv), Pf of the coal reservoir ascends with a rise in λ value. Under the stress state dominated by horizontal stress, the Pf of coal reservoir descends with the ascent in λ value. When the value of λ tends toward 1, that is, the Pf reaches the highest value under hydrostatic pressure, HF is unlikely to form. Fracture propagation and fracturing effect depend on the relative magnitude of σv, σhmax, and σhmin, and the propagation direction is perpendicular to the direction of σhmin. These results illustrate effective methods for improving the HF effect of CBM reservoirs and the production of CBM wells.