In-situ stress and fracture pressure of coal reservoir in Shizhuangnan Block and their coupling relations

In order to study the relations between in-situ stress and fracture pressure, 112 vertical coalbed methane wells were selected as the research object in Shizhuangnan Block. The in-situ stress and fracture pressure were obtained by hydraulic fracturing calculation method. The correlation models between the fracture pressure and the horizontal principal stress and effective stress were established respectively. The characteristics of the in-situ stress and fracture pressure of the No.3 coal reservoir were revealed. The relationships between in-situ stress and fracture pressure were analyzed. The results show that the No.3 coal reservoir in Shizhangnan Block was a medium-high stress area as a whole and the stress field types were converted in the vertical direction. The stress field in the buried depth within 400-640 m area was a reverse fault stress field, the stress field in the buried depth within 640-810 m area was a strike-slip fault stress field and the stress field in the buried depth deeper than 810 m area was a normal fault stress field. The side pressure coefficient was 0.38-1.99. With a burial depth lower than 600 m, most side pressure coefficient was greater than 1; within 600-800 m, the side pressure coefficient was 0.52-1.93, and within the depth deeper than 800 m was less than 1. The fracture pressure was 12.89-36.10 MPa and its gradient was 1.47-6.09 MPa/hm. The fracture pressure and the buried depth showed a reversed S-shaped change. It had a large discreteness with a burial depth lower than 810 m, which was negatively correlated with the buried depth as a whole. The fracture pressure was positively correlated with the buried depth deeper than 810 m. To some extent, the maximum horizontal principal stress, minimum horizontal principal stress and their stress gradient were positively correlated to the fracture pressure of coal reservoir, but the correlation was not too strong. Under the same buried depth, the fracture pressure varied with the horizontal stress, but the influence of the minimum horizontal principal stress on the fracture pressure was obviously greater than that of the maximum horizontal principal stress. The fracture pressure and the horizontal stress deviation showed a reversed N-shaped change, and it had positive correlation with effective stress. The fracture pressure decreased with the increase of the stress deviation when the horizontal stress deviation was 2-4 MPa. The fracture pressure increases with the increase of the stress deviation when the horizontal stress deviation was 4-8 MPa. The fracture pressure decreases with the increase of the stress deviation when the stress difference was higher than 8 MPa.