Determination of Formation Fracture Pressure under High Temperature and High Pressure in Deep Water of the South China Sea

The deepwater strata in the northern South China Sea exhibit a high degree of under-compaction, weak cementation, low pressure-bearing capacity, and other mechanical characteristics. At the same time, due to the existence of a temperature field, the heat exchange between the formation and drilling fluid will produce an additional stress field, which will lead to lost circulation in the process of deepwater drilling and seriously affect the drilling efficiency and cementing quality. Therefore, it is urgent to clarify the influence of the temperature field on the additional stress. On the basis of elasticity, the numerical simulation method was used in this paper to establish an analytical model for periborehole stress with temperature considered. According to the existing formation fracture pressure model, the dynamic change law of formation fracture pressure in the deepwater environment of the South China Sea was determined by comprehensively considering the influence of temperature and time. The research results showed that a temperature change of the borehole wall and formation along with additional thermal stress gradually increased with the increase in the cycle time of the drilling fluid. Further, the maximum radial thermal stress was at a certain distance from the axis of the borehole, and the maximum tangential stress and the maximum vertical stress were at the borehole wall. When the temperature of the borehole wall decreased, the contraction of the borehole wall produced tensile stress and the periborehole stress decreased; the tangential stress and vertical stress decreased the most at the borehole wall, and the radial stress decreased the most at 1.65 times of the hole radius from the borehole axis. The deviation of fracture pressure considering the influence of temperature change was much lower than when not considering it. The research results show that temperature change has an impact on periborehole stress, and considering the influence of temperature can improve the prediction accuracy of formation fracture pressure, thus improving the prediction accuracy of the safety density window of the drilling fluid. This research can provide theoretical guidance for safe and efficient drilling in deepwater in the South China Sea.