Formation Pressure Estimation Method for High Temperature and High Pressure Wells in Ledong Area of South China Sea

The Ledong area is a typical high temperature and high pressure area in the South China Sea's Yinggehai Basin. The geological structure of this basin is complex, with high formation temperature gradient and multiple formation pressure steps. Therefore, accurate evaluation of formation pressure is facing great difficulties. Based on geological and logging data, the genetic mechanism of abnormal high pressure in the Ledong area is as follows. Abnormal high pressure in the Yinggehai Formation and upper strata is caused primarily by under compaction, whereas abnormal high pressure in the Huangliu Formation and lower strata is caused primarily by hydrothermal pressurization and hydrocarbon generation. At the same time, according to the rock strength, mineral composition and rock physical parameters of the unloading mechanism layers in the Ledong area, the influence of effective stress, porosity and shale content on acoustic velocity was obtained by collecting field cores from the Ledong area and conducting rock acoustic testing experiments in laboratory. The acoustic velocity decreased exponentially with decreasing effective stress, and it had a linear relationship with porosity and the square root of shale content, according to the experimental results. Based on the principle of linear superposition, an improved Bowers unloading model for formation pressure evaluation was established. The original model was modified to overcome the shortcomings of the traditional Bowers unloading model, and the effects of effective stress, porosity and shale content on acoustic velocity were considered comprehensively. The improved Bowers unloading model was applied to evaluate the formation pressure of high temperature and high pressure wells in the Ledong area. The results showed that the improved Bowers unloading model had good applicability in the Ledong area, with average error less than 3% in formation pressure. This model will serve as a technical reference for determining formation pressure in the Yinggehai Basin's high temperature and high pressure region.