Abstract: The current formation pressure prediction methods based on the undercompaction theory are essentially empirical models, which are not generalizable, and the prediction accuracy is constrained by various factors. For this reason, the quantitative study of anomalous formation pressure was carried out from the perspective of rock pore volume and fluid volume. According to the theory of elastic mechanics of porous media, the quantitative evaluation model of anomalous formation pressure and pressure coefficient under the effect of undercompaction was established by comprehensively considering the factors such as temperature, stress, fluid composition, and depth change. The model is applicable to the evaluation of anomalous formation pressure with undercompaction as the main genesis, and has high accuracy. The simulation results show that the formation pressure increases under the effect of the undercompaction, but the pressure coefficient may increase, remain unchanged or decrease due to the increase of burial depth, and the influence of the increase of burial depth on the pressure coefficient shall not be neglected; the undercompaction can form both anomalous high pressure and anomalous low pressure; the changes of the formation pressure and the pressure coefficient are related to the physical properties of the fluid and the rock, and the increment of the formation depth as well as the initial depth; the influences of the compression coefficient of the fluid and the increment of the formation depth are relatively significant, while the influences of other factors are relatively weak. The research results break through the previous knowledge of the relationship between undercompaction and anomalous high pressure, and enrich and develop the theory of undercompaction.

Key words: undercompaction, formation pressure, anomalous pressure, pressure coefficient, compression coefficient, influencing factors, evaluation model