One of the major challenges during drilling in ultra-deep salt formation in Tarim basin is to predict pore pressure accurately. The salt formation buried at 6000-7800m, which had been affected by tectonic events. Currently, the industry mainly relies on the empirical relations, such as velocities and resistivity trends, to predict pore pressure. Consequently, many investigators have shown these methods are largely based on shale behavior, which reduces the reliability of purely trend-based methods in predicting salt formation overpressure.
In this paper, pore pressure mechanism can be obtained from the existing wells data--including the characteristics of geological structure, pore pressure distribution and logging response directly pertinent to the salt formation. A new model for pore pressure prediction in salt formation is proposed based on the small deflection bending theory of thin plate and poroelasticity theory. This model includes in-situ stress state, tectonic deformation curvature and elastic modulus which were not included in the existing models.
According to the field data analysis, the tectonic compression is the primary overpressure generation mechanism. This approach, based on a rock-physics model, is an improvement over existing pore-pressure prediction methods, which mainly rely on empirical relations. Moreover, it is the fundamental to understand the mechanism of the overpressure generation.
With more than 45 wells predicted with this proposed model, the average error is less than 8%. Thus, recommendations for drilling in salt formation can be given.