Establishment of Appropriate Normal Compaction Trend NCT: A Critical Aspect for Reducing Uncertainty in 3D Pore Pressure Modelling.

Abstract

In 3D Pore Pressure Modelling workflow, establishing appropriate Normal Compaction Trend (NCT) is not only critical but also requires the maximum extent of human interpretation and geological understanding. If not established appropriately, it can introduce substantial uncertainty in the final pore pressure prediction. Though, statistical algorithm techniques are available to establish it, the authors of this paper have demonstrated that establishing NCT manually based on geological logic and regional pressure understanding is much more reliable technique than pure statistical based approach. In this paper, authors utilizes two different approaches in establishing Normal Compaction Trend (NCT) for the study area. First, based on pure statistical technique and second, a manual one based on combination of 3D velocity trends and regional geological pressure understanding. The 3D pore pressure volumes generated from the above two separate NCT’s are then checked for their conformance and agreement with the regional pressure data and understanding, including validation with post drill measured pressure data in the study area. The results and analysis in the study area shows that, establishment of NCT based purely on statistical approach results in higher uncertainty in the 3D pore pressure estimation process. Whereas, manual NCT based on logic results in much more robust, reliable, and regionally consistent 3D pore pressure model with lower uncertainty. In our case study, the average uncertainty in the statistical NCT based 3D Pressures was ranging between 0.8 – 2.3 PPG when compared with actual pressures, while in the case of logic based manual NCT the average uncertainty was less than 1.0 PPG. This case study indicates that in the offshore areas, particularly in areas where there is transition from shelf to slope to deepwater, it is advisable to use all the regional pressure knowledge and geological understanding in establishing the NCT, rather than adopting only the pure statistical methods.