This paper presents the results of an investigation of shear and compressional velocities and their ratios for pore-pressure prediction in a subsalt environment. In this work, we introduced the state of the art in the analysis of acoustic methods—compressional velocity (Vp), shear velocity (Vs), and Vp/Vs ratio—for pore-pressure estimation. Because industry best practices of using shear velocity and Vp/Vs ratio are still being developed, we evaluated a number of existing techniques and methods, advanced new approaches, and developed the necessary database of modeling results.
This database contains data gathered from 17 subsalt wells and includes definitive pore pressure and effective stress directly pertinent to the subsalt sediments. Velocity and resistivity data were then crossplotted against effective stress to define the correlation among these variables. Based on several forms of published velocity effective-stress relationships, we worked to calibrate these relationships and to simplify some of these formulations to get a tractable direct solution for pore pressure.
To validate these relationships, we selected certain other relationships and performed tests, which resulted in varying degrees of success. These tests represent 17 wells that penetrated salt. Ten of these 17 wells have good pore-pressure control data, which is a good pool of cases for a screening analysis from which we can derive lessons learned for future improvements.
Based on these results, we drew the following conclusions:
The unconventional pore-pressure analysis approach of using Vp, Vs, and Vp/Vs with a modified Eaton method (1975) was successful. The unconventional analysis of Vp produces more consistent results than Vs-based pore-pressure analysis. Further tuning is required to develop area-specific model parameters.
The rock-physics-based pore-pressure transform derived directly from the simplified and calibrated Eberhart-Phillips equation (1989) showed better performance than the modified Eaton approach. This approach will eliminate the guesswork for defining normal-compaction trend for velocity.
This work accomplished the following benchmarks:
Development of velocity effective-stress transforms that are tailored for the Gulf of Mexico subsalt sediments.
Simplified the process of pore-pressure prediction using Vp, Vs and their ratios utilizing a universal rock-physics-based normal-compaction trend.
