Subsalt exploration is challenging because of the profound impact salt movement has on subsalt structure, fluid flow, and our ability to image and predict subsalt conditions. From a drilling perspective, exiting salt has proven to be problematic due to encountering abnormally high pressures when exiting base of salt, resulting in the loss of well control and likely necessitating the need to sidetrack the well at a substantial cost. Current methods of predicting pressure, which rely on velocities derived from surface seismic data or detailed subsurface geologic models, do not sufficiently reduce uncertainty pre-drill.
A walkaway VSP (Vertical Seismic Profile) recorded above the base of salt can be used to more accurately determine elastic parameters and a Vp/Vs ratio for the zone immediately below the base of salt. The elastic parameters can be used in empirical relationships to derive pressure. Two methods, which derive elastic parameters in unique ways, were tested on three post-drill tests and then applied in real-time to two complex subsalt wells. In all five cases, the VSP derived pressure correctly indicated a change in mudweight program, resulting in substantial cost savings and the first real-time application of the technology. These tests demonstrate potential to better estimate pressure exiting base of salt in complex deepwater subsalt prospects, eliminating the costly sidetracks due to well control issues.
Standard pressure prediction for wellbore planning involves the analysis of petrophysical and pressure data collected from analog wells, pressure conversion of velocities derived from seismic data, and basin models. There is considerable error in subalt areas due to the uncertainty each analysis brings. The velocity error is large in the subsalt sections due to poor seismic data quality, processing errors, anisotropy, and inadequate resolution. Conversion errors exist in poorly calibrated analog wells or inappropriate empirical density relationships, normal compaction, and pore and fracture pressure models. Currently the pressure range below salt is predominately based on basin models which are built from interpretations using poor seismic data, and analog wells that may be large distances away. Thus, given the range of uncertainties, the range of maximum and minimum mudweight is large and often not confident around the most likely scenario and enormous drilling resources continue to be spent on mitigating pressure related problems (Albertin et al. 2003).
Statistical data mining of industry wells in the Gulf of Mexico deepwater show that nearly 50% of the wells in the deepwater complex salt arena have problems exiting salt (Figure 1). A strong driver of improving our drilling efficiency is the potential cost and time savings in the expensive deepwater drilling environment. Another motivation for improving the non-productive time (NPT) is that industry in general is moving more into the complex subsalt arena in the Gulf of Mexico. Figure 2 shows subsalt industry activity from the 1990's through to 2003, and inherently for those active in the Gulf of Mexico a greater percentage of the portfolio will be below complex salt.
