The increase in subsalt exploration drilling activity, and the drilling problems associated with exiting salt, have led the oil industry to scrutinize subsalt drilling practices and the methods commonly in use for pre-drill pore pressure forecasting. Integrated drillability assessment utilizing a combination of offset well information, seismic derived velocities, and basin fluid flow modelling has improved theaccuracy of pre-drill pressure forecasts, but significant uncertainty still exists in quantifying the stress regimes immediately below salt.
Recently, 2D walkaway VSPs designed specifically to address the subsalt pressure uncertainty via Vp/Vs estimates and empirical models for Vp/Vs to pressure conversion, havebeen successfully used to impact drilling operational decisions and improve subsalt drilling success. The process of using lookahead VSP's can be subdivided into three distinct phases:
modeling to establish feasibility of a walkaway VSP for revealing reasonable Vp/Vs and for establishing an optimum 2D acquisition, and calibration of the Vp/Vs to pressure conversion model
acquisition of the VSP and real time processing using AVA and kinematic methods to establish thesubsalt Vp/Vs, identify inclusions, and refine the base of salt depth prediction, and
pressure prediction from the Vp/Vs and review and update of the salt exit Mudweight/salt exit drilling procedures. This paper will focus on the kinematic methods employed for estimating Vp/Vs in the second phaseof this lookahead VSP workflow.
In the last few years a number of workers have described or implied that shear velocity as well as Vp/Vs should be as sensitive or more sensitive to pore pressure of mud rocks asthe typical compressional velocities, Vp or travel time, we have used for years for pressure prediction and pressure detection while drilling (Huffman and Castagna, 2001; Huffman, 2002; Eberhart-Phillips et al., 1989). In some cases recommendations for drilling overpressure in deep water in the shallow, riserless section, most often referred to as the shallow water flow zone, have been based on the prediction of unusually high Vp/Vs in sediment derived from surface seismic data. The high Vp/Vs is interpreted to be due to very unconsolidated conditions likely reflecting relatively high overpressure. Our work focused on evaluating the response of shear velocity and Vp/Vs to overpressure, that is low effectivestress, and if possible create a workable technique to calculate the magnitude of overpressure (Heppard et al., 2002; Ebrom et al., 2003).
Our process began by reviewing shear (Vs) and Vp/Vs as measured by dipole sonic tools from a number of wells from six petroleum basins in the Americas, Africa, and Asia. Ourprocess was similar to the way we in the industry have evaluated compressional sonic travel time or velocity via comparing measured responses to an expectation of velocity under normal pressure conditions (Figure 1).