Real Time Velocity and Pore Pressure Model Calibration in Exploration Drilling

Abstract

When drilling in remote areas, in the absence of offset data, pore pressure is determined as a function of seismic rock velocity. The velocity model and corresponding pore pressure model form the basis for exploration well planning. These models directly impact the casing design, number of sections, and mud weight plan. For frontier exploration drilling, the relationship is not well established; this reliance is high risk and can benefit from a high-technology approach to reduce this risk. This study examines how velocity and pressure models updated using real-time measurements from a high-tech logging-while-drilling suite can enable key drilling decisions to be made, reducing risk and the time taken to drill the well. Seismic-while-drilling technology provides checkshot data while drilling, which is used to update the velocity model. The velocity model updates in the shallow sections allow modification to the forward trajectory to ensure targets are intercepted. Furthermore, real-time updates enable geostopping by refining the predicted depths of targets or marker horizons and ensuring no accidental penetration of bright seismic amplitudes that may be associated with potential pore pressure abnormalities. In addition, formation-pressure-while-drilling technology can be added to the seismic and sonic technologies to calibrate the predrill pore pressure model. The acquired formation pressures, coupled with the while-drilling petrophysical data, allow for the velocity to pore pressure transform and normal compaction trend lines to be calibrated and reduce the uncertainty of the pore pressure model ahead of the bit. The calibrated models permit the operator to confidently make decisions and continue drilling in a single section to intercept key targets, potentially eliminating casing strings, with the assurance that the kick tolerance and safe overbalance are maintained at an optimal level to reduce the risk of mud losses or kicks.