Modelling Seismic Velocities & Pressure Cells for Pre-Drill Pore Pressure Prediction and Application in Development Wells in Nigerian Deepwater Turbidites

Abstract

The use of quantitative techniques in the evaluation of pore and fracture pressures across entire well path plays an important role in successful well delivery. The ability to correctly predict geo-pressures before drilling and manage them during drilling is a critical success factor to safe drilling of wells and cost reduction especially in deepwater environments. Compressional velocities have been used to provide indirect estimate of pressures prior to drilling; the method is well established Rodney Littleton et al, (74487). Vertical effective stress-Compressional velocity models in offset wells when fitted with empirical models can be applied to calibrated seismic velocities in prospect well locations to predict pore pressures. The accuracy of this method however depends on factors not limited to geology, compaction trend, seismic anisotropy, consistent velocity picking/gridding, input to gathers, over pressure prediction, etc. To raise the confidence level in the seismic based prediction, a simple yet pragmatic methodology has being used to demonstrate how combining velocity based pressures with pressure cell modeling can yield formation pressures across entire well paths with much higher degree of certainty. This work therefore discusses a study done on a deep water Nigeria field; where Seismic velocities have been used to predict pore pressures. The outcome was combined with pressure cell based methodology (standard method for pore pressure prediction in sands) to reduce the inherent uncertainties in sand pore pressure predictions. The results shared in this paper have already been used in effective design of several deep water wells in offshore Nigeria.