AAPG Annual Convention and Exhibition

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Impact of Pore Pressure Estimation Uncertainty on Well Design and Execution: An Example From Deepwater West Africa

Abstract

Uncertainty in pre-drill pore pressure prediction (PPP) often has a significant impact on pre-drill well design, which in turn impacts the overall cost and the safe execution of a well. The uncertainty in pore pressure estimates are driven by: a) the geologic scenarios and velocity to pressure transform used; b) the input data. In this paper, we present the impact of uncertainty on the PPP for a deep-water exploration well in West Africa. The closest offset well was ~10 km away and in a different geology. The main uncertainties affecting the pore pressure prediction were: rock properties, separating out the impact of different lithology and pore pressure on log and seismic response, lack of sufficient calibration data, understanding pore pressure generating mechanisms, and hydraulic connectivity potential in different stratigraphic units. We show how the uncertainty in predrill estimate manifested in the final pore pressure plot (i.e. a low case, a base case, a high case and a walk-away case) and the impact it had on well design (i.e., contingency casing, planned mud weight, decision points including options to stop, etc.). We will also show how real-time pore pressure monitoring of LWD and PWD logs, gas behavior and drilling response has been used to manage and mitigate some of the predrill PPP uncertainties; however uncertainties on log signature due to lithology change still remain. Finally, we will share some of the learnings from our post-well analysis which indicate that the range of predrill scenarios needed to be wider and that during drilling lack of secondary indicators was only a result of very low permeability shale.