Abstract

We study how stresses change as a salt diapir evolves into a salt sheet with no concurrent sedimentation. We find a reversal in the stress direction, as the radial stress changes from being the maximum stress when the diapir is rising, to minimum when the salt sheet is advancing. We study an axisymmetric diapir with the large-strain FE program Elfen. We model the salt as solid visco-plastic, and the sediments as poro-elastoplastic materials. During the diapir rise, the salt applies a horizontal thrust load onto the upper parts of the basin. However, as the diapir evolves into a sheet, the effective height of the salt diapir drops, and so the stress within the salt decreases. At the same time, the vertical stress within the basin increases because of the weight of the advancing sheet. We hence predict a change in the orientation of the minimum principal stress within the wall rocks associated with a switch from horizontal shortening to extension. Furthermore, the ratio of minimum effective principal stress to effective overburden decreases after the salt emplacement, and shear stresses below the salt sheet change. Overall, our results highlight that forward evolutionary modeling can improve borehole stability calculations below salt.

1. INTRODUCTION

A salt sheet is an allochthonous salt body sourced from a single feeder, whose breadth is several times greater than its maximum thickness [1]. The most common type spreads laterally from the top of a feeder diapir, and forms when the upward rise of the salt in the feeder is much faster than the local sedimentation rate [2]. Salt sheets are a common feature in salt systems around the world, including a very strong presence in the Gulf of Mexico [1]. Prediction of stresses and pore pressure below salt sheets is increasingly important for the exploration stage of drilling programs. New plays have been discovered beneath allochthonous salt [3-6]. However, there are several documented examples of drilling problems in such locations that have led to additional expense or even abandonment [3-5, 7, 8]. Improving the prediction of the stress state below salt sheets will benefit the design of the most economic well trajectory, best casing plan and overall borehole stability.

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