Estimating fluid pressure below intrusive salt sheets; the factors causing uncertainty

In the Gulf of Mexico, uncertainties in salt sheet thickness, lateral salt speed, sub-sea floor depth of salt top, and lithology underlying salt all contribute to the uncertainty in assessing excess pressure development to be expected when drilling through a salt mass. In an example, the mean excess pressure is around 170 atm with + or -30atm uncertainty. The uncertainty in the estimate of excess pressure is shown to be dominated (58-65%) by uncertainties in salt sheet thickness, with salt speed and lithology of lesser importance, each contributing around 10-25% to the excess pressure uncertainty, depending on the case considered. The cumulative probability of obtaining an excess pressure greater than a specified magnitude at any depth is investigated, as is the volatility of estimates on mean pressure. The calculations provide information on which uncertainties-in salt thickness, salt speed or sub-salt lithology-need to be improved first in any attempt to narrow the range of uncertainty of predicted pressure.