Methane solubility in synthetic oil-based drilling muds

The development of new drilling technologies and the reinforcement of environmental legislations require improvements of drilling fluid formulations. In particular, mineral oil-based muds have been recently replaced by low toxic or substitution oil-based muds (SBM). The molecular composition of these oils is adapted to the fluid property requirements. Very little is known on methane solubility in these oils and in their invest emulsions under reservoir conditions, though this is of crucial importance for predicting gas eruption risks when drilling. This is particularly the case of offshore deep drilling areas for controlling gas hydrate formation. In this work, solubility measurements of methane in four base oils have been performed at 90 degrees C and pressures from 150 to 350 bar. The solubility in the fluid mixtures was found to be proportional to the solubility in water and oil, respectively. In order to extrapolate the experimental results to other conditions, a Peng-Robinson equation has been used. The critical parameters of the unknown constituents have been determined either by correlations based on the normal boiling point and the density or by a group contribution model. A single binary interaction parameter has been fitted between methane and the other oil components. Such results can be pertinent for predicting the risks related to gas/mud interactions. (C) 1999 Published by Elsevier Science B.V. All rights reserved.