Phase equilibria of (methane-long chain ester cuts) systems in drilling conditions

During the drilling operation, the drilling fluid interacts with the formation at the well bore leading to the dissolution of hydrocarbons in the mud. Berthezene et al. [J. Petroleum Sci. Eng. 23 (1999) 71] studied the methane solubility in different fluids representing the drilling mud. They concluded that the ester remains immiscible with methane up to very high pressures. Additional experimental work at pressures up to 100 MPa has been performed on different esters. The effect of chain length and the presence of the ester group onto the chain on the phase equilibrium with methane is investigated. The conclusions of this work are identical to that of Berthezene et al. for critical points: the high molecular weight esters yield very high critical pressures, but the lower the molar mass, the lower the critical pressure. The position of the ester group in the chain has been found to have a negligible influence on the phase envelope. Several cubic equations of state have been used to model the bubble pressure data. The Peng-Robinson (PR) and the Elliott-Suresh-Donohue (ESD) equations of state have been combined with the classical mixing rule using binary interaction parameters equal to zero (k(ij) = 0) and the Soave-Redlich-Kwong (SRK) equation- of- state has been used with the MHV2 and UNIFAC mixing rule. In all cases, group contribution methods have been used for predicting the critical properties of the pure components since we are looking for a completely predictive method. We have found that the SRK/MHV2/UNIFAC gives very poor results. The PR/k(ij) = 0 model gives the best results for the low pressures but the predictions are poor for high pressures. The ESD/k(ij) = 0 model appears to provide much better predictions, especially for high pressures, which is interesting for us. (C) 2002 Elsevier Science B.V. All rights reserved.