Summary
Bottomhole samples collected in wellbore systems using oil-based muds (OBMs) are likely to be contaminated by medium-to-heavy hydrocarbon fractions present in the OBM. Pressure/volume/ temperature (PVT) data measured for a contaminated fluid will not be representative for the clean reservoir fluid, and such PVT data are hence often ignored by the operator, which means loss of a considerable investment. A proper estimate of the representative clean reservoir fluid is essential for reserves estimation and facilities design. Unfortunately, no experimental methods exist for such estimations. It would be valuable for the oil industry to have options for numerical cleaning of OBM-contaminated reservoir fluids and to be able to carry out equation-of-state (EOS) modeling and regression for a contaminated composition in a way that would allow PVT data for a contaminated fluid to be corrected to represent the uncontaminated fluid. This paper describes such a methodology, which is integrated with EOS modeling procedures for numerically cleaned reservoir-fluid compositions. Thanks to this method, PVT data for contaminated samples do not have to be ignored and oil and gas operators can justify investing in PVT analyses for contaminated-fluid samples.
This paper details the process through which the available data can be used. The composition of the reservoir fluid is estimated from the composition of the fluid with a certain content of OBM contaminate, and, contrary to other proposed methods for numerical cleaning, it takes into consideration that the aromaticity of the reservoir fluid may deviate substantially from that of the OBM contaminate. A regression procedure is applied afterward using the available PVT data in order to develop ultimately an EOS model for the clean reservoir fluid. Compositional data and PVT data are presented for a real reservoir fluid contaminated with OBM. Because data are also available for the clean reservoir fluid, it has been possible to verify the validity of the suggested procedure. The numerical cleaning procedure does not require any nonstandard laboratory data, and the given method also is not restricted to any particular brand of OBM or well type.
