Compositional and PVT properties of reservoir fluids contaminated by drilling fluid filtrate

Oil-based drilling muds are widely used in offshore drilling applications mostly due to substantial cost savings. Wireline samples of reservoir fluids are taken without completing the well, and the sample will contain various amounts of mud filtrate, depending on the sampling conditions. The present paper describes methods to obtain compositional and PVT data for the initial reservoir fluid from wireline samples contaminated by mud oil. Two fluid systems have been studied, a black oil and a gas condensate, both typical fluids from the North Sea. The reservoir fluids were contaminated by two types of base oils, termed Ultidrill (a synthetic olefin-based oil) and EDC (a low aromatic mineral oil fraction), containing 0, 10, 20, and 40 wt.% of base oil in single flashed stock tank oil (STO). The composition of the contaminated STO was adjusted using a semi-logarithmic correlation. Procedures to obtain correct data for the plus-fraction are discussed especially. Actual fluid parameters were calculated from the adjusted fluid composition and used as input data to a commercial PVT-simulator. How to tune the simulator to obtain the correct saturation point for the uncontaminated fluid, using experimental data from the contaminated fluids at the reservoir temperature, is described. Then, fluid properties like gas/oil ratio, constant mass expansion, differential gas liberation, liquid drop-out in a constant mass expansion for the gas condensate, and fluid viscosities were simulated for the adjusted fluids and compared with experimental data for the uncontaminated samples. The simulated fluid properties correlated very well with the experimental data for the uncontaminated fluids. It is also described a method to quality-check mud oil contaminated wireline samples prior to further PVT studies.