Design of Experimental Test Method to Investigate the Effect of OBM Contamination on PVT Samples from Gas Condensate Reservoirs

Abstract

Abstract PVT samples captured by openhole formation testers in wells drilled with Oil Based Mud (OBM) will often suffer some degree of measurable contamination1-5. The contamination is even further pronounced for gas condensate reservoir fluid systems. Often many years will pass before production or the opportunity to re-sample the reservoir fluid and these contaminated samples are all there is to work with until that time. Improved understanding of the effect that OBM will have on these fluids will lead to improved characterization of the native reservoir fluid. The primary objective of this work was to design a new experimental laboratory method to contaminate a clean reservoir fluid PVT sample with OBM and capture the live single-phase fluid again under dynamic conditions. This experimental set-up will enable a laboratory to prepare samples representing PVT samples captured by Wireline Formation Testers (WFT) from an OBM drilled well, as well as the clean reservoir fluid to work with for comparison. It is envisaged that such an experimental setup will be able to be used to generate data to then study the effect of OBM contamination of PVT samples and ultimately improve the ability to correct PVT data on contaminated samples to obtain representative PVT properties. In order to test the experimental test method, base oil from an OBM was used to contaminate a gas condensate PVT sample with Condensate Gas Ratio (CGR) of 25 stb/MMscf. The data obtained from results from both contaminated and uncontaminated sub-samples were then used to investigate the effect of OBM on PVT properties. Sufficient sample was obtained in order to measure dew point, Z-factor, CGR and composition of the resulting contaminated samples to compare to the same properties from the original uncontaminated reservoir fluid sample. In this paper we present the following: A new experimental set-up was designed to create contaminated PVT samples from a clean sample under dynamic conditions – flowing within a porous media at reservoir conditions.With the experimental set-up we were able to obtain samples and data for comparison of a clean and OBM contaminated sampleResults indicate that under the right conditions, a condensing-vaporising partial miscibility dynamic is established between the gas condensate reservoir fluid and base oil.