Investigation of Partial Miscibility Between Gas Condensate PVT Samples and Oil Based Mud

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Abstract The measured PVT properties of gas condensate reservoir fluid samples collected while logging from wells drilled with Oil Based Mud (OBM) will often suffer due to contamination from the base oil. The objective of this work was to investigate the effects on reservoir fluid PVT properties of certain components dissolving preferentially into the gas stream and from the reservoir gas into the base oil. Using a modified slim tube set-up, a reservoir gas was contaminated with base oil from an OBM. The two fluids interacted within a porous media under reservoir conditions by injecting the reservoir gas into the slim tube that had been pre-charged with base oil. After displacing all liquids, gas samples were collected again under reservoir conditions creating a batch of contaminated samples. Using the results of compositional and PVT data obtained from contaminated samples at the back end of the slim tube and the original clean reservoir gas, a fully compositional numerical simulation model was created to simulate the experimental results. During the experimental work, after extended flow of the reservoir gas through the modified slim tube we noticed that the reservoir gas was being produced leaner, indicating that the heavier components of the reservoir gas were staying on the porous media. Using the compositional simulation model, we tested the sensitivities of conditions under which heavy end stripping of the gas condensate components may occur and how to obtain cleaner reservoir fluid samples faster from wireline formation testers in OBM drilled wells. The modelling work indicates that when the reservoir gas and base oil are more miscible cleaner samples are obtained sooner. Throughout this work we have been testing the idea that, if not completely miscible, the near wellbore region will retain an immobile phase of base oil and that this stationary fluid may hold back the heaviest components native to the reservoir gas. Furthermore, perhaps by altering the properties of the base oil, miscibility between the reservoir gas and base oil can be increased and improved samples obtained faster.