Formation fluid properties are critical inputs for field development planning. Acquisition of representative, low-contamination, formation fluid samples is key to obtaining accurate fluid properties from laboratory analysis. Quantification of oil-based mud (OBM) or water-based mud (WBM) filtrate contamination of hydrocarbon or water samples is still one of the biggest challenges, both during real-time formation tester sampling operations and with surface laboratory techniques. Laboratory sample analysis uses either the skimming or the subtraction method to quantify OBM filtrate contamination of hydrocarbon samples whereas tracers are typically required to quantify WBM filtrate contamination of water samples. Recently, a new real-time workflow has been developed to quantify OBM or WBM filtrate contamination of hydrocarbon or water samples with downhole multi-sensor formation-tester measurements. When discrepancies are observed between laboratory-derived and real-time contamination quantification, it can be challenging to uncover the source of the difference or to identify the most accurate method. This paper evaluates the applicability of different methods.
Surface laboratory methods to quantify OBM filtrate contamination crucially assume that the mole fraction of components in the C8+ portion of uncontaminated reservoir fluids and the corresponding molecular weights (or carbon numbers) follow an exponential relation. When actual fluid compositions deviate from the assumed behavior, a large error in OBM filtrate contamination quantifying can occur. In this paper, more than 20 laboratory-created mixtures of formation fluid and mud filtrate are analyzed to validate the laboratory methods. Errors of 2 to 3 wt% in OBM filtrate contamination quantification were observed for virgin reservoir fluids that follow the assumed behavior. However, much larger errors may be observed for biodegraded oil, oils with multiple charges from different sources, or oil with similarly wide ranges of compositions to OBM filtrate.
A new workflow allows quantification of OBM or WBM contamination using multiple downhole sensors, for real-time measurement, with unfocused and focused sampling tools for water, oil, and gas condensate. The new workflow comprises five steps:
data preprocessing;
endpoint determination for a pure native formation fluid using flow regime identification;
endpoint determination for pure mud filtrate and quality control of all endpoints using linear relations between measured fluid properties;
contamination determination in vol% and wt% on the basis of live fluids and stock tank liquids; and
decontamination of the fluid properties including gas/oil ratio, density, optical density, formation volume factor, resistivity, and compositions.
The new workflow has been applied to a large number of field cases, with very good results. For most of the cases, the downhole analysis is consistent with the surface laboratory results.
When discrepancies between methods are observed, a thorough understanding of the limitations of each technique, as described in this paper, will help to determine which data to bring forward and what to discard.
