Formation fluid sampling early in the life of a well ensures that vital information is available for timely input to field planning decisions. For example, in subsea wells, flow assurance is a major concern, and formation fluid samples from openhole logging help operators optimize investment in both upstream and downstream facilities. When a formation fluid sample is taken from a well drilled with oil-based mud (OBM), sample contamination by the OBM filtrate is a critical factor for the accurate measurement of the sample pressure/volume/temperature (PVT) properties. A technique of monitoring sample contamination from OBM filtrate uses optical means to monitor the buildup of both color- and methane-absorption signals during sampling. The technique provides realtime analysis of sample contamination. Methane detection is essential for condensates and lightly colored crude oils; for such fluids, the color buildup becomes difficult to detect, but the high methane content of these fluids makes possible a reliable methane-based OBM-contamination monitoring algorithm. Gas/oil ratio (GOR) is an important property of crude oil, and it is a vital input to the design of production facilities. Conventionally, GOR is measured at a PVT laboratory, and it may take many weeks before the laboratory can provide this critical information. In this paper, we describe the development of an in-situ GOR measurement technique, which uses the optical properties of methane and oil components in crude oil. With this technique, GOR can be measured downhole in real time, when the sample is taken, and without requiring phase separation. Downhole GOR has many advantages over the conventional GOR measurement techniques. It does not require tampering with the sample, which helps the operator maintain the fluid in a single phase during and after sampling. It also can aid in fingerprinting oils from different layers and provides early indications of GOR that can be compared to PVT lab results. Both the OBM contamination monitoring and the GOR algorithms work well for most crude oils. However, for heavy (dark) oils, the contamination prediction from the methane component and the GOR prediction become unreliable because of the color effect. In this paper, we describe the methodology for downhole GOR measurement, and we provide details of a decolorization technique to remove the color effect of dark oils from the methane, oil, and base channels in a downhole optical fluid analyzer tool. This technique significantly improves real-time contamination monitoring and GOR prediction results for dark oils.
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