Formation fluid sampling early in the life of a well ensures that vital information is available for timely input to field planning decisions. Particularly 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.
Oils have different color due to the amount of large, complex aromatic compounds they contain. Since synthetic and oil-base drilling muds contain simple aliphatic compounds, they absorb little in the shorter wavelength color channels. The Oil-Base Contamination Monitoring (OCM) technique uses optical means to monitor the buildup of color during sampling. The technique provides real-time analysis of sample contamination, as well as prediction of how long it will take to achieve an acceptably low level of contamination.
As reservoir fluid replaces filtrate in the flow line, the optical density (OD) of the methane signal also increases, in proportion to the oil's methane content. Methane detection is essential for condensates and for lightly colored crude oils. For such fluids, the color buildup becomes difficult to detect; however, the high methane content of these fluids makes possible a reliable methane-based OCM algorithm. Furthermore, the methane content can be used to determine the gas/oil ratio (GOR) of the sample. When these methane-based techniques are applied for darker oils, the color absorption of the crude extends to the near infrared region and covers the methane molecular vibration peak, resulting in a higher methane signal optical magnitude. If uncorrected for, this would result in errors in the methane-based contamination prediction and GOR determination.
In this paper, we show how to measure GOR downhole during sampling using the OD of the methane signal. We also describe a decolorization algorithm to remove the color effect from the methane optical channel. The algorithm is based on the exponential decay of color absorption toward the high wavelengths in the near infrared region. After decolorization, the methane channel contains only methane molecular vibration absorption, which is then used to derive an accurate crude oil contamination value and GOR.