Sampling in wells drilled with OBM and SBM is a particular challenge. The OBM filtrate is miscible with crude oil and cannot be separated. It is best to reduce the contamination to low levels during sampling, yielding clean samples that require little parameter extrapolation. However, the expense of pumping indefinitely to reduce contamination levels often is unacceptable. When samples are acquired during openhole logging, NIR measurements can be used to determine flowline coloration.
Coloration is used to distinguish different oils.
Nevertheless, the variation of coloration or electronic transitions of crude oils extends in wavelength far beyond where the eye can see. Generally, the coloration value of OBM filtrate is less than that of crude oils. Thus, the initial fluid extracted from the formation is rich OBM filtrate and is light in coloration. This time variation of coloration can be monitored to assess the reduction of OBM filtrate. When the change in coloration is sufficiently small, the contamination is deemed acceptably low and samples are collected.