Wireline formation testers are commonly used to obtain fluid samples and measure formation pressure during openhole logging operations. The identification of the fluid usually depends on the analysis of the sample chamber contents at the surface. A new approach is now possible using the measurement of in-situ optical fluid density. This analysis can be used to determine the formation fluids and to evaluate significant fluid characteristics in real time.
The optical fluid analysis technique utilizes a visible and near-infrared absorption spectrometer for fluid discrimination and a refractometer for free gas. The spectrometer measures the transmittance of a liquid at 10 different wavelengths and distinguishes between oil and water by comparing the resulting absorption spectra in the visible and near-infrared region. The spectrometer yields quantitative data on fluid phase volumes and qualitative data concerning fluid coloration. Experiments were performed to catalog the optical fluid density characteristics of typical hydrocalbons, formation waters, filtrates, drilling mud systems, and mixtures of these fluids.
Utilizing this data base, optical density measurements can do an excellent job differentiating between oil and water. Hydrocarbon responses also show a strong correlation trend with optical fluid density measurements and can be used to estimate the in-situ oil gravity. Furthermore, differentiation between oil~base drilling fluid filtrates and hydrocarbons is possible. This outcome was observed with diesel, synthetic oil, and various other oil-base filtrates.
A new technique has been developed, which performs a weighted regression analysis on the optical spectrum, to quantify the mud filtrate contamination, water, and formation hydrocarbon in real-time. Field examples are presented to illustrate the application of the optical density information to the interpretation of formation fluid characteristics. The technique reduces overall sampling time, minimizes sample contamination, and improves sample quality.
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