Characterization of In-Situ Fluid Responses Using Optical Fluid Analysis
Authors: Felling M.M. , Morris C.W. 
Published: December 2007

Abstract
Wireline formation testers are commonly used during openhole logging operations to recover fluid samples and to determine the type and distribution of formation fluids. Positive identification of fluid type usually comes from inspection and/or analysis of recovered samples at surface. A novel approach to identifying formation fluids in-situ is possible with the OFA optical Fluids Analyzer module of the MDT Modular Formation Dynamics Tester tool. Measurement of optical fluid density offers the opportunity to evaluate significant fluid characteristics downhole in real-time.
Experiments were performed to quantify the optical density and transmittance characteristics of more than 75 fluid samples, including hydrocarbons, formation waters, drilling muds and filtrates, and mixtures of these fluids. A catalog of characteristic OFA responses was developed to aid in real-time fluid identification applications. Hydrocarbon responses were found to show a strong correlation between oil gravity and optical density. Downhole oil gravity can be estimated from the OFA response by comparison with an experimentally determined database.
For the majority of hydrocarbon liquids studied here it was observed that there is a region in the range of wavelengths from 700–1600 mn where the optical density is constant. This observation is important for MDT sampling operations since, in particular, it can be used to estimate in real-time, oil sample contamination from water- and oil-based drilling mud systems. A new technique for minimizing (with respect to further pumpout time) oil sample contamination by various oil-base mud (OBM) filtrates is proposed. Furthermore, a means is suggested for distinguishing between oil-base mud filtrates and hydrocarbons having an oil gravity less than 400 API. Several field examples are presented to illustrate the application of the techniques introduced.

Introduction
Long-standing difficulties associated with wireline formation tester pressure measurements and fluid sampling operations include mud filtrate invasion, establishing and maintaining a seal between the probe and the borehole wall, and drawing down the pressure of the in-situ fluid below saturation pressure. These and other issues are addressed by the MDT, a third generation wireline formation tester tool. A typical pressure and sampling configuration is illustrated in Fig. 1; hydraulic communication with the formation is effected by means of probe or packer modules. The pumpout module is used to draw fluids from the formation, through the MDT flowline and out to the wellbore (if desired). As fluids pass through the tool their resistivity and temperature are measured in the probe module; their optical properties are measured in the OFA module. Optical data are processed in real-time to quantitatively determine flowing oil and water fractions, and to obtain a qualitative indication of the amount of free gas in the flow stream.
The OFA module has a visible and near-infrared absorption spectrometer for oil/water discrimination and a refractometer for free gas identification. Each sensor responds to one of two basic optical properties, namely absorption and index of refraction. These properties are measured by passing light through a window opening onto the flowline The measured absorption spectra depends on the composition of the sampled fluid. Fig. 2 shows OFA responses in terms of optical density - defined as the logarithm of the inverse of light transmittance - for several fluids. Notice in the figure that water and oil have considerably different light absorption spectra in the near-infrared region. Water has an absorption peak at 1450 nm and an absorption region from 1860 nm to 2080 nm; oils have an absorption peak at about 1720 nm.
P. 391^