In-Situ Optical Fluid Analysis as an Aid to Wireline Formation Sampling

Abstract

This paper describes the basic theory of operation for a newly developed wireline tool that uses optical principles for continuous analysis of fluid in the flowline of a wireline formation fluid sampler. The tool has a visible and near-infrared absorption spectrometer for fluid discrimination and a refractometer for free gas detection. The Effective Flow Stream (EFS) model has been developed to interpret the measured data and to estimate the volume fraction of water and oil in the tool's flowline for biphasic flow. For triphasic flow, the technique can give a quantitative estimate of water, along with a qualitative evaluation of the amount of oil and gas. We present an example of showing tool response in the laboratory, introduce applications of the technique for aiding formation sampling, and, finally, illustrate the implementation of this technique with field examples. Tool DescriptionThe Optical Fluid Analyzer 1 (OFA * ) module is designed to be integrated in a third-generation wireline-conveyed formation sampling tool with pumpout capabilities, the Modular Formation Dynamics Tester (MDT * ). (For a description of the MDT tool and its applications, see Refs. 2 and 3.) The OFA module allows the engineer on location to make an intelligent decision of when and whether to take a sample. Fig. 1 shows a representative formation sampling tool string incorporating the OFA module. In a typical operation, the engineer sets the sampling tool (selecting either the packer module or the probe module as the source for the sample) against the formation and performs a pretest to verify seal integrity. The engineer then starts the pumpout module to pull fluids from the formation into the flowline in the tool string. As the formation fluids flow through the OFA portion of the flowline, the real-time interpretations of the measurements indicate the proportions of oil and water, and give a qualitative indication of free gas. The engineer then uses these interpretations to decide when to divert the flow to the sample chamber, or whether to sample at all.The OFA module responds to two basic optical properties of the fluid in the flowline: (1) optical absorption in the visible and near-infrared region (used for fluid discrimination and quantification), and (2) change in the index of refraction (used for free gas detection). A spectrometer measures the optical absorption, and a refractometer identifies the change in index of refraction.For the optical absorption (spectroscopy) measurement, light shines through a 0.08 in. [2-mm] thickness of the fluid by sapphire windows mounted in the flowline. The amount of light absorbed at several specific wavelengths indicates the amount of oil and water in the flowline as well as the darkness (color) of the oil. (Oil color allows discrimination between formation oil and oil-base mud filtrate in many cases). For free gas detection, polarized infrared light shines at an angle on the interface between a sapphire window and the fluid in the flowline. The intensity of the reflected light, measured at ...

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