Method and apparatus for a downhole refractometer and attenuated reflectance spectrometer

#The invention relates to refractometry and spectrometry in a wellbore environment. Specifically, it pertains to a robust apparatus and method for measuring refractive index of fluids along a continuum (rather than in steps), of measuring attenuated reflectance spectra, and to the interpretation of measurements made with this apparatus to determine a variety of formation fluid parameters. The refractometer and attenuated reflectance spectrometer disclosed here uses a simplified design, which is very appropriate for a downhole environment.

#Oil and gas companies spend large sums of money in their attempts to find hydrocarbon deposits. They drill exploration wells in their most promising prospects and use these exploration wells not only to determine whether hydrocarbons are present but also to determine the properties of those hydrocarbons, which are present.

#For deep offshore fields, before any hydrocarbons can be produced, it is first necessary to spend several years building very expensive platforms with proper oil and gas handling facilities. The design specifications and cost of materials used in these facilities are strongly dependent on the properties of the hydrocarbons, such as gas to oil ratio, viscosity, bubble point pressure, asphaltene precipitation pressure, and so on. The exploration well itself is generally plugged and abandoned not long after it is drilled. However, the information that it provides is often used throughout the life of the oil or gas field.

#FIG. 1 shows the percentages of light reflected at a fluid/sapphire interface versus the index of refraction for normal incidence (perpendicular) to the interface, where the fluid is the refractive medium being measured;

#FIG. 2 is a diagram of a preferred embodiment of the refractometer of the present invention;

#FIG. 3 is an expanded view of the refractometer of FIG. 2;

#The present invention provides a method and apparatus for determining a number of formation fluid properties from a refractometer measurement. The present invention also provides a method and apparatus to more accurately distinguish between gas and liquid based on the much lower index of refraction of gas. The refractive index of a wellbore or formation fluid can be determined from the fraction of light, R, reflected off of the interface between the preferred transparent window, having a known refractive index and the formation fluid under analysis.

#FIG. 1 shows the percentage of light reflected from the fluid/sapphire interface, when that light is incident perpendicular to the plane of the surface of the transparent window and interface. In this figure, the window has a fixed index of refraction of 1.75 but the index of refraction of the fluid varies. The minimum reflection occurs when the fluid's refractive index equals the window's refractive index, which for sapphire, is 1.75.

#FIG. 2 shows a preferred refractometer design of the present invention that fits into the existing space within a downhole fluid-characterization tool, which can perform in situ analysis of formation and wellbore fluids (for example, a Baker Atlas SampleViewSM tool).