Optics: measuring and testing – Of light reflection
Patent
1997-02-06
1998-11-03
Font, Frank G.
Optics: measuring and testing
Of light reflection
356448, 385 85, G01N 2155, G02B 636
Patent
active
058317430
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical probes, and particularly (but not exclusively) to such probes as may be useful in multiphase fluid component discrimination. The invention concerns especially probes for component discrimination and volume fraction measurement of a fluid containing multiple immiscible phases flowing along a channel, and more specifically still the invention relates to measuring the flow of the components of the gas, oil and water mixture commonly encountered in the bores of oil wells.
BACKGROUND OF THE INVENTION
Once completed, an oil well normally has a concrete-lined elongate borehole with 7 inch (17.5 cm) diameter tubular casing disposed therein. Formation fluids and treatment fluids pass through holes in the lining/casing and up/down the core of the borehole via whatever valves, sampling chambers and other mechanisms may be contained therein, either downhole or at the top of the well. One such mechanism may be a device for measuring the flow rate of the formation fluids up the borehole, for after a well has been drilled, lined and cased, and is producing, it will on occasion be desirable to measure, and log (record), the rate at which fluid is flowing out of the geological formations through which the bore has been drilled and is passing into and up the casing. While such a measurement at the wellhead will be adequate for most purposes, it may also be desirable--because this fluid will often be issuing at two or more levels from the formations through which the bore passes, and thus can have a corresponding number of compositions and flow rates--to be able to measure each in situ rather than as a combination when the fluids all reach the top of the casing as a mixture of the individual fluids. Thus, there is a need for a flow measuring device which can not only be used at the wellhead (where the flow may be a mixture of oil, brine and as much as 90% or more by volume gas) but can also be lowered as part of a production-logging or -monitoring tool string down the bore hole to a chosen depth, where there issues the fluid of interest (which may be an oil/brine/gas mixture with, typically, only a few gas bubbles because of the great ambient pressure).
As might be expected, one simple type of flow measurer is a propeller (or fan) acting as a turbine and driven round by the fluid passing "through" it. Such devices, which, coupled to a suitable generator, provide a flow-rate-dependent output signal (usually electrical in nature) that can be fed to metering equipment either included as part of the downhole measurer package or sited at the wellhead, are in use, and in many circumstances work well. However, they do have their limitations, particularly when applied to the formation fluids commonly encountered in oil wells (which are usually two- or three-phase fluids, being for the employ, instead or as well, one or more point device such as an optical probe, which relies for its operation on the interface properties of a fibre optic waveguide projecting into the fluid being observed.
Light travelling along a guide, such as an optical fibre, is kept within the guide by total internal reflection at the guide's boundaries provided that there is a small difference--about 1% is normally satisfactory--between the refractive index of the guide material and the refractive index of the surrounding medium. Similarly, when that light reaches the end (or tip) of the guide it may, or may not, be reflected back from the tip/medium interface (rather than exiting the guide). Whether it is or not depends upon the tip reflectivity (the reflected power ratio, which in turn depends on the refractive index contrast; whenever light travels across an interface between two materials some of it is always reflected/scattered back at that interface), and also upon the shape of the guide's end face or faces (coupled with the refractive index values, this determines whether light reaching the interface undergoes total internal reflection thereat). This effect has been employed to construct so-called optical probes that a
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Vince, M.A. et al., Optical Prove for High-Temperature Local Void Fraction Determination, 2219 Applied Optics, vol. 21 (1982) Mar., No. 5, New York, pp. 886-892.
Fordham Edmund J.
Ramos Rogerio Tadeu
Font Frank G.
Lee Peter Y.
Schlumberger Technology Corporation
Smith Keith G. W.
Stafira Michael P.
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