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 ...
LntroductionFiber optic sensors and systems in the oil and gas industry have potentially significant advantages compared to conventional sensor systems, particularly for downhole permanent monitoring applications. The typical environmental requirements in well bore applications are 20kpsi and 175 degree C and measurements in these hostile environments need much higher pressure and temperature capability [ 11. Fiber optics sensors are electrically passive and do not require downhole electronics, resulting in potential operations at high temperatures with high reliability [2].Fiber Bragg gratings (FBGs) can operate at high temperatures, and are candidates for multifunctional and multi-point sensors for well and reservoir monitoring. Fiber optic Bragg grating sensors have been used to measure longitudinal and transverse strain [3-51, as well as longitudinal strain and temperature [6-81. Schlumberger demonstrated a Bragg grating pressure sensor with a single mode side-hole fiber [9] and superior p ressure response with low temperature sensitivity up to 300 degree C compared to the Bragg grating written onto conventional single mode optical fiber [lo]. The transverse load is applied to the core by the air holes in the fiber cladding under high pressure, resulting in stress induced birefringence and a dual peak spectral output. The peak to peak separation is sensitive to pressure, and veryinsensitive to temperature.In this paper, we describe a transversely loaded Bragg grating pressure transducer with mechanically enhanced the sensitivityutilizing fiber Bragg gratings written onto a single mode fiber [l I], and demonstrate pressure sensitivity up to 5k psi with low temperature sensitivity.Transducer structure and experimental results Figure 1 shows a schematic view of the transducer structure. The transducer and an optical fiber are set in an atmospheric chamber, and transverse stress is applied to the optical fiber by a piston. The force applied to the fiber is proportional to the area of the piston; therefore, the transverse load can be adjusted by changing the area. Two optical fibers without coating material are set between two quartz plates, one w i t h a FBG, and a dummy fiber, added to keep the symmetry and alignment of the plates. The balance of the transverse load between the FBG and the dummy fiber depends on the position of the fibers; therefore, the pressure sensitivity of the transducer changes slightly depending on assembly conditions. The surfaces of 0-7803-7289-1/02/$17.0002002 IEEE 535
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