Preliminary Testing of a Novel Downhole Fiber Optic Fluid Analyzer
Authors: Cobern Martin E. , Turner William E. , Cooper John , Aust Jeffrey F. 
Published: December 2007

A novel fiber optic down hole fluid analyzer has been developed for real-time determination of the oil, gas and water fractions of fluids from different zones in a multizone or multilateral completion environment. A near-infrared attenuation measurement provides differentiation among oil, water and gas at all but the highest water cuts. An induced fluorescence measurement unambiguously determines the oil fraction of the fluid as a validity check on the attenuation measurement. The only down hole components of the system are the fiber optic cable, lenses and windows, which may be installed in multiple locations in the well and operated with a single surface unit.

Introduction

As well completions become more complex, including multiple zones and multilateral legs, down hole monitoring of the production from individual zones becomes more essential. Intelligent completions give the operator the means to make changes in production rates, but to do so requires real-time information about fluid types. In particular, determination of the oil, gas and water fractions is necessary to make informed decisions on changes in production controls.

The system described in this paper involves a combination of two optical techniques to determine the mass fractions of oil, water and gas at remote locations within a well. These measurements are carried out remotely through the introduction of optical sensors into the production tubing; the sensors consist solely of inert optical components and are, therefore, extremely robust.

Initial testing of the two methods in the laboratory showed that the output signals produce excellent correlations with the known percentages of oil, gas and water (for the attenuation measurements,) and oil (for the fluorescence measurement.) The second phase of testing is being conducted in a low-pressure multiphase flow loop constructed for the purpose. Data are being obtained on system performance over a wide range of inclinations, flow rates and flow regimes.

Laboratory Testing & Results

Near-infrared attenuation.

Apparatus & Measurements. The NIR attenuation measurements were carried out in an analogous manner using the apparatus shown in Fig. 1. Light from a broad band source covering the near infrared wavelength range of at least 1000 - 1700 nm is sent down an ~ 1 mile long, to simulate field deployment, to a reflectance probe, which is immersed in the oil-water mixture. The reflected light is carried by a second fiber of the same length to the detector system.

The NIR absorption method relies on differences in the opacity of oil and water to the transmission of infrared light. As illustrated in Figs. 2 and 3, crude oil absorbs light preferentially at 1220 and 1390 nm; water has a very strong and broad absorption between 1400 and 1500 nm. It is, therefore, possible to distinguish between them based upon variation of the transmitted light intensity as a function of wavelength.

A key element of this apparatus is the solid state InGaAs spectrograph. This device permits the simultaneous determination of the intensity at all wavelengths of interest. Standard scanning techniques sample different wavelengths sequentially. Since the local composition and scattering are expected to vary rapidly with time, appropriate correction for scattering requires that the entire spectrum be acquired at once.

Two titrations were performed: water was titrated into oil to 50%, and oil was titrated into water to 50%, in steps of 2.5%. The fluid is constantly stirred to prevent separation. For each mixture, a spectrum was gathered over 0.1 sec, and 100 samples were averaged. Each titration was independently repeated ten times.

Near-infrared attenuation.

Apparatus & Measurements. The NIR attenuation measurements were carried out in an analogous manner using the apparatus shown in Fig. 1. Light from a broad band source covering the near infrared wavelength range of at least 1000 - 1700 nm is sent down an ~ 1 mile long, to simulate field deployment, to a reflectance probe, which is immersed in the oil-water mixture. The reflected light is carried by a second fiber of the same length to the detector system.

The NIR absorption method relies on differences in the opacity of oil and water to the transmission of infrared light. As illustrated in Figs. 2 and 3, crude oil absorbs light preferentially at 1220 and 1390 nm; water has a very strong and broad absorption between 1400 and 1500 nm. It is, therefore, possible to distinguish between them based upon variation of the transmitted light intensity as a function of wavelength.

A key element of this apparatus is the solid state InGaAs spectrograph. This device permits the simultaneous determination of the intensity at all wavelengths of interest. Standard scanning techniques sample different wavelengths sequentially. Since the local composition and scattering are expected to vary rapidly with time, appropriate correction for scattering requires that the entire spectrum be acquired at once.

Two titrations were performed: water was titrated into oil to 50%, and oil was titrated into water to 50%, in steps of 2.5%. The fluid is constantly stirred to prevent separation. For each mixture, a spectrum was gathered over 0.1 sec, and 100 samples were averaged. Each titration was independently repeated ten times.