New High-Temperature Optical Fiber Technology Successfully Provides Continuous DTS Data under Severe SAGD Conditions

Abstract

The use of optical fibers in the oil and gas industry is becoming more viable for several applications, particularly in permanent reservoir monitoring, such as distributed temperature sensing (DTS) and optical pressure transducers. However, poor long-term performance of fibers, especially at elevated temperatures, is still an issue yet to be fully resolved. This problem is critically important in steam assisted gravity drainage (SAGD) applications, where wells operate in extreme conditions of high temperatures, often exceeding 250oC, as well as in high pressures within a hydrogen-rich environment. Optical fiber performance is seriously affected by many factors, including:Hydrogen ingressionThermal resistance of the materialsMechanical resistance of the fiber Exposure of optical fibers to hydrogen changes the performance of the fibers through what is referred to in the industry as "hydrogen aging" or "hydrogen darkening." Hydrogen darkening is increased absorption or light loss due to various chemical species in the glass fiber resulting from the presence of hydrogen. Introduction In DTS applications, a series of short pulses is sent down an optical fiber. As a pulse propagates down the fiber, it interacts with microscopic defects in the fiber and a small fraction of the photons making up the pulse are scattered. Some of the scattered photons are recaptured by the fiber and return back toward the surface. The majority of these "backscattered" photons is at the same wavelength as the laser pulse and is called "Rayleigh" backscatter. A small fraction of backscattered photons undergo a non-linear event called "Raman" scattering, and return at two new wavelengths. The first Raman wavelength is referred to as the Stokes wavelength, which is longer than the laser wavelength, while the Anti-Stokes wavelength is shorter than the laser wavelength. The ratio of the number of Anti-Stokes to Stokes photons is a function of temperature. The Stokes and Anti-Stokes signals are detected at the surface, and by computing the ratio of the amplitudes of these two signals, the temperature distribution along the fiber can be calculated. In high-temperature environments, the physical structure of the fiber glass and the coating or jacket materials protecting the fiber can be compromised. Many of the protective materials of conventional fibers are not designed for the extreme temperatures associated with SAGD operations. Additionally, continuous thermal expansion and contraction with changes in temperatures during SAGD operations may cause conventional fibers to fatigue prematurely in these environments. In this paper, we focus primarily on addressing the effect of hydrogen ingression on the long-term ability of optical fibers to accurately record temperature responses in oil and gas wells - particularly in SAGD wells. The useful life of conventional multi-mode optical fibers with germanium-doped, graded-index cores is extremely short at the temperatures in SAGD wells, due to hydrogen darkening.

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