加拿大工程师测试论文.pdf

1 PAPER 2008 365 Successful Field Application in Continuous DTS monitoring under Harsh Environment of SAGD Wells Using Improvised Optical fiber Technology Case Study from Canada J KAURA WellDynamics J SIERRA WellDynamics This paper has been selected for presentation and publication in the Proceedings for the World Heavy Oil Congress 2008 All papers selected will become the property of WHOC The right to publish is retained by the WHOC s Publications Committee The authors agree to assign the right to publish the above titled paper to WHOC who have conveyed non exclusive right to the Petroleum Society to publish if it is selected 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 ingression Thermal resistance of the materials Mechanical 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 2 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 are at the same wavelength as the laser pulse and are 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 Results are presented from the first full scale field trial of a new single mode DTS system designed for long term reliability in these harsh conditions The system WellDynamics OptoLog DTS HT uses a new single mode optical fiber with a pure silica core and surface equipment modified to take DTS surveys with the new fiber Additionally the system uses a new algorithm for temperature calculations that takes into consideration the effects of minor light losses in the fiber resulting from hydrogen ingression The new OptoLog DTS HT system was installed alongside a conventional system in the same SAGD well Results thus far indicate that after bringing the well on line the conventional fiber failed after only a few days of operation however the system using the new fiber is fully operational almost six months into the field test This paper presents the following technical information Results from a successful field test of a new single mode DTS fiber that can withstand the high temperature SAGD environment A new single mode DTS fiber for SAGD applications Comparison of field performances of the new single mode fiber versus that of a conventional fiber in harsh SAGD environments Comparison of DTS High Temperature HT Fiber and Conventional Optical Fibers In this section we examine the performance of the new HT fiber and contrast it with that of many conventional fibers being used in the oil and gas industry The new fiber is a single mode pure core optical fiber rated for temperature applications up to approximately 300oC It is important to note that several single mode fibers are employed in commercial downhole applications as are several pure core optical fibers Similarly there are a few 3 hermetic and non hermetic fibers that are rated to 300oC However to the authors knowledge none of these conventional fibers has demonstrated sustained acceptable long term performance in a hydrogen rich SAGD environment a Descriptions of Various Optical Fibers Used in Oil Field Applications Optical fibers have generally cylindrical cross sections with an inner and outer layer The inner layer is referred to as the core while the outer layer is called the cladding In order for a fiber to act as a wave guide and support the long distance propagation of light the refractive index of the core must be greater than that of the clad A wide array of optical fiber products is currently being used in the oil and gas industry distinguished by the mode of operations compositional makeup of the glass fiber nature of protective covering around the glass fiber and the temperature ratings Regarding the mode of operations optical fibers are either single mode or multi mode depending on the number of optical modes that they can support A mode is a set of electromagnetic waves that is guided or propagates for long distances through an optical fiber Single mode fibers can support only one fundamental mode As the name implies multi mode fibers support multiple modes Some multi mode fibers allow propagation of hundreds or thousands of modes Single mode and multi mode fibers are usually manufactured from similar materials so the main difference between them is the size of their cores The most common type of single mode fiber has a core diameter of 8 to 10 m Multi mode fibers are usually manufactured with core diameters as small as 50 microns and as large as hundreds of microns Both single mode and multi mode fibers are subdivided into step index or graded index refractive index fibers depending on the profile distribution of refractive index from the core to the cladding as demonstrated in Figure 1 In a graded index fiber the index of refraction in the core decreases continuously between the central axis and the cladding Multi path or modal dispersion is lower in a graded index multi mode fiber than in a step index multi mode fiber Figure 1 Profiles of Step Index and Graded Index Single Mode and Multi Mode Fibers Compositionally the primary material used in the manufacture of optical fibers for downhole use is high quality silica SiO2 This glass contains very low amounts of impurities such as water or elements other than silica and oxygen Small amounts of elements or materials called dopants are typically added to the core region of the glass to increase its refractive index For the fiber reported here the core is pure undoped silica or pure core fiber and dopants are added to the cladding region in order to depress its refractive index Optical fibers also vary according to the nature of coatings protecting the glass fiber from the harsh environmental conditions These coating are made of a combination of materials including carbon polyamides silicone steel and many chemical alloys b Advantages of HT Fiber Pure Core over Conventional Optical Fibers Multi mode fibers have several advantages compared to single mode fibers The larger core sizes make it easier to launch light into the fiber and to make fiber connections During fiber 4 splicing core to core alignment becomes less critical than for single mode fiber These advantages have propelled many commercial applications of multi mode fibers in several industries Unfortunately multi mode fibers also have some disadvantages Each individual mode propagates along the fiber at different speeds This speed difference leads to modal dispersion which causes light pulses to spread as they propagate down the fiber As the number of modes increases the effect of modal dispersion increases Modal dispersion affects system bandwidth and fiber manufacturers have adjusted the core diameter refractive index profile and other parameters of multi mode fibers in order to minimize modal dispersion and maximize system bandwidth Single mode fibers also have their advantages compared to multi mode fibers Since a single mode fiber supports only one guided mode there is no modal dispersion The fundamental advantage is the ability of single mode fibers to carry a higher capacity of information over very long distances with minimal loss of spatial and spectral integrity This tremendous information carrying capacity and low intrinsic loss have made single mode fiber the ideal transmission medium for a multitude of longer distance and higher bandwidth applications The new single mode fiber presented in this work has added advantages as a result of manufacturing changes that eliminate the need for germanium dopants in the core for refractive index grading The pure silica core of the fiber makes it less susceptible to hydrogen damage especially at higher temperatures which is an obvious advantage for SAGD applications Laboratory Performances of HT Fiber versus Conventional Optical Fibers The new DTS HT system has demonstrated marked resilience over conventional fibers in the presence of hydrogen under controlled laboratory conditions As noted above this new single mode fiber has excellent performance against hydrogen compared with conventional single mode and multi mode fibers The heat resistant coating can withstand wellbore temperatures up to 300oC Hydrogen molecules negatively impact the performance of optical fibers and their ability to accurately determine temperature profiles in two critical ways hydrogen molecular absorption and molecular chemical reaction These are reviewed individually in the following sections a Hydrogen Molecular Absorption The SAGD operations designed for the production of high viscosity hydrocarbons represent a hydrogen rich environment Hydrogen molecules come from many sources including the produced hydrocarbons protective coatings around the glass fibers and corrosion of metal tube or cable exterior These free hydrogen molecules penetrate into glass and impact light transmission through the fiber Conv SM FiberConv SM Fiber HT FiberHT Fiber Figure 2 Measured Light Loss in Conventional and HT Fibers versus Wavelength as a Result of Hydrogen Absorption 5 Figure 3 Hydrogen Absorption Rate as a Function of Partial Pressure It is important to mention that this form of hydrogen damage affects all kinds of optical fibers including the new single mode fiber under consideration Figure 2 compares the wavelength dependent losses in conventional single mode fiber with that of the new fiber Note that the losses for these two fibers are essentially identical This example demonstrates that the effects of molecular hydrogen absorption exist in the presence of hydrogen regardless of fiber type Figure 3 shows that the rate of hydrogen molecular absorption is directly influenced by the temperature and partial pressure of hydrogen surrounding the fiber It is noted however that once the hydrogen source is removed and the unreacted molecular hydrogen has time to diffuse out of the fiber the light loss in the new fiber returns to its original value In other words hydrogen darkening from molecular hydrogen is reversible b Hydrogen Chemical Reaction Conventional single mode and multi mode fibers are made up of SiO2 and GeO2 since germanium is a common dopant in the cores of optical fibers used to increase the refractive index of silica When hydrogen diffuses into the core of a conventional fiber it can react chemically with the dopants and other impurities in the fiber to form stable chemical compounds These new chemical species can increase light loss or attenuation and remain in place even if molecular hydrogen is removed In other words hydrogen darkening in fibers due to chemical reactions is permanent or non reversible The new fiber on the other hand has a core region that is made of pure silica or SiO2 without dopants Since dopants such as germanium are not present hydrogen cannot react with these impurities to form absorptive molecules in the core Thus the new fiber is resistant to chemical reactions in the core due to hydrogen ingression even at elevated temperatures This resilience to chemical attack or non reversible hydrogen darkening is demonstrated in Figure 4 As shown practically no light loss resulting from hydrogen molecular reaction is present in the new DTS HT optical fiber However significant losses are observed in conventional single mode fiber with a germanium doped core DT S Conv SS Fiber DT S Conv SS Fiber HT FiberHT Fiber Figure 4 Rate of Light Loss for Different Wavelengths as a Result of Hydrogen Chemical Reaction 6 Field Tests of DTS HT versus Conventional Optical Fibers Extreme conditions of SAGD operations require optical fibers in DTS applications to be able to resist hydrogen ingression under high partial pressures as well as to withstand high temperatures JACOS provided a typical SAGD well in Alberta Canada as test case for the new DTS HT optical fiber a Overview of the Field Trial This Jacos SAGD well is typically operating within a temperature range of between roughly 200oC and 300oC The well is equipped with a U type capillary tube through which optical fibers could be run It is also being monitored with thermocouples which provide temperature readings at three fixed points along this horizontal well Figure 5 is a wellbore schematic showing the locations of the thermocouple pairs at approximately 786 meters 1143 meters and 1518 meters from the wellhead A conventional multi mode fiber was pumped into one side of the looped capillary tube that was previously installed in the well via coiled tubing The new single mode fiber was installed in the opposite side of this same looped capillary tube This configuration provides an excellent setting for side by side comparisons of DTS system performance over time Figure 5 Wellbore Profile of the SAGD Horizontal Producer Throughout the trial which to date has progressed almost six months temperature data for both the conventional multi mode fiber and the new single mode fiber were compared to the multiple thermocouples This field trial is currently ongoing b Optical Time Domain Reflectometry OTDR Data Multiple OTDR readings have been recorded at fixed time intervals throughout the duration of this field test to ascertain the degree of light loss occurring across the entire fiber in these harsh conditions over extended periods of time For the new DTS HT single mode fiber the OTDR information was recorded using 850 nm 1310 nm and 1550 nm wavelength laser pulses Similarly OTDR data was recorded for the conventional multi mode fiber at 850 nm 1300 nm as well as 1550 nm wavelengths The 1550 nm wavelength curves recorded from the new HT Fiber on August 27 2006 September 05 2006 September 26 2006 October 24 2006 December 06 2006 and February 06 2007 are presented in Figure 6 The 1310 nm wavelength profiles recorded at the same time periods are shown in Figure 7 while similar curves recorded using 850 nm wavelength light pulses are presented in Figure 8 Identical curves were recorded from the conventional multi mode fiber and are presented in Figure 9 Figure 10 and Figure 11 for the 1550 nm 1300 nm and 850 nm wavelengths respectively In Figure 6 we observe by comparison of light losses over the five month period that there is only minimal change of approximately 0 33 dB km in the light loss characteristics of the HT Fiber using the 1550 nm wavelength light pulses Within the same time period we observe from Figure 9 that the performance of the conventional multi mode fiber has deteriorated quite significantly and light losses in the fiber have already reached unacceptable levels over approximately 13 dB km It should be expected that subsequent loss readings from the conventional multi mode fiber in this extreme environment should continue to severely deteriorate For the new HT Fiber however we expect minor light changes in losses in the OTDR readings in the long term 7 that is almost six months after the installation of the fiber Figures 7 and 10 demonstrate long term light losses in the two optical fibers using 1310 nm 1300 nm wavelengths We observe clearly that at these wavelengths again there are severe light losses in the case of the conventional optical fiber while only minor light losses are seen in the case of the new HT Fiber Even worse performance is observed from comparison of Figures 8 and 11 for 850 nm wavelengths In this case the light losses are so severe that it is unlikely that any useful information can be received at the bottom half of the fiber c Stokes and Anti Stokes Readings The Raman Stokes readings from the new HT Fiber are presented in Figur