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Heading Font Arial 16 PT Downhole Fiber-Optic Distributed Temperature Sensing System Executive Summary Weatherford offers permanent fiber optic Distributed Temperature Sensor (DTS) monitoring systems to provide high quality, real-time temperature profiles of the well bore for the life of the well. Temperature data is collected every meter along the well bore with capability to achieve measurements with 0.1 C temperature resolution. The upper end capability of the system not a function of the DTS instrument but is a function of the operating limit of the optical cable, which is currently 175C and 15,000 psi. The DTS fiber optic sensing fiber is an integral part of the Weatherford Completion Systems optical downhole cable, allowing the unique capability to mix distributed temperature sensing with fiber optic pressure, temperature, flow and seismic measurements. The optical downhole cable is deployed with the production tubing string, providing the total coverage of complex and multilateral wells. DTS measurements can also be extended to monitor risers and flow lines to support flow assurance applications. Since the fiber is the sensor, there are no electronics, it is electrically passive, reliable and overcomes the design limitations imposed by harsh downhole environments. Table of Contents Executive Summary...1 1. Introduction...2 2. Distributed Temperature Sensor Technology...3 3. WFT Fiber Optic DTS System...4 4. System Performance...6 5. Surface Instrumentation...7 6. Data Applications...8
1. Introduction Development and exploitation of oil and gas resources in increasingly difficult operating environments such as deepwater raises many technical challenges. Among these is the ability to provide assurance on the completions and production from high-cost and complex wells. Real-time, permanent production and reservoir monitoring is a critical technology for providing assurance and maximizing profitability of these fields. Recent developments in fiber optic sensing technology have resulted in reliable alternatives to conventional electronic systems for permanent, downhole production and reservoir monitoring. Permanent, in-well fiber optic sensors are now being developed and deployed in the field at an increasing rate. Fiber Optic Distributed Temperature Sensor (DTS) based fiber optic systems provide the unique ability to provide a reliable, non-obtrusive means of continuous temperature logging of wells, enabling full coverage of complex and multilateral wells with a single optical fiber cable. These systems are being installed worldwide in a variety of operating environments for a variety of applications. Temperature logging of wells is a well-established procedure in the industry. It is used to gain insight on the production characteristics of a well. Wellbore temperature data is used to analyze fluid flow; characterize oil, water and gas production; and monitor water, gas and steam injection production performance. Historically, temperature logging of wells has been done by placing a probe in a well, usually by wireline, and recording the temperature as it is lowered into the well. Due to the disruptive nature of some wireline services, the assessment of wells is usually done only if there is a problem with the well. To support operators desires to know immediately when there is a change to a well s production characteristic and to minimize production disruption, permanent temperature logging capability has been developed based on fiber-optic DTS technology. Distributed temperature sensing was one of the first applications of fiber optic sensors for inwell monitoring. This technique has been deployed in 100+ wells and has gained wide scale acceptance by providing valuable information for well optimization. These truly distributed systems can provide a mapping of the temperature along the wellbore with ~1 m axial resolution. In these systems, the optical fiber itself is the fundamental sensing element. Weatherford has incorporated DTS capability into its downhole fiber optic infrastructure, which also supports downhole fiber optic pressure, temperature, flow and seismic sensing systems. Integrating DTS data with measurement parameters, such as pressure and flow rates, the operator can attain a new level of understanding of the well s production characteristic and performance. The information attained from well production performance monitoring using multiple measurement parameters is not limited to the points where the discrete pressure and flow measurements are made, but along the total length of the well. In addition, incorporation of DTS capability to a well can be leveraged, adding riser and flow line monitoring to support flow assurance management with an incremental cost of additional fiber optic cable. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 2
Weatherford's DTS system extends the line of high performance fiber optic downhole sensors available to give the customer a complete toolbox to tailor exceptionally reliable downhole fiber optic sensing systems to meet their unique requirements. 2. Distributed Temperature Sensor Technology The primary means to measure the distributed temperature of an optical fiber is to send a pulse of light down the optical fiber and record the returning light. Distributed temperature sensing technology is derived from technology used in the telecommunication industry to measure the loss in fiber optic communication cables. As light travels down an optical fiber, a portion of the light is reflected back to the source of the light much like the blinding light a driver sees when driving through fog with your headlights on full beam. The reflected light the driver sees is caused by small water droplets suspended in the air acting like tiny glass beads, refracting and reflecting the light back to the car. In an optical fiber, the reflections back to the light source are caused by imperfections and materials added to the optical fiber. Along with the reflected light, light referred to as backscatter light can be measured from the optical fiber (figure 1). Backscatter light is generated when the source light interacts with molecules in the optic fiber s glass core, generating light with different colors than the reflected light. By measuring the relative intensity of two of the backscattered light s colors, temperature of the optical fiber can be measured. Since the speed of light in glass is known, it is possible to determine, by tracking the arrival time of the reflected and backscattered light, the precise location of where the light came from. With knowledge of where the backscattered light came from and measuring the relative intensity of the backscatter light colors, a DTS instrument can produce a plot of the temperature versus distance along the optical fiber. fiber ity Scattered optical signals Scattering Process TEMPERATURE, T Propagating light Figure 1 Light pulses travelling down an optical fiber produces Intens Anti-Stokes Raman Band Brillouin Raleigh Stokes Raman Band The primary backscatter measurement technique used by Weatherford DTS instruments is Raman Backscatter (figure 2), which measures the molecular energy state of an optical fiber s glass core. The energy state is a measure of the optical fiber core s molecular vibration, which increases Wavelength Figure 2 Raman backscatter is comprised of two bands, Anti- Stokes which changes with temperature and Stokes band, which does not. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 3
and decreases with temperature. When a pulse of light is transmitted down an optical fiber, Raman Backscatter it generated. Raman Backscatter is comprised of two components, the Stokes and the Anti-Stokes. The intensity of the Anti-Stokes color band changes with temperature while the intensity of the Stokes color band remains practically constant. The relative intensities of the Stokes and Anti-Stokes signals are used to calculate the temperature in the fiber. By sampling at a time increment of 10ns, a DTS instrument can collect temperature data points approximately every meter along the length of a fiber. The distance through an optical fiber that a DTS instrument can provide high-resolution temperature measurements is dependent on the instruments ability to detect the very weak returning Raman Backscatter signals. DTS instruments utilized by Weatherford can provide temperature measurements with 0.1 C resolution at lengths of fiber up to 10kilometers long. 3. Weatherford Fiber Optic DTS System Weatherford's permanent, in-well fiber optic DTS monitoring system consists of three subsystems, as shown in Figure 3: instrumentation unit; wellhead outlet and surface cable; and inwell cable and connectors. Instrumentation. The DTS instrumentation unit can be used independently for short-term logging applications or can be integrated with a microprocessor, monitor, keyboard, associated power supplies, disk drives, and data communication interfaces for permanent monitoring applications. The DTS instrument also contains the software required to control the data acquisition, conversion, storage and interfacing. In standard permanent monitoring implementations, the instrumentation is designed to reside in a control room environment and interface with an external data management system. Wellhead Outlet and Surface Cabling. The wellhead outlet provides for feedthrough and exiting of the fiber optic cable Wellhead Equipment Cables & Connectors Qualified for high temperature & pressure, H2S, corrosives Surface Instrumentation Transducers Rugged, Scalable, Distributed Sensing, Multi-well Pressure, Temperature, Flow, Liquid Fraction, Seismic Fig. 3 - Components of Weatherford's permanent, in-well fiber optic monitoring systems with optional transducers. from the well in a safe and reliable manner and is similar to that for an electrical system. The standard wellhead outlet contains a minimum of two sealing barriers to every potential leak path and is rated to a working pressure of 15,000 psi. It has been DNV certified, both for design and manufacture. The connection from the wellhead outlet to the instrumentation unit is made with the optical surface cabling. On multi-well installations, a multi-core surface cable can be run from the instrumentation unit to a junction box in the well bay and separate surface cables run from the junction box to each well. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 4
Cable and Connectors. The in-well fiber optic cable and connector system provides for light transmission to and from the downhole sensors. It is specifically designed for mechanical and environmental robustness, as well as functional redundancy, and incorporates multiple protective barriers between wellbore fluids and the optical fiber. Every attempt has been made to give the cable a look and feel similar to its electrical counterpart. Mechanical strength and protection of the cable is provided by a ¼-inch metal capillary tube, encapsulated in a polymeric buffer. The tubing encases a specially coated, small-diameter stainless steel fiber in metal tube (FIMT) surrounded by a buffering material. The optical fibers are packaged in the FIMT with a hydrogen gettering grease, which provides high striction forces for holding the fiber in place. Together with the cable, high-reliability optical connectors and cable fusion splicing techniques have also been developed for long-term survival in harsh downhole environments. Weatherford is uniquely positioned to combine DTS with discrete sensors onto a single armored fiber optic cable to surface. This unrivalled capability is made possible by incorporating single-mode fibers employed by the discrete sensors together with a multi-mode fiber for the DTS into a single package as shown in cross section below. Single Mode DTS Multi-Mode Optical Fibre.. Figure 4 Cross-Section of Downhole Fiber Optic Cable Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 5
4. System Performance Weatherford baseline DTS monitoring system utilizes a high performance DTS instrument to support characterization of horizontal wells and distributed flow analysis. Specifications for the baseline DTS instrument are given below. For applications that do not require the baseline instrument s high level of performance, such as vertical well injection breakthrough monitoring, Weatherford can implement a DTS system that is tailored to meet the applications specific requirements. Table 1. Performance Specifications for Baseline DTS monitoring system. Baseline DTS System Instrument Environmental Condition Temperature 0 C 40 C Relative Humidity 85% max Supplied Power AC100/200V, 50/60Hz, 160 VA Instrument General Specification Optical Fiber 50/125 Graded Index (ITU-T G.651) Dimension 425Wx222Hx450D (mm) Supplied Power AC100/200V, 50/60Hz, 160 VA Downhole Sensor Operating Range Temperature -40 C 175 C Pressure 0 15,000 psi Temperature Measurement Specification Max Measurement Range 15 kilometer (7.5 db optical budget) Sampling Resolution 1 meter Spatial Resolution <2 meters (10% - 90% rise time) <4 to 5 meters (to achieve Temperature Precision) Temperature Precision +/- 1 C Averaging Time 80 minutes 20 minutes 5 minutes Temperature Resolution at 10km 0.1C 0.2C 0.4C A differentiating feature of Weatherford's fiber optic DTS monitoring system is that when the DTS monitoring system is implemented with Weatherford downhole pressure/temperature transducers, they can be utilized to eliminate drift and inaccuracies associated with competing DTS monitoring systems. This means that as downhole conditions change over the life of the well, the operator is assured that the DTS system will continue to deliver accurate temperature logs. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 6
5. Surface Instrumentation The surface instrumentation for the Weatherford fiber optic DTS monitoring system is available in three different models: standalone DTS instrument, RMS1, and RMS2. The standalone DTS instrument is primarily for single well, short-term logging applications. The DTS instrument produces distributed temperature data files that can be converted to standard API temperature log reports and data files. Data is retrievable from the DTS instrument by a variety of means, including download onto an external laptop computer. Configuration of the system, setup of data collection parameters, and system diagnostics are also achieved with an external laptop computer. The unit is capable of being powered remotely, such as by batteries or solar panels, and of operating in an external environment up to 40ºC. Fig. 6 - RMS1 surface instrumentation system. The RMS 1 surface instrument, shown in Figure 6, is designed for control room or other controlled environment applications. It uses the baseline DTS instrument to produce DTS traces. An optional optical switch can be added to monitor a high number of wells sequentially. Fiber optic pressure and temperature gauges can also be incorporated into an RMS1 system for integrated well monitoring. The RMS2 surface instrument offers the greatest flexibility and capabilities for integrated distributed temperature and multiple fiber optic pressure and temperature gauges. RMS2 can contain multiple pressure and temperature gage interrogator units to monitor up to 18 gages. An on-board computer controls data acquisition, monitors gage temperatures and pressures, writes data files to a hard drive, and communicates data to other systems. Several output options are available, including: Local monitor; Modbus via serial cable to the platform RTU/SCADA system; Modbus TCP/IP; Remotely accessible via WAN directly to PC; Separate, remotely accessible hard drive attached directly to customer LAN/WAN. Fig. 7 - RMS2 surface instrumentation system. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 7
Other options for RMS2 include an uninterruptible power supply, optical modem, and a line power conditioner. The unit can be supplied in a stand-alone cabinet, as shown in Figure 7, or in a 19-inch rack mountable configuration. 6. Data Applications The past several years have seen a great increase in the development, deployment and application of permanent in-well monitoring systems. Drivers behind this increase include new field developments in much more challenging, costly operating environments; the requirement to provide assurance on the production from these new fields; and the desire to optimize management of production and reservoir recovery. Cost. Many large, new fields coming on line today and in the near future are being developed with relatively few high-cost, high-rate, complex wells. Intervention costs in these wells will be high or even prohibitive. This puts a premium on the value of real-time downhole data during production and on the use of this data to foresee and prevent well problems. Assurance. The large, up-front capital investment for many new field developments, such as deepwater, puts a tremendous importance on the assurance of producing the anticipated volumes of oil and gas in the anticipated timeframe, in order to make the required return. Downhole monitoring systems provide data to continuously assess the health of the well, optimize well operations, and provide assurance on the flow of oil and gas. Optimized Production and Reservoir Management. Real-time downhole data offer many opportunities to greatly improve production management and reservoir recovery. These include production and injection profiling in horizontal and multi-zone wells to identify and control fluid flow to and from different parts of the well; optimizing drainage; and increasing overall field recovery. In most, if not all cases, the value derived from real-time, downhole monitoring systems greatly exceeds the cost and can be recovered early in the life of the well, IF these systems are reliable and perform as specified over the life of the well and IF the data are managed properly and used to their fullest potential. Fiber optic-based sensing systems being deployed today offer the promise of achieving the level of performance required to achieve this value. 6.1 GENERIC APPLICATIONS Gas Lift Optimization. In order to optimize gas lift production it is imperative that the entry point for the lift gas is precisely controlled and monitored. The Joule-Thompson effect on gas flowing through a gas lift mandrel cools the producing fluid, providing a means for detecting which mandrel(s) are passing gas through acquisition of temperature logs. In addition, a mandrel that is slugging gas, rather than operating normally, would be readily identified using time lapse thermal monitoring. Sub-optimal efficiency can thus also be rectified. The most common conventional method for acquiring temperature logs is to run pressure and temperature memory gauges on slickline and conduct a flowing temperature gradient Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 8
survey. However OPEX associated with this activity is appreciable and involves considerable logistics. An alternative technique for acquiring temperature profiles is to incorporate fiber optic cable with the completion down to the lowermost gas lift mandrel. This fibre optic cable would be secured to the outside of the completion using conventional cable clamps and exit the wellhead in the same fashion as other control/chemical injection lines. The cable is terminated into a pressure-blocked wellhead outlet (junction box), with armoured fibre optic deck-cable used to route the downhole DTS signal to a data collection point in the control room. Well Temperature Log 0 10 20 30 40 50 60 70 80 90 100 110 120 0 400 800 1200 1600 Onset & Location Of Water Production. In highly deviated/horizontal wells there is often a great desire to detect onset and location of water production in the open hole lateral(s). The most common traditional technique involves use of production logging tools that, in this instance, would need to be deployed on coiled tubing owing to the high hole angles. Again however, OPEX associated with this activity would be extremely high and would involve considerable logistics. 2000 2400 2800 3200 Perforated Interval An alternative technique would be to again use DTS fiber optic cable deployed along the Figure 8 Well Temperature Log (custom format) length of the main lateral and again analysis acquired temperature profiles on a time-lapse basis. Of course, variations in temperature along the horizontal lateral will be small compared with the profiles acquired in vertical sections of the wellbore. However, subtle variations do exist owing to heating of oil through Joule-Thompson effects. The presence of water, which has significantly greater thermal conductivity and is less affected by the Joule-Thomson effect, will tend to flatten temperature profiles. Through superposition of consecutive time-lapse temperature profiles, it is thus possible to qualitatively determine both onset and migration of water production along the lateral. Perforated pipe is employed to convey the DTS fiber optic cable along the length of the lateral to the toe. The exact choice of tubing would be determined from torque and drag calculations. This technique has been successfully employed in extended reach snake well for an operator in Brunei in June 2001 and again in May 2002. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 9
Estimation of Water Production through Modeling. Use of thermodynamic models that exploit the difference in thermal conductivities and Joule Thomson heating-cooling effects can be employed to yield first-order quantitative estimates of water production from analyses of the acquired temperature profiles. WCS has access to DTS analysis and interpretation software that is offered as a service in conjunction with acquiring DTS profiles. Downhole Fiber Optic DTS E. J. Zisk - September 23, 2002 - Page 10