Improved Monitoring System for Heavy-Oil Steam-Assisted-Gravity-Drainage Wells
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The complete paper provides an overview of the development of fiber-optic sensing for steam-assisted-gravity-drainage (SAGD) applications, including a review of more than 10 years of work in development and field applications in western Canada. Information provided in this paper is applicable beyond SAGD applications. The fiber-optic monitoring systems might also be used for intelligent, subsea, and unconventional wells.
One area that has benefited from the unique advantages of fiber-optic sensing is thermal monitoring of SAGD wells. One particular fiber-optic technology that has proved successful in these thermal-monitoring applications is wavelength-domain multiplexing of Bragg-grating-based fiber-optic sensors. Commonly referred to as array temperature sensing (ATS), the Bragg-grating arrays can be manufactured splice-free for long-term reliability. The number of thermal sensing points can range from a few to well over 100 for a single cable. Each Bragg-grating sensing point provides a real-time, accurate measurement of temperature at that location along the wellbore. ATS systems have been used successfully to monitor the steam-injection and production processes in SAGD wells. ATS systems are capable of performing more-accurate temperature measurements and are able to resolve much smaller temperature features compared with distributed temperature sensing (DTS).
Bragg-grating optical sensors not only provide temperature measurements but also can be packaged in a transducer to provide pressure and temperature (PT) readings. The optical PT gauge is compatible with the surface instrumentation that records the ATS responses, allowing for the PT gauge to be integrated at the end of the ATS optical fiber for single-wellhead penetration, providing multiplexed DTS and PT measurements.
Environmental Effects on Downhole Fiber-Optic Sensors
The requirements for a successful monitoring system in SAGD applications are dictated by the harsh environments experienced downhole. In the past, the presence of hydrogen in these applications has caused issues with various fiber-optic sensing technologies. Improvements in optical-fiber design and downhole cable construction have helped to mitigate some of these issues, but any downhole monitoring system must be able to cope with the presence of hydrogen in relatively high-temperature environments.
Because of the known hydrogen effects on optical fibers, a downhole monitoring system was designed on the basis of Bragg-grating technology. In order to predict expected lifetime of in-well Bragg-grating sensors under different conditions, an empirical model was developed to understand long-term attenuation effects. The model provides a worst-case scenario for attenuation of Bragg-grating sensors at various temperatures. Bragg-grating sensors function by relating a wavelength-shift response to temperature or pressure, so the attenuation effects do not affect the ability to provide accurate measurements. Application of the model is demonstrated in Fig. 1, where attenuation changes from a downhole Bragg-grating-based PT gauge are plotted vs. time. The attenuation model is applied to the trend of the data to provide a prediction of remaining life of the PT gauge as influenced by the presence of hydrogen attenuation effects.
Bragg-Grating-Based PT Gauge
Recently, a new-generation design for the Bragg-grating PT gauge has been introduced for the SAGD pressure-monitoring application. The basis for the new-generation optical PT gauge stems from glass technology developed in the late 1990s. The previous-generation optical PT gauge was based on a bonded Bragg-grating-sensor design, which allowed for a small form factor of ¼ in. in diameter. The small form factor enabled the PT gauge to be integrated easily into a coiled format for installation in the wellbores. However, the bonded Bragg-grating design posed performance limitations in terms of maximum pressure measurement, a nonlinear pressure response, and poor performance in high-vibration environments. With the use of Cane-glass technology for the optical PT gauge, the response to pressure is highly linear and the overall design of the gauge has improved manufacturability and increased the operating pressure range.
Development of any new sensor requires evaluation under expected operating conditions. The qualification-test plan used for the SAGD PT gauge included shock and vibration testing, thermal cycling, and pressure loading. Testing also included ensuring that the PT gauge continued to perform within specifications after experiencing an overpressure to 1.2 times the maximum operating pressure. Multiple prototype PT gauges were subjected to the qualification procedure to ensure that the design was valid across multiple samples. After each test of the procedure, the prototype gauges were tested at room-temperature and -pressure conditions to determine if any damage had been incurred during the testing. Additional testing of the PT gauges included extreme overpressure and overtemperature testing as well as testing to failure.
SAGD Applications for Fiber-Optic Sensing
The use of Bragg-grating sensors for thermal monitoring in SAGD applications began in 2007 with the first installation of a 40-point sensing array. The use of these permanent monitoring systems grew to more than 50 installations per year in 2014. In 2009, an optical PT gauge based on Bragg-grating sensors was introduced to the market. The gauge had the ability to serve as a standalone sensing point at the termination of the optical cable or to be integrated on the same optical fiber and cable as the Bragg-grating sensing arrays for thermal monitoring. For more than a decade, Bragg-grating technology has expanded the number of applications for SAGD wells.
In-well ATS sensors were used to monitor the steam front as it progressed along the length of the horizontal wellbore. The spatial separation and real-time data acquisition of the individual sensors provided a means of determining the velocity of the steam front along the wellbore. Also, the fast data-sampling rate provided an understanding of how fast the region near each temperature sensor reached a saturation value. Thermal-response data can be used by reservoir engineers to model the reservoir and plan for future production operations. This type of data is evaluated not just during circulation, but also following shut-ins to examine changes in the reservoir. Fine tuning of the steam-injection rates, temperature, and pressures has led to a lowering of steaming requirements and to operational savings. Reductions in the amount of steam required also lead to a reduction in the amount of greenhouse-gas (GHG) emissions from the SAGD operation.
During the production phase, thermal monitoring is used to measure the temperature difference between the injection well and the production well. Keeping an optimal temperature difference between the injection well and the production well is a key parameter in optimizing SAGD wells through the steam/oil ratio (SOR) and is referred to as subcool monitoring. Managing the local or zonal subcool with a reliable optical ATS and PT monitoring system along the lateral section of the well optimizes the production rate. Real-time monitoring can enable proportional-integral-derivative feedback-loop control, to reduce the cumulative SOR and GHG emissions and to increase the production rate.
Field Trial of the Improved SAGD Cane-Glass PT Gauge
The ¼-in.-outer-diameter (OD) downhole sensing cable with 40-point ATS optical fiber and the SAGD Cane PT at the tip of the cable was installed in a producer well. The first ATS sensor (ATS 1) was located near the heel of the well; ATS 40 was landed near the toe. The ATS monitoring cable was pumped into a 2⅜-in.-OD guide string using a 1.81-in. polymer pig. The pig was attached to the tip of the 1600-m capillary where the SAGD Cane PT was located. This assembly was delivered through the wellhead hanger, was secured, and was integrated with the sheave mechanism. A capillary spooler pump truck was used to pump the liquid until the pig and cable began to move and the SAGD Cane PT was landed at the toe. The entire surface data-acquisition system and the surface fiber-optic cables were installed within a 4-hour window after the safety meeting. The capillary pumpdown technique used for this installation is not the only available method to install the SAGD Cane PT gauge. There are two other installation methods: (1) running the capillary together with coiled tubing and (2) clamping to the outside casing or tubing.
The measured temperatures between the PT gauge and the nearest ATS sensors were found to be in good agreement. The readings from the ATS sensors displayed an increasing temperature along the wellbore to a certain depth. After this point, the thermal response is observed to decrease at the toe of the well.
The pressure reported by the PT gauge at the toe is consistent with the pressure reported at the heel pump with synchronized evolution and variations and a pressure drop of approximately 100 kPa over the horizontal section. Also, the temperature reported by the toe-located PT gauge is consistent with the temperature reported by the ATS sensor several meters away. The data show that the ATS and PT-gauge in-well system has maintained full functionality for more than 2 years of continuous operation. Some of the earliest-installed in-well systems are still operational after 8 years.
Another application for the ATS and SAGD PT gauge that has been explored recently is coiled-logging operations. The coiled-logging system provides the high spatial resolution of a DTS system with the accuracy and data frequency of the ATS thermal-monitoring system. A SAGD PT gauge is integrated at the end of the ATS cable in the bottomhole assembly, and both ATS and DTS cables are encased in a single coiled tubing. Rigless coiled-tubing deployment provides a cost-effective solution for acquiring accurate temperature and pressure data in the challenging environments of SAGD wells.
Improved Monitoring System for Heavy-Oil Steam-Assisted-Gravity-Drainage Wells
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