A major industry objective for measuring permanent downhole pressure and temperature in extreme wellbores was achieved with the successful field test of an advanced optical gauge system. Conducted in the demanding environment of a multistage hydraulic fracturing application, the test marks the latest advancement in a decades-long pursuit of reliable HP/HT monitoring.

The achievement is key to the deployment of life-of-well downhole pressure and temperature monitoring systems in HP/HT wellbores. The candidate well was the first exploratory shale gas well in the Cooper basin in Australia. Bottomhole temperature in the vertical wellbore was more than 200°C (400°F). Hydraulic fracturing, done in seven stages, required more than 18,000 of psi downhole pressure.

The combination of high injection rates and downhole temperatures was very challenging, particularly with respect to the extreme, rapid pressure-temperature swings encountered when injecting colder surface fluid into the very high-temperature well.

Installation objectives, results

The objectives of installing the optical gauge system in the well were twofold: understanding elevated nearwellbore pressure loss due to tortuosity and accurate downhole monitoring of temperature data during the fracturing operations at extremely high pressures and temperatures.

The Weatherford OmniWell optical sensing system, installed on 4½-in. production tubing, has a pressure-temperature gauge and cable rated to 200°C and 20,000 psi. The system was subjected to 191°C (376°F) and 18,100 psi during the test. Maximum gauge depth was 3,060 m (10,039 ft) in the 3,483-m (11,427-ft) wellbore.

The optical gauge system performed flawlessly in the harsh conditions of the stimulation and later when the well was produced at steady gas flow rates of approximately 57 Mcm/d(2 MMscf/d) of gas.

Real-time pressure-temperature monitoring enabled early screenout detection and prevention to avoid incomplete stimulation treatments. The timely, high-quality pressure-temperature data provided critical information to modify fracturing designs, avoid screenouts, and optimize the stimulation.

Superior quality data relative to surface-acquired data also enhanced near-wellbore pressure loss analysis. When combined with other diagnostic tools, the high-quality data were instrumental in improving the understanding of fracture geometry.

Since the field trial, the new optical gauge has been installed successfully around the world in some of the most challenging environments, including HP/HT wellbores and very high-rate fluid flows in producers and injectors.

Downhole P/T monitoring

The company provides both electronic and optical sensors for a variety of downhole monitoring applications, including pressure-temperature gauges, multipoint temperature arrays, distributed temperature sensing, multiphase flow meters, and multicomponent seismic sensors. High-performance downhole pressure-temperature measurements have historically been acquired using electronic gauges with quartz transducers. These gauges have been deployed with some success, working in certain environments better than others. However, the longevity and reliability is particularly problematic in more demanding wellbore environments.

All-optical approach

To remove failure-prone electronics from the well, a unique nonelectronic all-optical approach has long been considered attractive for monitoring extreme applications and providing life-of-well pressure-temperature data. The system installed in Australia is a third-generation optical pressure-temperature gauge that is the product of about 19 years of industry optical experience.

Much of that experience involves the Bragg grating sensor technology that is key to the current gauge system. Bragg grating technology provides a unique platform on which to build robust multipoint sensors.

Bragg gratings are integrated in the optical fiber to selectively reflect particular light wavelengths. This makes them useful in communication applications, and because these are sensitive to strain, Bragg gratings also make effective sensing elements. In this application the optical devices are photo-imprinted in the core of the fiber to sense strain in the fiber.

However, the fiber Bragg grating-based sensor designs could not achieve the requirements for high-performance downhole monitoring applications. To improve measurement performance and service life, an alternative to fiber was developed. Weatherford’s glass waveguide technology, referred to as “cane,” is larger in diameter than standard fiber and offers unique qualities for stability, sensitivity, and strength while still employing the robust Bragg grating technology.

The Bragg sensor, imprinted in the larger diameter wave-guide, measures pressure- and temperature-induced strain using advanced surface-based optoelectronics. These surface measurements detect wavelength shifts that are reflected in the Bragg grating located deep in the wellbore.

The first such Bragg grating optical pressure-temperature gauge was installed commercially in 2000 for longterm reservoir pressure monitoring in the Gulf of Mexico and is still operational.

The sensor design proved reliable, but limitations existed in some higher temperature wells. One of the weaknesses of the early gauge design was the high-pressure penetration point required to link fiber from the cable at atmospheric pressure to the wave-guide sensor exposed to high wellbore pressure. The original method of protection used high-temperature epoxies to seal the fiber within the cable/gauge transition point inside the protective housing. However, bonding epoxy to the fiber and the metal housing proved tricky and less than ideal.

Wave-guide enhancement

For the second-generation gauge a new method for sealing the glass to the metal was enabled by the availability of the larger diameter glass wave-guide. The sealing method resulted in a more robust high-pressure penetrator to protect well integrity at this critical interface for downhole gauges, where high well pressure meets the signal transmission cable.

Globally, the company installed more than 450 of its early-generation gauges in temperatures up to 240°C (464°F) and at maximum pressures of 16,800 psi with a 95% success rate.

While the second-generation gauge was largely successful, including in applications in ultra-high temperature wells, some potential weaknesses in the design still existed. Using a separate sensor and penetrator required the use of a short piece of fiber to link the two glass elements together. This interface required precise, delicate handling of the fiber, glass, and gauge parts.

Third-generation sensor

To simplify the assembly process, a third-generation gauge was developed that uses the wave-guide as both the sensor and the high-pressure penetrator. This simplified design uses only a single glass element, now affectionately referred to as the “light bulb.”

Confidence in the design was so high that the standard gauge qualification test program was expanded to include extraordinary testing conditions that represent persistent downhole concerns. These included high-level, repeated pressure pulses and rapid depressurization – such as going from 20,000 psi to 0 psi in 1 second. The design verification, qualification, and extended testing – including highly accelerated lifetime testing – performed on the third-generation optical gauge are believed to comprise the most extreme set of tests ever performed on a down-hole gauge.

Monitoring yields knowledge

Testing in HP/HT shale gas hydraulic fracturing has shown the success of the new design optical sensor in extreme downhole environments and marks the culmination of decades of design, testing, and lessons learned in the challenging downhole environment. The reliability and high-quality, real-time data achieved by the latest technology provide the performance demanded for effective life-of-well pressure-temperature monitoring to better understand and optimize production and reservoir performance over the life of the well.