Hybrid measurements improve reliability in gas storage application

ENRICO BARBIERI and ANDREA GABBIANI, Stoccaggi Gas Italia SpA; and GILES POWELL, Schlumberger

Image courtesy of Stoccaggi Gas Italia SpA.

Italy leads Europe in investment in underground gas storage (UGS) projects and is continually seeking technological advancements that render the fields safer and more reliable for the long term. One new technology recently investigated was a hybrid optical-electrical system to increase reservoir monitoring and facilitate higher-capacity UGS in one of the earliest UGS fields in Italy. As part of a strategic energy plan to increase the volume of UGS within the available reservoirs, an operator initiated field studies to determine how to efficiently increase the redundancy of the permanent monitoring in use. Current Italian legislation required certified downhole data, and environmental legislation required constant monitoring of downhole pressure.

The system that was installed combines hybrid cable and connectors to join a premium quartz electrical gauge to a fiber-optic pressure gauge. As a result, one cable provided access to both gauges. By using two gauges with totally different modes of functionality yet similar metrology, the operator could greatly increase confidence about what pressure existed in the reservoir and how the reservoir was behaving over time. This information ensured that the gas storage facility was run safely and within its prescribed pressure limits. The hybrid permanent monitoring solution successfully demonstrated that combining multiple measurement techniques and principles into one cable can provide an alternative method of introducing measurement redundancy. This alternative has great potential value for operators that must reliably demonstrate their reservoir integrity with limited budgets.

BACKGROUND

UGS helps utility suppliers maintain their inventories of natural gas and reduces the risk of interruptions in service that might result from technical, political or other problems. It is used to address seasonal and other fluctuations in local demand, balance flow in pipeline systems, mitigate some of the effects of variations in global prices, and meet contractual and regulatory demands.

UGS systems can be constructed in salt cavern formations, porous rocks and abandoned mines. They do not require costly construction and need very little surface area. Porous rock facilities include saline aquifers and brownfield oil and gas fields, which benefit from having existing infrastructure. In Europe, most UGS sites are depleted gas reservoirs into which natural gas, purchased from elsewhere, has been injected. Gas is injected and extracted either through preexisting wells or new wells drilled specifically for the UGS facility. Important characteristics of a UGS reservoir include its capability to retain the inventory securely for future use and its ability to deliver gas at rates sufficient to meet peaks in demand. UGS wells have long lifetimes (80 years or more) compared with oil and gas production wells, and must tolerate high injection pressures, high production rates and frequent changes in pressure and temperature.

UGS in Italy. The first UGS activities in Italy started in 1964 at Cortemaggiore, northern Italy, about 80 km (50 mi) from Milan. The country has several UGS facilities, mostly located in depleted gas production reservoirs at average depths of 1,000–1,500 m. Increases in production levels are required to meet high gas demand. These increases are expected to come from better use and management of existing facilities and new storage resources. As part of a strategic national energy plan, studies are ongoing to increase the capacity of UGS within existing reservoirs. These facilities are often strategically located close to large cities, where HSE regulations demand high levels of confidence to ensure secure storage and leak-free wells. Reservoir and well integrity over the short and long term are key requirements.

Monitoring downhole pressure. Italian environmental legislation requires certified data from continuous monitoring of downhole pressure in UGS wells. Stogit, operator of one of the earliest UGS fields developed in Italy, reviewed the technology it was using to meet regulatory requirements. Experience with electrical gauges proved that they had delivered stable and accurate measurements over decades. However, with the planned increase in storage facilities’ pressure ranges, Stogit was looking for cost-effective options to efficiently increase the redundancy of its permanent monitoring solution.

Fig. 1. The hybrid cable employed delivers measurements from a dual-gauge electrical quartz system and a fiber-optic pressure gauge.

PROVIDING METROLOGICAL REDUNDANCY

A unique fiber-optic pressure gauge has recently been introduced to the market, exhibiting similar metrological specifications to an electrical quartz gauge. Another new development is a hybrid cable specifically designed to withstand the harsh conditions to which a wellbore can subject both electrical and optical cables. The decision was made to combine these with the field-proven electrical quartz gauge. The result was three independent permanent monitoring systems deployed in a UGS well, delivering measurements from a dual-gauge electrical quartz system and a fiber-optic pressure gauge via a single hybrid cable, Fig. 1. Using two gauges with totally different modes of functionality yet similar metrology would increase confidence in reservoir working pressure, and its behavior over time, in turn ensuring that the UGS facility was run safely and within its prescribed limits. In addition, sufficient space was made available for an additional fiber, giving the opportunity for distributed temperature sensing (DTS) measurements to be taken on a survey basis should this ever be required as part of wellbore troubleshooting, such as leak detection.

Permanent monitoring connections. Most monitoring system failures can be attributed to fluid ingress through a compromised cable connector, usually at the gauge cable head or a splice. A new-generation of premium connectors has been shown to virtually eliminate the chance of system failure, providing significantly more protection for the permanent downhole monitoring system. Based on the proven success of these connectors, and following thorough environmental qualification, the principles of the new premium connections have been applied to the hybrid solution. The high level of reliability that is now achievable allows multiple connections along a cable without compromising overall system reliability.

The connectors come in two families: the enhanced redundant metal-to-metal seal, or a fully welded connector. A unique feature about these connectors is the ability to test each seal during make-up. Efficiency is further supported by a system design that allows the majority of the connections to be made up during manufacturing, thus reducing the required rig time without compromising system integrity. Multi-drop gauge systems are delivered directly from manufacturing to the field in fully welded assemblies, Fig. 2. In the particular case discussed in this article, only a single hybrid cable splice was required onsite, as everything else was manufactured.

Quartz sensors. Quartz sensors provide measurements with high accuracy and stability and with very low drift. Pressure and temperature are measured by precise quartz crystal resonators. The resonance frequencies of two quartz crystals, one mainly sensitive to pressure and a second one sensitive to temperature, are digitized downhole by means of a very stable quartz crystal reference. This quartz crystal reference is used for mixing and digitizing pressure and temperature frequencies. Quartz sensors used by the service company are sourced to a high specification and packaged with proprietary high-temperature electronics. All designs are tested to high degrees of shock, vibration and pressure/temperature cycles.

Fig. 2. Manufacture of the hybrid gauge system.

Fig. 3. The electrical and fiber-optic gauges demonstrated very similar pressure monitoring performance.

Fiber-optic pressure sensors. The fiber-optic sensor used by the service company in this installation employed a proprietary side-hole fiber technology for pressure measurement. This technology relies on the use of an optical fiber that contains holes drilled adjacent to the core and is only attached to the gauge housing at one end, which reduces temperature sensitivity in the pressure measurement and improves gauge drift. When wellbore pressure is transmitted to the fiber, there is a change in the light path, as measured along the side-hole axis. A beam of light transmitted through the pressurized fiber sensor travels at different speeds depending on the polarized axis, creating a birefringent pattern that is dependent on the applied well pressure. This pattern can be measured by appropriate surface instrumentation and analyzed to derive the well pressure. The fiber-optic pressure gauge has high stability, at a measured drift of less than 1 psi/year at 250°C and 15,000 psi.

Hybrid cables. Hybrid cables are a technological extension of the state-of-the-art electrical cables currently used with premium quartz gauges and DTS fiber-optic permanent downhole cables. Similarly, they have been designed to instrument the tubing from the wellhead to the reservoir. A particular feature of the hybrid cables used in this case study is that hydrogen barriers for optical life have been optimized. The hybrid cable technology is designed for environmental temperatures up to 175°C, with a targeted five-year optical life under demanding operating conditions.

The cable contains an 18 American wire gauge (18AWG) electrical conductor and a proprietary fiber loose tube. The fibers are located inside the loose tube in a protective medium. The number of fibers and the stranded construction are adapted to the application required and to the downhole conditions. The external part of the cable includes an outer armor package that helps protect the enclosed fiber(s) and electric line(s) from environmental conditions, ensuring electrical and optical performance for the specified life. The armor is typically an Inconel 825 tube, providing long-term performance in aggressive environments, protected against abrasion and shocks by an 11-mm square elastomeric encapsulation. The particular method of encapsulation is chosen as a function of the downhole fluids and temperature.

JOB EXECUTION AND RESULTS

A new well was drilled into the UGS reservoir by a ram rig, creating a 9⅝-in. cased hole and 8½-in. openhole section. Expandable screens and the lower completion were then run into the hole. The upper completion was stabbed into a sealbore assembly. Included within the upper completion was a gauge mandrel that secured two permanent gauges, both reading the tubing pressure—one based on quartz technology and the second using a fiber-optic method.

The hybrid cable and associated hybrid connectors successfully integrated the premium quartz electrical gauge to the proprietary fiber-optic pressure gauge, delivering measurements from both types of gauge to surface. Similar performances of the two gauges (Fig. 3) have allowed the operator to closely monitor reservoir pressure with increased confidence in the measurements, ensuring UGS operations within the storage system safety limitations while managing to increase efficiency. Sufficient space was made available for installation of an additional fiber in the event that DTS measurements are required in the future. 

ACKNOWLEDGMENT

The authors would like to acknowledge Stoccaggi Gas Italia SpA, the operator of the UGS site. The authors would also like to acknowledge Robert Greenaway, operations manager for onshore and offshore surveillance at Schlumberger, for his contribution to this article.