Tracking natural gas storage in hollowed-out salt domes is a new application for downhole pressure and temperature measurement tools.

In 2001, a natural gas company decided that measurement in a Texas salt-dome gas-storage cavern could be improved for more precise operation and improved gas inventory tracking. Better data measurement would also help engineers ensure cavern integrity.

Wood Group Production Technology managed the project, designed and engineered the new system, and supplied gauge hardware, downhole cable, feed-through seals and the topside data interface. Wood Group Pressure Control supplied wellhead equipment, including a specially designed tubing hanger. Wood Group Logging Services provided a logging truck and blowout preventer (BOP) lubricator equipment. The combined forces allowed for a single invoice and a single point of contact for technical assistance, system installation and troubleshooting, if needed.

The team installed the new system in February 2002. It consists of a quartz pressure and temperature gauge with weight bars and it is attached to a downhole cable by a special connector designed to handle the tensile stresses involved. Workers lowered the gauge into the cavern through the BOP lubricator. With the gauge at the required depth, the team installed a specially designed tubing hanger and fed the cable through to an exit block. They connected the system to the topside interface system. A surface microprocessor-based unit, which directly inputs data into the SCADA system, provided data gathering and handling. As the system is permanently installed, real-time data can be gathered and analyzed at will, and the need for quarterly wireline runs is eliminated.

Research and development

While the cavern-measurement system has proved successful and is a unique application of an existing product, the client was interested in expanding the capabilities of the system by utilizing research already under way at Wood Group Production Technology. The traditional method used for cavern surveys is single-point measurements obtained at various levels as a wireline runs a gauge in and out of the cavern. The objective is to develop a cost-effective, easy-to-use system for measuring not just pressure and temperature at a specific point, but to determine the complete temperature profile over the entire length of the production tubing. Wood Group Production Technology uses its permanently installed special TEFAC (tubing encased fiber and conductor) cable that will collect real-time, continuous temperature profiles along its full length, in addition to the conventional single-point pressure and temperature measurements.
Such a system also will serve a wide range of applications, from coalbed methane to ultradeepwater wells. In deepwater applications, a temperature gradient can be especially helpful in managing chemical-injection operations. Since critical temperatures for the formation of waxes, hydrates and other flow-impediments are known, knowing temperatures throughout a flowing system obviously will allow more intelligent, effective and economic use of injected chemicals.

The next step is to begin field tests on an available well conveniently near the Wood Group complex in west Houston. Wood Group and the client will continue their close liaison so the client can take advantage of any developments in the total project that may benefit its operations.

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