ToreOVD Helps Maintain High Deliverability of Gas Storage in California

Tuesday, 19 January 2010

Temporary storage of natural gas is an important component of energy supply systems around the world. In the USA, approximately 400 facilities provide over 8 trillion cubic feet(TCF) of gas storage capacity to smooth out variations in supply and demand between the winter heating and summer storage seasons. The majority of these facilities are depleted gas or oil reservoirs that utilize existing wells, piping, and infrastructure that are left behind after the original reserves have been produced.

While storage capacity is an important metric, deliverability and reliability are key factors that affect the economics of a gas storage operation. Deliverability is a measure of the rate at which gas can be delivered to market from a storage facility and is usually expressed as Million Cubic Feet per Day(MMCFD). Deliverability is most affected by the number and condition of the wells that access the storage reservoir. Higher deliverability rates are of greater value to gas storage customers.

Reliability represents the operator’s ability to make the gas available for delivery to the market or to make the facility available to receive gas for storage. Customer demands for storage and withdrawal change on an almost daily basis, and the facility operators must be able to accommodate storage and withdrawal nominations on very short notice.
In the withdrawal cycle, the gas that is delivered from the reservoir must pass through a gas-liquid separator to remove entrained liquids prior to delivery to the pipeline network. Sand production from the wells poses the risk of separation problems from reduced residence time, blockage of critical level sensing instruments, and sand carryover that can damage downstream equipment. These problems can lead to inconvenient and unplanned shutdowns of the facility if not monitored and managed. While sand production can be partially controlled by reducing individual well production rates, this approach reduces overall deliverability.

Lodi Gas Storage, L.L.C. owns and operates the Lodi Gas Storage facility near Sacramento, California. This depleted reservoir storage facility has the capacity to store 21 billion cubic feet of natural gas. The reservoir is accessed through 15 wells which extend to approximately 2,200 feet beneath the surface and cover approximately 1,500 acres. Lodi experienced an ongoing problem of solids deposition in the separators during heavy withdrawal periods. To maintain reliability of the system required extra manpower to manually blow out the sand from the bottom of the separators and to keep the level instruments operating. To meet the needs of their customers, Lodi would man the separators around the clock when necessary.

Lodi Gas Storage retained an engineering contractor to study the solids issue and provide recommended solutions to improve their operation. Alternatives consisted of installation of cyclonic desanders to remove the solids from the gas stream before the two-phase separators, and Merpro’s Tore® Online Vessel Desanders (OVD) installed in the two-phase separators. The cyclonic desander option could be installed without taking the facility offline, but carried a much higher capital investment, and could not guarantee to keep all of the solids from entering the separators. TORE®OVD was selected as the best option.

Merpro provided a complete sand handling system which included the TORE®OVD vessel internals, an Inline Pressure Reducing Cartridge(IPRC) to allow the sand to be discharged from the vessel while online, and a Solids Bagging System for collecting the sand and returning the produced water back to the system. The entire system was designed in Houston, Texas by Merpro Americas. The IPRC and bagging system were built in Houston, while the TORE OVD was fabricated at Merpro’s ISO 9001 approved alloy welding facility in Montrose, Scotland.

The Tore® OVD system will be operated approximately once per week during normal operation, and more frequently if necessary during heavy withdrawals. It will be fed with recycled produced water from the available water injection pumps. The feed water will develop a vortex beneath the Tore®s, which will fluidize and hydro-transport the solids from the vessels, delivering them to the sand bagging system.
The challenge for the TORE®OVD option was to deliver the equipment within the small window of time between the injection and withdrawal seasons, and to minimize the amount of time the vessels would be out of service for installation. Merpro delivered the TORE®OVD vessel internals in 7 weeks from receipt of the order, meeting the critical installation window. The modifications to the vessel included the addition of feed and discharge nozzles and support clips. Total time out of service was 5 days per vessel to make the vessel modifications, recertify the vessels, and install the internals. The IPRC and bagging system were delivered at 10 weeks, and the entire system was ready to be commissioned within 4 months of the decision to install the Merpro sand handling system.

“Merpro’s cooperation with our engineering consultant and vessel contractor helped us achieve a quick and clean retrofit,” said Scott Wilson, Vice President Lodi Gas Storage, L.L.C. Mark Wolf, President of Merpro Americas, Inc. also said, “The Lodi project gives Merpro a deeper understanding of the gas storage industry. We look forward to helping more facility operators improve reliability and deliverability of gas storage networks in the USA and elsewhere.”