Conversion of LPG Salt Caverns to Natural Gas Storage "A TransGas Experience"

Abstract

Abstract Large volume underground storage of hydrocarbons has been developed and practiced in Saskatchewan, Canada for many years. Use of these facilities has gained importance in recent years, due mainly to economical and environmental reasons. Trans Gas Limited currently has 1,259 million cubic metres (106.m3) of working storage capacity. Of this total 450 106m3 is held in solution mined bedded salt caverns and the remaining 809 106m3 is held in depleted gas pools and gas caps. The total gas storage capacity is 2,473 106m3 of which 571 106m3 is held in caverns and 1,902 106m3 is held in depleted pools. All of the depleted gas pools are located on the western side of Saskatchewan, mainly in Lower Cretaceous sandstone reservoirs. The washed salt caverns are scattered across Saskatchewan, all developed in the halite portion of the Middle Devonian Prairie Evaporite salt bed. Traditionally, Trans Gas Limited has developed its solution mined salt storage caverns from start to finish to supply Saskatchewan customers. However, in the increasingly competitive North American natural gas storage market, Trans Gas Limited has had to look for innovative methods to develop cost effective storage facilities while providing new market development opportunities. One method of providing natural gas storage service, used in two recent cases, is to convert existing salt caverns, previously used to store Liquefied Petroleum Gas (LPG) products, to natural gas storage. Trans Gas Limited currently owns, leases and operates 24 bedded salt caverns, of which six caverns are converted LPG caverns. This paper presents a general overview of Trans Gas Limited's present experiences with the conversion of LPG salt caverns to sweet natural gas storage service. Introduction Trans Gas pioneered the development of underground solution mined bedded salt caverns in Canada (and the United States as well) in the early 1960s at Melville, Saskatchewan. The sole purpose was for the dry or brine-free storage of natural gas. The first equivalent caverns in the United States were developed at Eminence, Mississippi in 1970. Prior to that time, salt caverns had been utilized by other companies for the storage of Liquefied Petroleum Gas (LPG) and other hydrocarbon liquids. Salt caverns permitted the highest rate of gas delivery; provided a secure and environmentally safe form of storage; permitted full product recovery; and required the least cost for construction; factors which still hold true today. In the 1990s, Trans Gas purchased outright or leased a total of six former solution mined bedded salt LPG caverns and converted them to natural gas storage. Three of these caverns are located at Regina North and three at Melville South as shown on Figure 1. One quarter of all Trans Gas cavern storage comes from converted LPG caverns. The pre-requisites for cavern conversions are identified in Table 1. Corporate Profile Until 1988, the gas and electric utilities in Saskatchewan were part of one organization, called the Saskatchewan Power Corporation. Recognizing the inherent competition between gas and electricity as energy sources, the utilities separated in 1988 and SaskEnergy Incorporated (SaskEnergy) was created.