Optimization scenarios for integrated storage systems can be completed quickly and efficiently by utilizing a variable state modeling technique. This technique uses consecutive steady state analyses to simulate flow from storage pools and through transmission pipelines to demonstrate the subsequent inventory change and pressure loss over multiple days of injection or withdrawal. The approach can be used to simulate scenarios from a single pool with limited facilities to a complex system of multiple storage reservoirs, pipelines and compressors. Applying unique reservoir characteristics to the storage flow equation, the flow from each storage reservoir at a given rock pressure can be determined. However, at any time the pressures in different pools will vary depending on the intended operation and current inventory of the pool. As each pool has unique flow characteristics and facilities, an optimized operation will exploit the potential of an integrated system while taking into consideration physical constraints, operational limitations, risks of loss of supply, and the potential to utilize any excess deliverability that may be available. Applications for this modeling technique include seasonal planning, facilities design and identification of potential shortfall or excesses resulting from short term changes in supply or demand. The model can be used to provide an operating plan prior to the beginning of the season or can be used during an injection or withdrawal season as a troubleshooting tool to aid in daily or short term operating decisions. These applications are discussed in this paper.
Union Gas (Union) is a wholly owned subsidiary of Westcoast Energy Inc based in Vancouver, British Columbia. Since 1942 Union has been operating underground storage reservoirs in southwestern Ontario to supplement supply to its distribution customers and ex-franchise transportation customers. With 128 Bcf of working storage capacity, in sixteen depleted pinnacle reef natural gas reservoirs, Union is the largest owner and operator of underground storage in Canada, The focal points for injection and withdrawal to and from storage are the Dawn compressor station and the Union owned Trafalgar transmission lines. (Figure 1: Map of Union's storage pools) Located at the Dawn compressor station is approximately 130,000 hp in six different compressor plants. In addition, Union has several compressors that are dedicated to, and located at, specific storage pools to supplement injection and/or withdrawal. The plan for Union's storage system is to turn over the entire working inventory on a yearly basis. Over the withdrawal season the pools are emptied and during the injection season they are again filled to maximum capacity. The complexity of this operation, while ensuring optimized compression and supplies, requires that Union Gas operate the sixteen pools as one integrated system. Each pool is not considered individually for specific performance, instead, all of the pools and facilities are considered together in order to determine the overall effect. Demand from storage is examined and the system is operated to ensure that the pools available can meet the demand.