The Use of Inert Base Gas in Underground Natural Gas Storage

Abstract

Abstract The use of less expensive inert gas to substitute all or part of the base gas requirements in underground natural gas storage fields is a promising technology that has been successfully field tested in France. This paper discusses geological, reservoir, and operational factors that need to be considered in selecting an underground storage field for inert gas use. A storage field in the U.S. has been selected to illustrate detailed data collection and analysis that will lead to formulation of a plan to inject inert gas and predict long term field performance using reservoir models. High base gas fraction, the presence of closed structure or an isolated area away from the injection/withdrawal wells, absence of large scale heterogeneity, and availability of adequate data for reservoir modeling are favorable features that would make a field an attractive candidate for inert gas use. The degree to which natural and inert gases will mix during the storage field operations can be predicted with the help of reservoir simulators now available. Introduction In an underground storage field, a large part (40 – 70%) of total gas stored is used as cushion to provide a desired deliverability, particularly at the end of the withdrawal season. When a storage field is abandoned, a significant portion of the base gas is not economically recoverable. Base gas costs are a major cost item for new storage fields. This is particularly true for aquifer storage fields, which tend to have a higher-fraction of base gas (about 60% in 1983) and at the same time lose more gas at abandonment than depleted fields. In new storage fields where cushion (base) gas must be supplied at present prices, substantial reduction in cost can be achieved by using less expensive inert gas instead of natural gas as base gas. The use of inert gas to replace all or a part of existing storage field base gas requirements may also be feasible when viewed with the following perspective:The replaced natural gas will be available for use by consumer.There may be a possibility of sharing the differential cost (cost difference between the current selling price of the replaced gas and initial acquisition price) between consumer and storage company.Replacement of natural gas by inert gas will result in saving of valuable natural -gas which would have been lost at abandonment. A number of investigators, have examined the feasibility of an inert gas cushion in gas storage. The key technical issue identified by these studies is mixing 1 between inert base and working gases. One study expressed concern that mixing between inert and pipeline quality gases may reach such proportion that separation would be required to yield pipeline quality working gas. This would erode some advantages gained by less expensive inert base gas. Application of inert gas in fields with natural fractures, large scale heterogeneity and irregularly placed injection/withdrawal wells (scattered all across the reservoir) would undoubtedly lead to quick production of inert gas. In view of this, the concerns expressed by earlier investigators may be genuine to some extent. However, careful planning to avoid quick breakthrough of inert gas and utilize those criteria that can keep the heavier inert gas away from the location of active storage wells would reduce the chances of mixing inert gas with natural gas. P. 353^