Storage of LPG in Gas Reservoirs

Abstract

To store great volumes of LPG at minimum cost, nonassociated gas reservoirs have proved satisfactory in the Borregos field of South Texas. For maximum success, this method of storage requires an exhaustive geologic description of the reservoir, or, lacking this, careful monitoring of reservoir pressure. Introduction The petroleum industry has rapidly responded to the increased demand for refined LPG to serve as feed stocks for the petrochemical industry. Many of the new natural gasoline plants being constructed have been designed to recover a maximum amount of the lighter products in anticipation of the expanding market for these products. Since the demand for these products products. Since the demand for these products in most cases is highly variable, the continuous output from the gasoline plants necessitates some form of temporary storage facilities. This paper reviews field experience with underground storage of LPG in gas condensate reservoirs in the Borregos field near Corpus Christi, Tex. In 1960, Humble Oil and Refining Co., realizing that markets soon would become available for the temporarily surplus ethane, propane and butane from the King Ranch Gas Plant, propane and butane from the King Ranch Gas Plant, began storing these products in two virgin nonassociated gas reservoirs, using dual-purpose injection and production wells. Peak storage volumes ranged from 275,000 bbl of butane to 2.5 million bbl of propane. Injection rates as high as 7,000 B/D and withdrawal rates as high as 4,500 B/D were achieved. Storage Reservoirs Although three reservoirs have been involved in storage operations, only the two with significant history are considered in detail here. The productive limits of the two major storage reservoirs, Borregos Zones H-5 and H-7, are shown in Fig. 1. These reservoirs are two of the many found in the large anticlinal structure of the Borregos field. They are shallow Frio sands deposited in a strandline type of environment and are considered to be near-shore or lagunal deposits. Originally, both reservoirs were of nonassociated gas; the properties of the native gases are shown in Table 1. Fig. 2 is a representative cross-section of the storage reservoirs. Borregos Zone H-5 is found at an average depth of 5,275 ft and varies in thickness from 0 to 25 ft, averaging 10.4 net ft in the storage area. This sand grades from a clean sand of moderate thickness on the flank of the structure to a tight (lower permeability) thin sand on the crest. Borregos Zone permeability) thin sand on the crest. Borregos Zone H-7 occurs approximately 50 ft stratigraphically below Zone H-5. Zone H-7 ranges in over-all thickness from 0 to 24 ft, averaging 5.5 net ft in the area of LPG storage. The continuity of this reservoir is complicated by re-entrant shale-outs and rapidly changing lithology. The exact depositional environment has not been established but is probably related to Zone H-5. Both Zones H-5 and H-7 dip from west to cast at an angle of about 1 degrees. Borregos Zone D-5, not discussed in detail in this report, is found at an average depth of 4,700 ft and averages 9.7 ft in thickness. A summary of storage reservoir properties is shown in Table 2. JPT P. 587