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WesTex Gas combines storage facilities.

Combined data from numerical simulations, test wells, and open hole logs were among the many parameters used in the planning and construction of a new gas storage facility in west Texas. Cavern storage (Salado), vertical well reservoir storage (Loop), and horizontal well reservoir storage (North Felmac) were all integrated into a single complex that offers high deliverability and storage capacity.

WesTex underground gas storage fields, caverns, and operations facilities are located approximately 60 miles from Lubbock in Gaines County, Texas. When completed, the site will have 18 Bcf of working gas with a facility-limited withdrawal capacity of 600 MMcfd and an injection capability of 300 MMcfd. Total available compression onsite is 7,800 bhp. Direct pipeline interconnects allow receipts and deliveries with El Paso Natural Gas, Northern Natural Gas, and Red River pipelines. Connecting with Red River Pipeline allows access to multiple pipeline interconnects at WAHA and North Texas.

Loop Gas Storage Project

The Loop Gas Storage project is a converted, depleted gas reservoir in the Yates Sand at 3,350 ft. During primary depletion, the field produced 12.24 Bcf of gas from a reservoir containing 13.25 Bcf. It was converted to storage operations in 1981 with the addition of six new wells, workovers of four existing producing wells, and installation of compression and metering facilities. At that time, a working gas total of four Bcf and a withdrawal capability of 50 MMcfd, was available at the site. Prior to its acquisition, a previous owner/operator-provided status report showed that the expectations under storage operations deviated significantly from past history primary depletion performance.

A reservoir evaluation of the storage field was commissioned to resolve these volume discrepancies. Numerical simulation techniques were used to provide an accurate description of the sand reservoir. A valid working model of the reservoir was obtained after numerous simulation runs of the primary and storage performance operations. This model showed that the storage field actually contained two separate reservoirs which were divided by a four foot, low permeability layer. These results were confirmed by a test well which was drilled, cored, and field tested.

The facility was expanded during 1991-1992 by drilling and completing 16 new injection/withdrawal wells and the added workover of eight original wells. Increased volumes of gas were handled by additional metering and dehydration facilities. This expansion increased the working gas from four Bcf to 9.4 Bcf and the withdrawal capacity from 50 MMcfd to 200 MMcfd.

Salado Gas Storage

These caverns were constructed in bedded salt formations and are the first in the U.S. to be used exclusively for natural gas storage. Bedded salt is found throughout the Permian Basin of West Texas with the Midland section containing up to 900 ft of gross salt. Construction of the caverns is in the lower "dirty" section of the Permian Salado Salt beds. Historically, liquified petroleum gas (LPG) has been stored in the upper "cleaner" section of these beds.

A test well was drilled, cored, and logged through these salt beds and results showed that the caverns could be constructed for natural gas storage. The acreage was being used for gas storage in the depleted Loop Yates reservoir which meant that the existing injection and withdrawal facilities could be used to handle surface gas operations.

Primary components used in the solution mining operation are: fresh water wells, cavern wells, disposal wells, and a pumping plant. Fresh water for solution mining comes from 14 wells which were completed in the Ogallala formation at 200 ft depth. It is piped by a poly and steel line gathering system to the pumping plant consisting of four gas engines which supply 1,550 bhp to 4-ten stage vertical turbine pumps. Removal of solids from the water is done with combined hydroclone and sock filtration units. Diesel fuel is used as a blanket fluid during mining and is stored in two 210 bbl tanks onsite.

At present, three cavern wells have been completed and are undergoing solution mining. An initial cavern well was converted to gas storage in November 1993 but was returned to solution mining mode in February 1994. Total solution mining capacity stands at 2,000 gpm.

Disposal of produced brine is done with two 14,000 feet wells completed open hole in the Devonian Lime/Dolomite formation.

The first well, Salado Cavern #1 was drilled and completed in June 1992. A 20 inch surface casing was set at 350 feet through the fresh water horizons. A 16 inch intermediate casing was set into the Rustler Anhydrite above the Salado Salt at 2,350 feet to isolate the Permian Redbeds. Also, a 13-3/8 inch production casing was set and cemented at 2,700 feet to provide required depth for the permitted maximum operating pressure. A 7 inch minimum string and 10-3/4 inch blanket string were installed for solution mining. The other two cavern wells (Salado Cavern #2 and #3) were similarly completed.

Salado Cavern #1 (Loop G.S.U. No. 9-28 GC) to date has a cumulative mined space of over six million cubic feet. Three sonar runs were made to evaluate the cavern size, shape, and solution mining process since initial startup. Cavern #2 was drilled in April 1993 and has a cumulative mined space of two million cubic feet. Salado Cavern #3 was completed in October 1993 and has a cumulative mined space of 700,000 cubic feet.

In October 1993, a mechanical integrity test was completed on Salado Cavern #1. Injection of natural gas at the rate of 60 MMcfd began in November 1993 and a total of 550 MMcf was injected up to early December 1993. Complete fillup of the cavern was thwarted by a ledge fall and subsequent shearing of the tubing string. The cavern continued to be used for storage and injection/withdrawals until February 1994. It was then returned to solution mining mode in anticipation of total completion of the permitted 11.23 million cubic feet (2 million bbls) in November 1994.

North Felmac Gas Storage Project

North Felmac Gas Storage Project is a reservoir facility in the depleted Felmac (Yates Sand) Field. During primary depletion, the 3,360 reservoir produced 8.4 Bcf of the 9.24 Bcf capacity. Only one of the original producing wells remained at the facility. This well was shut in due to insufficient pressure to enter the gas pipeline. After its acquisition, it was converted to a pressure observation well. Seven wells which remained were plugged and abandoned after depletion by the original operator. These wells were re-entered to confirm reservoir integrity and that proper plugging and abandonment procedures had been used.

A numerical simulation study of the field was used to determine an economical development plan. Open hole logs, test data, and completion reports were used as parameters. Also, a successful match during primary depletion history was completed after several iterations. Both vertical and horizontal well completions were included in the storage performance evaluation. From this analysis, horizontal well technology was chosen since it offered the most effective and economical development of the field.

Two in-line horizontal wells with a wellbore diameter of 6-1/2 in. and an open hole extension of 2,000 ft. were recommended. From the numerical simulation it was determined that the storage reservoir would have working gas volume of 5.6 Bcf and a withdrawal capacity of 200 MMcfd.

In October 1993, the first horizontal injection/withdrawal well was completed. A 16 in. surface casing was set through fresh water sands to 350 ft. An 11-3/4 in. intermediate casing string was set to the Rustler Anhydride at 2,350 ft. to isolate Permian Redbeds sands. After this, the well was vertically drilled to a total depth of 3,500 ft. Full bore cores were taken through the Yates Sands from 3,330 ft. to 3,390 ft. Open hole logs and formation pressure testing was used to evaluate reservoir properties.

In addition, a cement kick off plug was set at 2,800 ft. and a medium radius curve of 12 degrees per 100 ft. was built to the formation at 3,360 ft. When this was completed, the well had a horizontal displacement of 500 ft. at 90 degrees. Then a 7-5/8 in. production casing string was set and cemented to the surface. For final completion, the well was drilled horizontally to 2,000 ft with a 6-1/2 in. bit; a 4 1/2 in. slotted liner set in place; and a 5-1/2 in. tubing sting and packer set at the end of the curve in the production casing. In February 1994, a second horizontal well was drilled.

Compression, metering, and dehydration facilities were completed in October 1993. At present, the facility has a total of 4,500 bhp, withdrawal capability of 140 MMcfd, and an injection capacity of 150 MMcfd. Injection of natural gas started in March 1994, at the rate of 50 MMcfd.

Integration into a Single System

Loop, Salado, and North Felmac were integrated into a single storage complex by tying in gas piping, measurement, dehydration, control, and compression systems from each of the projects.

Also, a 22 mile, 20 in. gas pipeline was built from the facility to Red River Pipeline to allow for greater market flexibility and access. Continuous monitoring of the operation is done with state-of-the-art telemetry. When completed, the combined gas storage caverns will have a working gas of 18 Bcf, withdrawal capacity of 600 MMcfd, and injection capability of 300 MMcfd.
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Author:Evans, L. Jay, Jr.
Publication:Pipeline & Gas Journal
Date:Sep 1, 1994
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