Case History of Aquifer Gas Storage Development in a Silurian Dolomite at Glasford, Illinois
Authors: Oborn R.W. , Ryan Robert 
Published: December 2007

Abstract
All aquifer gas storage fields in northern and central Illinois have been developed in sandstone formations of Cambrian or Ordovician age. Due to the highly disordered nature of pre-Silurian sediments within the Glasford structure, storage is being developed by Central Illinois Light Co. in a dolomite of Silurian age at about 800 ft. This dolomite was neglected because of its suitable reservoir properties and because of the adequate caprock characteristics of the overlying Devonian-Mississippian formations. After defining the structure through gravity exploration and structure testing, gas storage leasing operations were conducted and various reservoir tests were performed prior to approval of the project by the Illinois Commerce Commission. Presently, the field is approximately one-third developed with 11 injection-withdrawal wells, 1,465 hp in compression and total gas inventory of 3 Bcf.

Introduction
Central Illinois Light Co.'s gas distribution market consists of two major areas: Peoria and Springfield, about 70 miles apart. There are approximately 90,000 gas customers in the Peoria area and 40,000 in Springfield and environs. The sole gas supplier for Central Illinois Light is Panhandle Eastern Pipeline Co. The present demand-commodity contract supplies a maximum total of 70,000 Mcf/D for June, July and August. Other months vary, with November through March having a maximum daily purchase of 250,000 Mcf. The contract stipulates a demand charge of $1.99/Mcf and a commodity rate of 21.7 cents/Mcf. Both market areas are included under the same gas purchase contract so that peak shaving in either market area affects the single-purchase contract. The company's supplemental gas supply consists of two propane-air plants which together can supply 20,000 Mcf/D of gas to the Peoria market, in addition to the recent supply made available to that market out of underground storage.

Exploration for Suitable Structure
The exploration methods through which the Glasford structure was discovered were
- a reconnaissance gravity survey,
- a study of surface geology, well logs and regional geology and
- a structure test drilling program.
Data from gravity work performed during 1958 and 1959 indicated the possible existence of several structures in the Peoria market area. These prospects were followed up with structure test drilling. The right to drill each structure test was obtained from the individual landowners for a payment of $50, plus damages incurred as a result of drilling. A +7/10 milligal residual gravity anomaly was discovered near Glasford, Ill., approximately 15 miles southwest of Peoria. A dry hole drilled 10 years prior to the company's exploration was located on the crest of the structure. Surface geology, coal maps and the gravity picture, along with previous well data, all indicated a steep structure steep, in fact, that structure test drilling and caprock and storage zone evaluation were deferred because of the possibility of faulting. Other gravity prospects were evaluated with structure test drilling. After it was determined that no other prospect had sufficient structural closure for storage. the Glasford prospect was evaluated. In 1960, five structure test wells were drilled: one in each quadrant around the anticipated periphery of the structure and on the crest to confirm the formation tops obtained from records of the existing well. This drilling program verified existence of a domal structure about 2 1/2 miles in diameter with a minimum closure in excess of 100 ft. None of the five test holes completely penetrated the Cedar Valley formation (Devonian). Four were drilled into the top of the Devonian. In every case, 7-in. OD surface casing was set and cemented to about 100 ft through the drift zone. After being drilled and logged, all the holes were completely plugged with cement.
The surface casing was then capped below plow depth and covered with top soil. This left no obstruction to farming in the area and still provided a means of readily locating the holes for observation or re-entry if necessary.

Leasing
On the basis of structural data obtained in 1960, it was decided that the Glasford structure warranted further evaluation and leasing operations were commenced.