Low-Tension Waterflood Pilot at the Salem Unit, Marion County, Illinois Part 1: Field Implementation and Results
Authors: Whiteley R.C. , Ware J.W. 
Published: December 2007

A tertiary recovery pilot of a low-tension waterflood process was conducted in the Salem Unit. A 5-acre, five-spot pilot was installed inside a previously waterflooded 20-acre, five-spot pattern in the Benoist reservoir previously waterflooded 20-acre, five-spot pattern in the Benoist reservoir to provide afield test of a chemical flooding process found successful in laboratory experiments.

Introduction
A tertiary chemical process for recovery of oil remaining after conventional waterflooding has been pilot tested in the Benoist sand reservoir of the Salem field, Marion County, Ill. The pilot was applied in a 5-acre five-spot pattern that consisted of four injection wells, two pattern that consisted of four injection wells, two observation wells with bottom-hole locations 67 and 164 ft from one of the injectors, and a center producer.
This was a joint Texaco Inc./Mobil Oil Corp. test, slice the low-tension waterflood process is under license from Mobil, which is a co-owner in the Salem Unit. Four separate injection phases comprised the process: process: Phase 1. A 0.5-PV, three-part, aqueous reservoirconditioning slug consisting of (1) 0.2 PV of softened fresh water to initiate displacement of formation brine, (2) 0.1 PV of fresh water containing 6,000 ppm sodium carbonate to reduce, through precipitation, undisplaced divalent ions, and (3) 0.2 PV of fresh water containing 6,000 ppm sodium carbonate, 1,000 ppm sodium tripolyphosphate to reduce further the divalent ion concentration, and 6,000 ppm salt (NaCl) to maintain the desired level of salt concentration for optimum surfactant reaction.
Phase 2. A 0.285-PV surfactant slug composed of 20,000 ppm of petroleum sulfonate mixed in softened fresh water containing 6,000 ppm each of sodium carbonate and sodium chloride and 1,000 ppm of sodium tripolyphosphate.
Phase 3. A 0.3-PV, two-part polymer drive slug:0.1 PV of 700 ppm biopolymer hydrated in salinity-adjusted, fresh and softened water, and0.2 PV of biopolymer-thickened water, as above, with polymer concentration reduced gradually from 700 to 0 ppm.
Phase 4. 1.0 PV of Salem field injection brine to provide the final drive. provide the final drive. The desired injection rates into all chemical injectors were maintained over the life of the pilot test, with the exception of a 40-day period of reduced injection during the surfactant phase caused by an interruption in sulfonate supply. However, all chemical injectors were treated selectively twice to improve vertical distribution of injected fluids. It was also necessary to acidize the center producer, Well 5-2, on two occasions to maintain total producer, Well 5-2, on two occasions to maintain total fluid withdrawal.

Location
The Lake Centralia-Salem pool is located between the cities of Salem and Centralia in Marion County, Ill., about 70 miles east of St. Louis (Fig. 1). The field contains 8,800 surface acres and is 6 1/2 miles long and 2 1/2 miles wide. The field was discovered in 1938 by Texaco Inc. and 2,400 wells have been drilled. Eight horizons have produced under primary conditions and four have been subjected to full-scale waterflooding operations. The field was unitized for waterflooding on Sept. 1, 1950, with Texaco as unit operator (Fig. 2).
Geology
JPT
P. 925