Scale Removal in the Virden, Manitoba, Area
Authors: Charleston James 
Published: December 2007

In this particular area of Manitoba, ethylene diamine tetraacetic acid has proved successful in treating wells whose capacity to produce has been reduced because of calcium sulfate scale. Although experiments are being conducted on the prevention of scale deposition it appears that removal may prove to be as effective as prevention, and more economical, too.

Introduction
The Virden area is midway between Regina, Sask., and Winnipeg, Man., and is 60 miles north of the international boundary between Canada and the U.S. There are three main fields in the immediate vicinity of the town of Virden: North Virden Scallion, Virden Roselea and Routledge (see Fig. 1). Production is from the Virden and Scallion members of the Lodge-pole formation, which is of Mississippian age and is found locally at a depth of approximately 2,000 ft. The pay zone is a crystalline limestone grading to dolomite with natural fractures and a considerable amount of anhydrite infilling. The producing mechanism, prior to waterflooding, was a producing mechanism, prior to waterflooding, was a combination of solution gas drive, natural water drive along the western flanks of the individual pools, and limited bottom-water drive. North Virden pools, and limited bottom-water drive. North Virden Scallion and Virden Roselea are currently being produced by waterflooding. produced by waterflooding. The reservoirs were undersaturated at discovery, with producing GOR's less than 100 scf/STB. The bottom-hole temperature is approximately 80 degrees F. Production declines were initially attributed solely to the Production declines were initially attributed solely to the declining bottom-hole pressure; but it became evident from the scale deposition found on bottom-hole production equipment that part of the production decline production equipment that part of the production decline was due to scale deposition. This deposition is most severe in pressure-depleted and high-drawdown areas. The mechanics of deposition is believed to be caused by chemical instability that results from the evolution of gas from solution or through the mixing of water from the separate members of the producing zone.

History of Development
Initially, 500 to 1,000 gal of 15 percent hydrochloric acid was used to stimulate the scaled wells. It was concluded that this type of treatment did not remove the scale, but did open new flow channels, and in many instances restored the producibility of the well. The production increases from the acid treatments were not sustained much longer than 3 months. It was evident, therefore, that alternative, less expensive methods of stimulation should be evaluated. An analysis of the scale indicated the following composition:
CaSO4 - 93.4 percentCaCO3 - 4.3 percentFe2S3 - 2.3 percent
Various commercial sulfate scale removal agents were used with limited success, until an experimental treatment using a heated solution of a sodium salt of ethylene diamine tetraacetic acid, commonly referred to as EDTA, was used to treat the well designated Routledge 11-22. The treatment consisted of 2 bbl of EDTA in 10 bbl of fresh water. The treatment cost $600 approximately 60 percent of the cost of a normal acid treatment and resulted in a seven-fold production increase from 4 to 29 BOPD. With such production increase from 4 to 29 BOPD. With such encouraging results, similar EDTA treatments were tried on other wells and this became a standard treatment for scale removal. The average production increase of a successful treatment is 10 BOPD, and it lasts from 6 to 12 months.
JPT
P. 701