During the summer of 1995, National Fuel, through a Department of Energy (DOE) sponsored prqject for storage deliverability enhancement, began a stimulation project on three wells in Galbraith Storage Field. Galbraith, located in Jefferson County, Pennsylvania, consists of 26 active injection/withdrawal wells completed in the First Sheffield Sand at an average depth of 2,800 feet. Three of the active wells were fractured in the early 1980's using a gelled water fluid and approximately 20,000 pounds of sand. Several years of cleanup were required in all of these wells before the peak stimulation deliverability was experienced. The combined delivery from two of these wells was responsible for more than 40% of the field total. The fracture stimulation provided a significant increase from pre-frac deliverability. However, roughly 10 years of storage use and a coiled tubing cleanout were required before this level of increase was realized.
Three wells from the remaining 23 active were selected for the first liquid carbon dioxide fracture stimulations conducted in a storage field. Preliminary well test analysis indicated that permeability in the field was greater than anticipated, and deliverability was being adversely affected by a large skin. The fracture procedures were designed to produce short propped fractures to extend beyond near wellbore skin damage. The fracture gradient experienced was greater than 1, pushing the treating pressure to the maximum allowable for all of the jobs. As a result, the sand concentrations did not get as high as designed. The formation breakdown in one well was not sufficient for proppant to be run and very little deliverability increase was achieved at this well. However, deliverability increases at the other two wells were about 5 fold, roughly 5 MMcfd. Concerns that the carbon dioxide would not be recovered in a timely manner were put to rest after a few days as the concentration in the gas quickly fell below 3% and the wells were put back into service.
The level of immediate deliverability increase is what makes this technique attractive to storage operators. Because the pumped proppant was lower than anticipated, the longevity of the increase is suspect. Eighteen-month follow-up well testing has shown good maintenance of deliverability at the two propped wells and an increase in the deliverability at the unpropped well. Flow testing was conducted in July 1998, to evaluate the third-year performance. Additional details regarding the planning, testing, pumping, and evaluating of these jobs are discussed in the paper as well as a general review of the costs.