This paper describes the mechanics and operation of a new selective acid washing tool and its application in successfully stimulating the Arcs member of the Nisku formation in the Youngstown Field of Alberta. Case histories and recommendations are provided as to the wide ranging adaptability of the acidizing method.
Acid stimulation in the Arcs member had, in the past, consisted of batch washes until the zone would no longer hold a fluid column. Recent recompletions had resulted in the need for a method of selectively acid washing to ensure that new perforations were open to allow inflow. Selectively squeezing acid into the formation was rot recommended due to a limited pay zone and the proximity of an underlying aquifer. Premature "watering out" of wells had historically been a problem due to acid squeezing.
Mechanical methods of diverting acid have been recognized by the oil industry since the mid 1940's. Selective acidizing equipment utilizing cups and/or packers have been utilized on an ever increasing trend due to the recognized production benefits. A tool capable of washing selected pay in discreet intervals while isolating remaining pay fran fluid contact was not available from the current selection of well service tools. A selective acid wash tool was developed as an alternative stimulation method.
The Youngstown Nisku Arts Pool is located in southcentral Alberta, approximately 40 km east of the town of Hanna (Figure I). Suncor has a high working interest percentage in the developed portion of the pool and currently operates 32 oil wells with an average daily oil production rate of 8 m3 OPD (Figure II).
The Nisku formation at Youngstown was deposited as a low relief carbonate build-up. The productive horizon has teen referred to locally as the Arcs Member. The Nisku is overlain by 75 m of interbedded dolomites and anhydrites of the Stettler Formation. The Youngstown Nisku oil pool is one of the shallowest Nisku oil pools in central Alberta at an average depth of 1150 m. (The average Nisku oil pool depth is 2000 m in Alberta). The oil pay zone averages 4 - 5 m in thickness and is underlain by an active aquifer which has provided continuous pressure support over the history of the reservoir. The Nisku structure is shown as figure III.
The requirement for acidizing after perforating the wells at Youngstown was recognized early in the life of the reservoir. Infow from the reservoir was severely restricted by perforation damage until an acid treatment was carded out. Acid stimulation in the Arcs remember had, in the past, consisted of retch washes until the zone would no longer hold a fluid column. Squeezing acid into the formation resulted in high water cuts and in sane cases premature "watering out" of wells. The water problem was due to loss in fluid control in combination with a limited pay zone, the proximity of an underlying aquifer, low matrix (fracture pressure and treating the entire perforation interval at one time (the treating fluid taking the path of least resistance.)