Alberta's Small Gas Pool Reserves

Abstract

Abstract Small, single-well pools presently contain about 28% of Alberta's remaining marketable gas and their share is steadily increasing. A performance study was conducted for 2668 small gas pools producing from a variety of geological zones throughout the province. Reserves from performance are 25% lower than initial volumetric reserves. Much of the difference is attributed to depleted pools and pools where water has curtailed gas production. The pools studied are grouped into 29 zones to examine results and to compare with small shut-in pools. Also included are revised productive areas that the ERCB has adopted for estimating small gas pool reserves. Introduction A decreasing trend in the size of gas pool discoveries is well established in Alberta(l). As large pools become more depleted, small pools will be called upon to replace increasing volumes of reserves and production. What is a small pool? There is no completely satisfactory definition because estimates of a pool's size or reserves can change over time. In this paper, a small pool is defined as a non-associated, single-well pool; therefore, the terms "pool" and "well" are used interchangeably. Presently, some 15 000 (71%) of Alberta's gas pools are in this category, and together they contain about 435 billion m3 (28%) of the province's remaining marketable gas. With the oversupply of gas in Alberta, most small gas pools remain shut in. Between late 1985 and early 1987, the Energy Resources Conservation Board (ERCB) reviewed manually the performance of 2668 small pools that had produced. The initial volumetric reserves based on a nominal 200-hectare area were found to be overstated by about 25%. While performance methods can sometimes be misleading, most of the net reserve reduction is accounted for in depleted or watered-out pools. These findings raisedsome important questions about the large inventory of reserves booked in small shut-in pools. Following its review, the ERCB revised the productive areas it assigns to single-well pools in certain Cretaceous zones, The.se revisions have dropped reserves in small shut-in pools about 11%. Further adjustments may be necessary if the early indications for some zones are confirmed by additional performance monitoring. Historical Development The total number of small gas pools in Alberta (Fig. 1) climbed rapidly after 1975 in response to increased gas prices. Other factors that may have contributed to this growth were several large area-purchase contracts issued by major buyers; tough surplus tests(2.3); and resource basin maturity. Most (12 000) of these small pools have never produced because of the slow growth in gas markets(4), However, the annual production from small pools has increased nearly five fold since 1975 (Fig, 2). In 1987, small pools contributed about 8 billion m3 (l2%) of Alberla's total marketed gas production. To support the growth in annual production, over 200 new small pools were tied in each year (Fig. 3). The average initial rate of a new well is 44 000 m3 per day. When the ERCB began its study (late 1985), the pools reviewed had an average produced life of four years.