Changes

All basin-centered gas accumulations (BCGAs) reservoirs require carefully designed drilling programs and some type of artificial stimulation for commercial production rates. Reservoir continuity is an important consideration in the design of an appropriate drilling and completion program. Single, lenticular reservoirs have limited volume and are generally not commercial, whereas single, blanket reservoirs have much larger volumes and may be commercial, but, because blanket reservoirs commonly have better reservoir quality than lenticular reservoirs, they may be water bearing, as discussed previously.

All basin-centered gas accumulations (BCGAs) reservoirs require carefully designed drilling programs and some type of artificial stimulation for commercial production rates. Reservoir continuity is an important consideration in the design of an appropriate drilling and completion program. Single, lenticular reservoirs have limited volume and are generally not commercial, whereas single, blanket reservoirs have much larger volumes and may be commercial, but, because blanket reservoirs commonly have better reservoir quality than lenticular reservoirs, they may be water bearing, as discussed previously.

In lenticular, fluvial-dominated reservoirs, such as those in the Jonah field in the northern part of the Green River basin of Wyoming or the Rulison field in the Piceance basin of Colorado, it is imperative to stimulate as many reservoirs as possible to attain commercial rates of gas production. The completion practices in the Jonah field provide a good example of commingling production from multiple, lenticular reservoirs (Finch et al., 1997; Eberhard, 2001); as many as 28 sandstones are perforated and fractured (Montgomery and Robinson, 1997). In a similar manner, gas production from multiple sandstone reservoirs in the Upper Cretaceous Williams Fork Formation in the Piceance basin of western Colorado is commingled following multiple fracture treatments in an interval about 2400 ft (732 m) thick (R. E. Mueller, 2002, personal communication).

In lenticular, fluvial-dominated reservoirs, such as those in the Jonah field in the northern part of the Green River basin of Wyoming or the Rulison field in the Piceance basin of Colorado, it is imperative to stimulate as many reservoirs as possible to attain commercial rates of gas production. The completion practices in the Jonah field provide a good example of commingling production from multiple, lenticular reservoirs;<ref name=Finchetal_1997>Finch, R. W., W. W. Aud, and J. W. Robinson, 1997, Evolution of completion and fracture-stimulation practices in Jonah field, Sublett County, Wyoming, ''in'' E. B. Coalson, J. C. Osmond, and E. T. Williams, eds., Innovative applications of petroleum technology in the Rocky Mountain area: Rocky Mountain Association of Geologists, p. 13-24.</ref> <ref name=Eberhard_2001>Eberhard, M. J., 2001, The effect that stimulation methodologies ha(ve) on production in the Jonah field, ''in'' J. W. Robinson and K. W. Shanley, eds., Tight gas fluvial reservoirs: A case history from the Lance Formation, Green River basin, Wyoming: Rocky Mountain Association of Geologists Short Course Notes 2, unpaginated.</ref> as many as 28 sandstones are perforated and fractured.<ref name=Montgomeryandrobinson_1997>Montgomery, S. L., and J. W. Robinson, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1049/1049.htm Jonah field, Sublette County, Wyoming: Gas production from overpressured Upper Cretaceous Lance sandstones of the Green River basin]: AAPG Bulletin, v. 81, p. 1049-1062.</ref> In a similar manner, gas production from multiple sandstone reservoirs in the Upper Cretaceous Williams Fork Formation in the Piceance basin of western Colorado is commingled following multiple fracture treatments in an interval about 2400 ft (732 m) thick (R. E. Mueller, 2002, personal communication).

Early attempts to produce from blanket reservoirs were mixed. Massive hydraulic fracturing techniques using 300,000 lb of proppant were used in an attempt to create long fractures. However, the large fracture treatments commonly resulted in shorter fracture lengths than predicted because of fracturing out of the reservoir into adjacent, nonreservoir rocks (Spencer, 1989a). This problem has, in some cases, been modified by adjusting pumping rates of the fracture fluids.

Early attempts to produce from blanket reservoirs were mixed. Massive hydraulic fracturing techniques using 300,000 lb of proppant were used in an attempt to create long fractures. However, the large fracture treatments commonly resulted in shorter fracture lengths than predicted because of fracturing out of the reservoir into adjacent, nonreservoir rocks.<ref name=Spencer_1989a>Spencer, C. W., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0005/0600/0613.htm Review of characteristics of low-permeability gas reservoirs in western United States]: AAPG Bulletin, v. 73, p. 613-629.</ref> This problem has, in some cases, been modified by adjusting pumping rates of the fracture fluids.

Natural fractures are important factors in successfully completing a well. The probability of a vertically drilled hole intersecting fractures is considerably less than horizontal or slant holes. For example, at the U.S. Department of Energy Multiwell Experiment site in the Piceance basin of Colorado, a slant hole was drilled through lenticular gas reservoirs. The hole was then deviated to horizontal in a blanket reservoir. Fifty-two fractures were reported from 266 ft (81 m) of core taken from the slant hole part of the hole. In contrast, a nearby vertically drilled hole penetrating the same slant hole interval encountered one fracture, and, in the horizontally drilled part of the hole, 37 fractures were reported from 115 ft (35 m) of core (Lorenz and Hill, 1991). In a more recently drilled 14,950 ft (4557 m)-deep well in the Green River basin of Wyoming, more than 400 open fractures were detected on a Formation MicroImager log from a 1750 ft (533 m)-long horizontally drilled leg in the Upper Cretaceous Frontier Formation (Krystinik and Lorenz, 2000). In the same well, approximately 76 natural fractures were recorded from a 78.2 ft (23.8 m)-long core taken from the same horizontal leg (Lorenz and Mroz, 1999). From these two examples, the probability of encountering fractures in slant or horizontal wells vs. vertically drilled wells is well documented. The cost of drilling nonvertical wells, however, is considerably greater than the cost of drilling vertical wells.

Natural fractures are important factors in successfully completing a well. The probability of a vertically drilled hole intersecting fractures is considerably less than horizontal or slant holes. For example, at the U.S. Department of Energy Multiwell Experiment site in the Piceance basin of Colorado, a slant hole was drilled through lenticular gas reservoirs. The hole was then deviated to horizontal in a blanket reservoir. Fifty-two fractures were reported from 266 ft (81 m) of core taken from the slant hole part of the hole. In contrast, a nearby vertically drilled hole penetrating the same slant hole interval encountered one fracture, and, in the horizontally drilled part of the hole, 37 fractures were reported from 115 ft (35 m) of core (Lorenz and Hill, 1991). In a more recently drilled 14,950 ft (4557 m)-deep well in the Green River basin of Wyoming, more than 400 open fractures were detected on a Formation MicroImager log from a 1750 ft (533 m)-long horizontally drilled leg in the Upper Cretaceous Frontier Formation (Krystinik and Lorenz, 2000). In the same well, approximately 76 natural fractures were recorded from a 78.2 ft (23.8 m)-long core taken from the same horizontal leg (Lorenz and Mroz, 1999). From these two examples, the probability of encountering fractures in slant or horizontal wells vs. vertically drilled wells is well documented. The cost of drilling nonvertical wells, however, is considerably greater than the cost of drilling vertical wells.