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<br />(1995) are Texaco Exploration and Production, Inc.
<br />(Aneth Unit); Mobil Exploration and Producing, U.S.,
<br />Inc. (McElmo Creek and Ratherford Units); and U.S.
<br />Oil and Gas, Inc. (White Mesa Unit).
<br />Shortly after the major discoveries in the late
<br />1950s, field pressures decreased and water injection
<br />was necessary to increase productivity; consequently,
<br />beginning in 1961, secondary recovery operations
<br />began (Moore and Hawks, 1993). Because the volume
<br />of water needed for injection purposes exceeded the
<br />volume of water produced with the oil, numerous shal-
<br />low water wells were drilled into the alluvium along the
<br />floodplain of the San Juan River to provide the neces-
<br />sary "make-up" water. From 1984 to 1991, about 316
<br />million barrels of water from the alluvium and "pro-
<br />duced" (formation) water from oil wells, were injected
<br />back into the Paradox Formation, although yearly
<br />injection rates have declined substantially (Daniel
<br />Jarvis, Utah Division of Oil, Gas, and Mining, written
<br />commun., 1991). Water from the alluvium is mixed or
<br />"comingled" with produced water prior to injection, or
<br />alternately injected with the produced water. Injection
<br />of produced water takes place through wells drilled
<br />specifically for injection back into the oil-producing
<br />formations or by using wells that have been converted
<br />from previously producing oil wells or dry holes. In
<br />recent years, carbon dioxide also has been used to
<br />increase field pressures and enhance oil and gas recov-
<br />ery in parts of the Greater Aneth Oil Field (Moore and
<br />Hawks, 1993).
<br />Producing oil strata in the Greater Aneth Oil
<br />Field and vicinity are primarily in carbonate units ofthe
<br />Pennsylvanian-age Paradox Formation (lsmay and
<br />Desert Creek zones) at depths generally ranging from
<br />5,000 to 6,000 ft. Producing intervals within these
<br />zones average 40 to 50 ftthick but vary in thickness in
<br />the Greater Aneth Oil Field and between fields (Moore
<br />and Hawks, 1993). The reservoir lithologies consist of
<br />oolitic and fossiliferous (algal) limestones and dolo-
<br />mites that have formed bioherms and bioclastic mounds
<br />up to 200 ftthick (Clem and Brown, 1984, p. 18).
<br />Minor amounts of oil and gas also have been obtained
<br />from the Mississippian-age Leadville Limestone. The
<br />Greater Aneth Oil Field contains more than 48,000
<br />acres of proven reserves, with about 450 producing oil
<br />wells on a 40-acre spacing (Moore and Hawks, 1993).
<br />Total cumulative production from primary and second-
<br />ary recovery as of 1992 is estimated to be about 310
<br />million barrels of oil, 336 million cubic feet of gas, and
<br />more than 400 million barrels of water (Clem and
<br />Brown, 1984, p. 18; and Moore and Hawks, 1993).
<br />
<br />10
<br />
<br />Water Use in the Study Area
<br />
<br />The existence of artesian ground-water condi-
<br />tions in southeastern Utah was recognized as early as
<br />1908 during drilling of scattered oil-test wells (Lof-
<br />gren, 1954, p. 107). Artesian conditions, characterized
<br />by hydraulic pressure greater than atmospheric pres-
<br />sure, also existed in wells drilled in Bluff, Utah, in
<br />1909, at depths of 800 ro 1,100 fl. These wells yielded
<br />water from the Navajo and Wingate Sandstones for
<br />domestic and irrigation use. Water in these sandstones
<br />was under sufficient hydraulic pressure to discharge at
<br />land surface. Two of the earliest wells to yield freshwa-
<br />ter in the study area are located northeast of Hatch
<br />Trading Post in T 39 S., R. 25 E., Sec. 5 (fig. I). Drilled
<br />in September 1951, these wells indicated that an
<br />untapped source of water in the Entrada, Navajo, and
<br />Wingate Sandstones was present in this area.
<br />After the discovery of the Aneth Oil Field in
<br />1956, a number of water wells were drilled to provide
<br />water for oil-drilling operations and to provide a source
<br />of "relatively" freshwater (dissolved-solids concentra-
<br />tions less than 10,000 mglL) to mix with produced (for-
<br />mation) water for injection purposes. Most of these
<br />wells yielded water from the Entrada, Navajo, and Win-
<br />gare Sandstones (Barnes, unpub. data, table 3, p. 18).
<br />After 1961, however, these bedrock wells could not
<br />provide the volume of water necessary for oil-field
<br />operations, and numerous shallow wells subsequently
<br />were drilled in the floodplain of the San Juan River
<br />between Aneth and Montezuma Creek. The volume of
<br />water withdrawn from wells in alluvium along the San
<br />Juan River for use in injection operations in the Anerh,
<br />Ratherford, and McElmo Creek Units from 1962-90
<br />has ranged from a maximum of about 12 acre-ftJd dur-
<br />ing 1963-67 to about 2.8 acre-ftJd during 1988-90
<br />(James Vanderhill, Mobil Exploration and Producing,
<br />U.S., Inc., written commun., 1994). Generally, the vol-
<br />ume of required make-up water from the San Juan
<br />River has decreased through time because a larger vol-
<br />ume of produced water has been pumped relative to the
<br />volume of oil, and because of recent carbon dioxide
<br />injection. Most of the water wells in bedrock aquifers
<br />that were drilled originally for oil-field operations have
<br />been plugged and abandoned or given to the Navajo
<br />Nation for domestic and stock use. Some oil-test holes
<br />(dry holes or previously producing oil wells) also have
<br />been plugged back to water-bearing formations and
<br />converted to water wells. Water that discharges from
<br />converted oil-test holes and water wells generally is
<br />artesian and saline. The volume of warer yielded from
<br />
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