WATER'S ARRIVAL TO PROMPT DRILLING IN NEVADA'S GRANT CANYON FIELD

G. Alan Petzet
Exploration Editor

Water has sharply reduced the oil flow in Nevada at what for several years has been the highest producing rate well in the Lower 48 states, and a well will be drilled in an attempt to reestablish higher oil flow rates.

Apache Corp., which operates three well Grant Canyon field in Railroad Valley 65 miles southwest of Ely, staked two close in locations but plans to drill only one well.

Estimates of the areal extent of the structure are from less than 200 acres to about 240 acres, making the new well a risky proposition, said Jerry Mayer, exploration manager.

Grant Canyon field has been important ever since its discovery in 1983.

Four authors involved in the field's discovery, in an exploration paper published in this magazine in early 1988, indicated that the field's ultimate reserves might be about 13 million bbl of black, 26 gravity crude oil with 0.5% sulfur and a pour point of 10 F.

The producing zone is intensely fractured Devonian Guilmette dolomite at about 4,400 ft (OGJ, Mar. 28, 1988, p. 67).

Through February 1991 it had produced more than 8.7 million bbl through the 3 Grant Canyon and 5.1 million bbl through the 4 Grant Canyon.

One well Bacon Flat field, a Guilmette reservoir in a separate, structurally lower fault block, is shut-in. It was discovered in 1981.

WHAT HAPPENED

The 3 Grant Canyon averaged 4,511 b/d of oil and the 4 Grant Canyon 2,786 b/d of oil during February. The No. 1 well has been shut-in several years.

The 3 well began producing water in April. The open hole completion was flowing by heads at rates of 100 b/d of oil and 700 b/d of water in early August.

The water production increased to its present level in about 45 days, said Alec Nash, production manager.

Apache is convinced the producing mechanism is mainly a strong bottom water drive, with other technical aspects it does not feel it can discuss for publication, and that ideas that the reservoir is being recharged with oil from greater depth do not apply, Mayer said.

The company believed water would encroach but did not know when. The open hole contains junk, possibly with some fill on top, and has no casing or tubing across the pay zone.

Therefore the lowest open position in the wellbore is not known, and remedial work is not possible.

Meanwhile, Apache has enlarged the choke at the 4 well, which is now making about 5,000 b/d of oil. Water will reach it, too, in time.

The company has not published its estimate of recovery factors for the wells and says it does not know with great confidence the exact location of some of the reservoir's bounding faults.

APACHE'S APPROACH

Apache staked locations for the 7 and 7A Grant Canyon to obtain the permits while it processed one final seismic line across the field.

Present plan is to drill only the 7 location, and the well is to start this week. Projected depth is 4,500 ft.

Apache hopes to position the well higher on the structure and believes the new well will drain a portion of the reservoir that the existing wells cannot.

Drilling will be risky because the bottomhole location is intended to be near a fault that forms the updip terminus of the reservoir.

"We want to be on the correct side of the fault and structurally high," Mayer said.

Holding working interests in the 7 well are Marathon Oil Co., North Central Oil Corp., Houston, Balcron Oil Co., Billings, Robertson Finley Co. and Finley Co. of Delaware, both of Texas, Wolford Butler Group, Denver, and American Cometra Inc., Fort Worth.

Apache's Nevada exploration program is limited by company budget/manpower constraints and its greater participation in other plays, such as Pennsylvanian Minnelusa in Wyoming.

The company participated in three wildcats in Nevada in 1990, all unsuccessful. It continues to upgrade its prospect inventory there, but had no other firm drilling plans as of Aug. 2.