Two become one in Alpine reservoir
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AOGCC agrees that one of the oil pools in the Nanuq satellite is connected to the Alpine oil pool in the Colville River unit
Alan Bailey Petroleum News
When in 2005 the Alaska Oil and Gas Conservation Commission authorized pool rules for the Nanuq satellite of the Alpine oil field on Alaska’s North Slope, the commission recognized two distinct pools: the Nanuq pool in the Torok formation and a deeper Kuparuk River formation pool. It now appears that the Nanuq-Kuparuk pool is, in fact, contiguous with the Alpine oil pool, the main pool in the Alpine field.
And in a somewhat unusual move, given that the defining of oil pool rules typically involves delineating multiple pools within a field, AOGCC has approved the extension of the Alpine pool into what has hitherto been considered the Nanuq-Kuparuk pool, thus eliminating the Nanuq-Kuparuk pool as a separate entity within the Colville River unit. Pool rules are designed to ensure optimum resource recovery and to protect resource ownership rights.
Nanuq lies to the south of the Alpine field and is produced from the CD-4 well pad.
Elevated pressure In a letter dated Dec. 10 to AOGCC Commissioner Dan Seamount, ConocoPhillips, operator of the Colville River unit, said that drilling, well log, reservoir pressure and production log data indicate reservoir pressure communication between the Kuparuk reservoir at Nanuq and the Alpine interval in the main Alpine field, indicating that the two reservoirs are connected to each other.
Two recent wells in the Alpine field encountered elevated reservoir pressures in the Kuparuk sandstone, directly above the Alpine sandstone that reservoirs the Alpine oil pool, ConocoPhillips said. And the Char No. 1 exploration well, drilled in the winter of 2007-08 to the north of the field, encountered similar elevated pressures in the Kuparuk. On the other hand, the Iapetus No. 2 exploration well, drilled in 2005 a short distance to the northwest of the Char well location, encountered normal pressures in the Kuparuk.
ConocoPhillips attributes the high pressures in the Kuparuk to the impact of water and gas injection in the Alpine field, injection that caused fluids to flow through faults and fractures connecting the Alpine sand and Kuparuk sandstone reservoirs. A loss of drilling fluids when drilling two wells to the east of CD-1, the central Alpine well pad, confirmed the presence of a system of faults and fractures, the company said. The Iapetus well encountered normal pressures in the Kuparuk because it was drilled before the pressure-elevating injection began.
Subtle changes Subtle changes in reservoir pressures since the start of oil production out of the Nanuq Kuparuk interval also suggest pressure communication between the Kuparuk and Alpine sands, ConocoPhillips said. And well CD2-02, about 5.5 miles west of CD-1, “encountered a sand-on-sand contact between the Kuparuk and underlying Alpine,” the company said.
The pressure communication observed within the Alpine and Kuparuk sands does not appear to extend into other formations above and below these units.
AOGCC agreed with ConocoPhillips’ interpretation of the reservoir and well data and approved the expansion of the Alpine oil pool to encapsulate the Kuparuk sand and, hence, encompass a significantly larger area than specified in the original pool rules for the Alpine field.
And the commission has also agreed to relax the well spacing requirements for the Alpine pool, to accommodate the well spacing needed for efficient production from the Kuparuk at CD-4 — the commission has eliminated a 500-foot interwell spacing requirement, while preserving a minimum well setback of 500 feet from property boundaries.
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