Abstract

The Beluga River gas field is a large, shallow gas accumulation located approximately 40 miles (64 km) west of Anchorage in the northern Cook Inlet Basin in south-central Alaska. ConocoPhillips Alaska, Inc., operates the Beluga River gas field on behalf of itself and its co-owners, Hilcorp Alaska, and Anchorage Municipal Light and Power. Each owner has a 33.33% working interest. The Beluga River gas field is a major gas supplier for local electric utilities as well as for home and commercial usage in the greater Anchorage area. The Beluga River gas field was discovered in 1962 while exploring for a deeper oil objective. Production commenced in March 1968, and more than 1.2 TCF have been produced as of 2011 (AOGCC, 2011).

The Beluga River gas field is approximately 7.5 mi long by 2.5 mi wide (12 km × 4 km). The trap is the Beluga River structure, a broad north-northeast-trending fault-propagation fold with a steeply dipping reverse fault along the west side. Basin fill consists of a thick sequence of nonmarine, fluvial-dominated, volcanic to arkosic sandstone, siltstone, mudstone, and coal deposited in a rapidly subsiding basin during Eocene to Pleistocene times. The field produces from two formations: the overlying high-net-to-gross Pliocene-age Sterling Formation and the underlying low-net-to-gross Miocene-age Beluga Formation. Gross reservoir thickness is up to 3400 ft (1036 m) and consists of dozens of stacked channel belt and crevasse splay sandstone beds separated by laterally continuous, relatively impermeable flood basin siltstone, mudstone, and coal. The dominant reservoir sandstone facies are characterized as relatively discontinuous channel belt or fan-shaped geometries. Reservoir connectivity is controlled by net-to-gross ratio, channel belt size and orientation relative to well spacing, and the presence of thin but widespread coal zones. Produced gas is biogenic and is thought to originate from nearby interbeds of coal.

The field is in the late stage of development, with many of the reservoir sandstones depleted to less than 40% of original pressure and with most down-structure wells experiencing water encroachment in reservoir sandstones. Declining and differential pressures, water breakthroughs, and sand production present significant operational issues. Pressure measurements indicate that much of the remaining gas resource resides in lower-quality, low-net-to-gross channel belt sand bodies in the lower portion of the reservoir.