ABSTRACT
The West Sak (Upper Cretaceous) sands, overlaying the Kuparuk field at 2,000-4,500 ft depths, contains heavy oil (12-23 API) at a low GOR. With a range of OOIP estimates of 13 to 25 Billion Barrels, this reservoir, covering 300 sq.miles (777 km2), would rank among the largest known oil fields in the US, but technical difficulties have so far prevented its commercial exploitation. Steam injection is the most successful and the most commonly-used method of heavy oil recovery, but its application to the West Sak presents the following major problems:
steam injected from the surface would have to traverse the 2,000 ft (600m)-thick Permafrost layer. Unavoidable heat losses from the steam and production tubings have two detrimental effects:
the injected steam quality is greatly reduced, and
melting the Permafrost around the cemented casing could cause the well to sink, with potentially disastrous effects;
the reservoir consists of 6 major layers, relatively thin and of medium permeability (ca.150 md);
the West Sak formation contains some swelling clays, which, when exposed to steam condensate, further reduce the rock permeability;
high well rates, generally required for economic operations in the Arctic, make it imperative to prevent degradation of permeability.
Such difficulties may be overcome by using a novel approach, in which steam is generated downhole in a catalytic Methanator, from Syngas made at the surface from endothermic reactions (Table 1). The Methanator effluent, containing steam and soluble gases resulting from exothermic reactions (Table 1), is cyclically injected into the reservoir by means of a horizontal drainhole while hot produced fluids flow from a second drainhole into a central production tubing. The downhole reactor feed and BFW flow downward in two concentric tubings. The large-diameter casing required to house the downhole reactor assembly is filled above it with Arctic Pack mud, or crude oil, to further reduce heat leaks.
A quantitative analysis of this production scheme for the West Sak required a preliminary engineering of the downhole and surface facilities and a tentative forecast of well production rates. The results, based on published information on the West Sak, have been used to estimate the cost of these facilities, per daily barrel of oil produced. A preliminary economic analysis and conclusions are presented together with an outline of future work. Economic and regulatory conditions which would make this approach viable are discussed.
