Recompletion Workover Program at the Kuparuk River Field

Abstract

SPE Member Abstract An aggressive workover program was undertaken from 1984 to 1987 at the Kuparuk River field on the North Slope of Alaska to recomplete older single completions with the more versatile selective single design. The two oil producing horizons (" C" and "A" Sands) have contrasting reservoir properties which complicate reservoir management. The recompletion allowed fracture stimulation of the "A" Sand which was easily damaged during drilling.. In addition, selective zone management in each production or injection well maximized reserves development under waterflood and gas reinjection. Eighty-six (86) of the field's 529 wells (16%) have been recompleted since 1984 at a total cost of $68 million (1987 dollars). This case history summarizes oil rate benefits and workover cost trends for use in future evaluation of remaining Kuparuk workovers. An economic evaluation approach for justifying injection workovers in a limited-capital environment is also included. Introduction The Kuparuk River field is the 2nd largest oil field in North America lying 40 miles (64 km) west of the Prudhoe Bay field on the North Slope of Alaska Approximately 1.5 billion STB oil (238 × 10 stock-tank m oil) are expected to be ultimately recovered with the aid of waterflood from an original oil-in-place (OOIP) of 4.5 to 5 billion STB oil (710 to 790 × 10 stock-tank m" oil)". Proposed field development could exceed 200 sq miles (518 × 10 m using 700 to 900 wells. To date 529 wells have been drilled on 160-acre well spacings (647 × 10(3) m2). Current oil production rate is 290,000 BOPD (46 × 10-1 m3/d oil) from over 300 producers. Full-field waterflood was commenced in late 1985 using sea water injection. Full field injection currently averages 550,000 BWPD (87 × 10 m3/d water) using over 170 wells on direct-line-drive and nine-spot patterns. Artificial lift is provided through gas lift. Approximately 250 million scf/D (7.1 × 10 std m3 /d) of produced gas is reinjected into the oil rim due to lack of a gas cap or sales market. Each of the current 36 gravel drill sites typically has 16 wells directionally drilled with an average hole angle of 45 degrees. The Kuparuk River sandstone is an oil-bearing Lower-Cretaceous sand/shale sequence divided into 3 major lithological units (" A", "B", and "C" Sands). An example type log is shown in Figure 1. The formation is a slightly-dipping anticline with depth varying from 5600 to 6300 ft (1707 to 1920 m). An impermeable siltstone overlays the "C" Sand providing the formation cap rock. The primary production mechanism is solution gas drive. Reservoir fluids were originally undersaturated preventing a formation gas cap. The upper unit (" C" Sand) is naturally fractured and most productive, ranging in rate from 1500 to 5000 BOPD (238 to 795 m3/d oil). Typical permeability values are 150 to 400 md with a net pay thickness up to 80 ft (24 m). Grain size for this sandstone ranges from very coarse, to very fine with siderite and quartz cementation. The "C" Sand has contributed most of the full-field rate since field startup in late 1981. This moderate permeability zone is typically stimulated using hydrochloric acid. The lower zone (" A" Sand) is less productive than the "C" Sand but areally more extensive providing 60% to 65% of the recoverable reserves. Average permeability is 60 md with a net pay thickness up to 35 ft (11 m). P. 529^