Surveillance & Optimization of a Waterflooded Fractured Carbonate Reservoir

Abstract

Abstract The surveillance and optimization of Norman Wells, a waterflooded fractured-carbonate reservoir in northern Canada, is presented. Discovered and on-production since 1920, Norman Wells is one of Canada's largest conventionally produced oil fields. Full development of this resource was undertaken in the early 1980's when directional and horizontal drilling technology allowed access to the majority of the reservoir, which underlies the Mackenzie River. Several artificial islands were constructed to provide pads for directional injection and production wells. The five-spot waterflood patterns were aligned and elongated to take advantage of the directional permeability associated with the natural fracture system. To optimize production and increase ultimate recovery, a sophisticated multi-disciplinary approach to reservoir and production surveillance has been employed. Basic surveillance methods including frequent well testing, fluid sampling, and gas sampling to ensure the accurate allocation of volumes. Surface pressure measurements have been used to accurately allocate production/injection volumes and monitor the status of the wells. Pressure measurements, including static, build-up, fall-off, flowing, and interference well tests have been utilized to monitor reservoir pressures, inflow/outflow performance and reservoir connectivity. Waterflood conformance has been assessed through the use of tracers, cased-hole production logging and injection logging. Waterflood effectiveness has been optimized through the use of voidage replacement analyses, Hall plots, the imposition of injection targets, and staging of fresh and produced water volumes. A holistic assessment of the surveillance information gathered has been accomplished through the use of streamline and floodfront analyses, material balance models, and several generations of full-field reservoir simulations. Ongoing optimization of the depletion of this complex reservoir has resulted in a 20% increase in the expected ultimate recovery since full-field start-up. An example of how the surveillance information has been utilized to improve reservoir performance is presented. prevalent Introduction The Norman Wells oilfield is located approximately 150 kilometers south of the Arctic Circle and lies 450 meters beneath the Mackenzie River in Canada's Northwest Territories (Fig. 1). Oil staining and hydrocarbon seepages had been reported by fur traders and explorers along this portion of the Mackenzie River since the early 1700s. In 1920, Imperial Oil Limited drilled the Norman Wells discovery well on the north bank of the river near these seeps. A refinery was built on-site and from 1921 to 1944 three wells were produced on a seasonal basis to supply the modest needs of the local market (Journal of Canadian Studies, 1981). The advent of World War II and the subsequent threat to Alaska posed by the Japanese occupation of the outer Aleutian Islands prompted the Canadian government, the United States Army, and Imperial Oil to sign the CANOL agreement. The CANOL project was designed to provide a supply of oil to Alaska free from the submarine threat to tankers. This agreement provided for the drilling of 63 additional wells on the mainland and two natural islands, construction of a refinery alongside the Alaska Highway in Whitehorse (1000 km southwest of Norman Wells) and a pipeline between Norman Wells and Whitehorse. Completed in February 1944, the CANOL pipeline was only in operation for 13 months before being shut in at the end of the war. Following the war, the field reverted back to its former level of production as a local supplier. Annual average production at Norman Wells, which had peaked at nearly 900 m3/d in 1944, quickly dropped to about 80 m3/d in 1946. In the following three decades, local demand for Norman Wells products increased to approximately 450 m3/d. In 1981 Imperial Oil initiated a full-field expansion of the Norman Wells field. This expansion used the latest technology in artificial island construction and directional drilling to fully access the remainder of the reservoir beneath the river. A field-wide five-spot waterflood program was implemented and in 1985 pool production increased from 450 m3/d to over 4000 m3/d. A total of 237 new directional wells (producers and water injectors) were drilled (Fig. 2) and a 870 km pipeline was constructed to bring the Norman Wells crude oil to markets in Alberta.