Selection, Operation, and Evaluation of High Temperature Oil in Water Monitor for Two-Phase Extra Heavy Oil (Bitumen) Test Separator Service at Peace River

Abstract

Abstract Since 1986, Shell Canada has been using two-phase separators for well test application in bitumen production service. As bitumen (8 to 10 API) at Peace River has a density similar to that of produced water, depending on operating temperature, the use of three-phase test separators is not practical. In bitumen service, a three-phase separator requires either heating or cooling plus diluent to reduce the bitumen density so that classical gravity separation can be achieved. With a two-phase system, a means is required to determine the amount of oil in the produced emulsion. This is often achieved by either obtaining a representative fluid sample from the separator and determining the water cut in the lab, or by an online instrument. The two-phase well test system operates at temperatures ranging from 80 to 200 °C at an operating pressure of 1500 kPa. The water cuts from the wells range from 10 to 90 percent with an average of 40 percent. Agar's OW-201 water-cut monitors, in conjunction with Coriolis mass flow meters, were selected for test separator systems at Peace River. This paper describes the field tests performed to verify the Agar OW-201 meter operation for Peace River service, Shell's operating experience to date and the performance achieved during one year of operation. Introduction The Peace River development, shown in Figure 1, is located in northern Alberta, Canada, approximately 450 km north of Edmonton. The Peace River bitumen deposit lies 600 m true vertical depth (TVD) and covers approximately 37,000 hectares (142 sq mi). Total bitumen in place is estimated at one billion cubic meters. The bitumen is an extra heavy deposit having density of 1018 kg/ m3 whilst the connate water has a density of 1006 kg/ m3. Insitu, the bitumen has a viscosity approaching 100,000 cp at a reservoir pressure of 3600 kPa. The bitumen is essentially immobile insitu and steam is used to heat the bitumen and reduce the viscosity such that it can flow to the wellbores. The density of the bitumen and the water changes at different rates resulting in the water and the bitumen having quite similar densities at temperatures from 100 to 160 °C as shown in Figure 2. Because of this phenomenon, classical three-phase separation is not possible without the addition of a diluent (natural gas condensate used at Peace River) to lower the bitumen density such that natural gravity separation can occur. The distance from the central plant facilities to the pad locations precludes either running a condensate line from the plant to the field facilities or installation of tankage for condensate storage due to cost. In addition, the cost of process equipment associated with three-phase separation is significantly higher. Hence two-phase separation was selected for well test application. In 2001, two pads of soak radial wells were developed to minimize costs while significantly increasing well productivity. As shown in Figure 1, the two pads (pad 30 and 31) consist of 8 wells each. The wells on Pad 30 have approximately 1000 m of total horizontal section while Pad 31 has eight wells with 1,500 m of horizontal section. The wells operate in a cyclic steam stimulation mode wherein the wells on each pad are steamed in unison for two months. Steam is injected into each well at rates of up to 800 t/d cold water equivalent (CWE) until a cumulative steam injection of 35,000 t has been attained. During steam injection cycles, surrounding reservoir pressure is increased from 1,000 (3,500 kPa for the first cycle) to 13,000 kPa. Once steam injection is complete, the wells are immediately placed on production. As the reservoir pressure is high, the wells will naturally flow without artificial lift for anywhere from 2 weeks to in excess of 2 months. During flowback, production temperatures up to 220 °C are seen. The wells are choked back to limit excessive fluid production and to minimize the risk of sand production.