Single-Well SAGD: Overcoming Permeable Lean Zones and Barriers

Abstract

Abstract Performance of conventional steam-assisted gravity drainage (SAGD) with horizontal well pairs is impacted significantly in formations with high-permeability lean zones. These lean zones rapidly halt the vertical growth of the steam chamber and spread the chamber laterally, resulting in water-loss issues and a significant impairment of the system to overcome barriers (i.e., low-permeability horizontal layers). The operating-steam pressure is lowered to reduce the water loss to the lean zone, and this in turn accentuates the impact of any barriers, thus giving rise to high steam-oil ratios (SORs) and lower production rates. A vertical single-well injector/producer was proposed that consists of six vertical propped planes installed at varying azimuths from the bottom to the top of the pay. Steam would be injected at the top of the pay, and liquids would be extracted at the bottom. The well would be operated immediately in SAGD mode (i.e., the continuous injection of steam and the continuous extraction of liquids), resulting in peak production achieved within 30 to 45 days. The system would be very efficient because of the immediate drainage available from the propped vertical planes, and also as a result of the full gravity effect on the drainage height at startup and a favorable steam-pressure gradient. Reservoir simulations show that the single-well system’s performance with high-permeability lean zones within the pay would be virtually unimpaired, both in terms of production rate and SOR. Clearly, the system’s operating pressure must be lower to reduce water loss to the lean zone; but, because of the top-down growth of the steam chamber, the permeable lean zone’s impact would be minimized. The net present value (NPV10) of the single-well SAGD system in 35-m thick Athabasca bitumen pay with a 5-m permeable lean zone was estimated to be greater than 6×NPV10 of conventional SAGD. The single-well SAGD system would be much easier to pressure balance and operate than conventional SAGD, resulting in a more robust system, provided the multi-azimuth, propped vertical planes are constructed continuously throughout the pay height.