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.
Introduction
Horizontal well-pair SAGD has limited options to overcome geological variability within the pay zone (e.g., permeable lean zones, shale barriers, and low vertical permeability). In many cases, because of the presence of such geological variability, the only option available is to relocate the SAGD well pair and thus not recover those reserves in poor geology. An alternate system is proposed that consists of installing vertical, propped permeable planes at varying azimuths from a single vertical well, and complete the well as an injector/producer operating in SAGD mode immediately on startup. Steam-trap control is maintained by an adequate liquid head over the production tubing, as in conventional SAGD. The steam chamber develops immediately at the top of the pay, with performance enhanced because of the full gravity effect on the drainage height at startup and a favorable downward-acting steam gradient. Provided the propped vertical planes are installed continuously throughout the pay, shale barriers and low vertical permeability have zero to minimal impact on performance (i.e., production rate and SOR). Also, because the steam chamber initially migrates laterally and then downward, permeable lean zones have minimal impact on the system's performance. Upward-growing steam chambers in conventional horizontal well pair SAGD are considerably impaired because of shale barriers, low vertical permeability, and the presence of permeable lean zones, resulting in a low production rate and high SOR. If the operating steam pressure must be significantly lowered because of shallow depth, caprock integrity, or water-loss issues, it could result in further impaired conventional SAGD performance over and above that due to the slower drainage of the more viscous bitumen at the lower steam temperature. Water- and steam-loss issues can potentially occur in areas associated with top gas, bottom water, permeable lean zones, and outcrop proximity.
