Highly permeable lean zones that halt the growth of steam chambers through lateral spreading significantly affect the performance of steam-assisted gravity drainage (SAGD). Nexen's Long Lake and Suncor's Firebag SAGD projects reported existence of intersecting lean zones that behave as thief zones, causing the operating pressure to be lower than the desirable value to reduce heat loss in the lean zones. Given a high steam-oil ratio, long and continuous lean zones located vertically above or near a wellbore pose a challenge. Log analyses and core experiments reveal that the distribution of lean zones is close to being periodic in the upper and middle parts of the McMurray Formation in Athabasca. A simulation model with periodic lean zones is established to analyze the effects of these lean zones on SAGD performance.
We investigate the effects of vertical distribution, horizontal spacing and sizes, and spatial relationship with SAGD horizontal wells. We classify the locations of lean zones as above an injector (AI), between an injector and a producer (BIP), and below a producer (BP). For each location, we assign different spacing for neighboring lean zones of different sizes. A lean zone must reach a critical size before this area can act as a thief zone, which is most significant in AI and BIP situations. In general, BP cases are hardly influenced by lean zones. We further control the vertical distribution of these zones using a triangular periodic function that depends on layer depth and connate water saturation. Changing the period of the dominant function yields different kinds of lean zone distribution patterns; we run simulations for each pattern. Results show that lean zones with small periods have significant effects on SAGD performance. We vary the injection pressure to obtain the highest net present value (NPV) based on the fact that more steam is released through the lean zones at a higher pressure.