On the Ratio of Energy Produced to Energy Injected in SAGD: Long-Term Consequences of Early Stage Operational Decisions

Abstract Steam-Assisted Gravity Drainage (SAGD) is a recovery process used to extract bitumen from underground reservoirs. The efficiency of SAGD is determined by the volume of steam injected (reflecting cost and greenhouse gas emissions) relative to the volume of oil recovered (revenue). Another way to look at this relationship is as a ratio of cumulative energy produced to energy injected (EPEI). First described in this study, EPEI ratios offer a novel perspective on SAGD because they reduce the inter-well comparison bias caused by operational choices such as steam temperature, steam quality and well length. There are numerous studies on SAGD from an experimental, analytical or numerical simulation perspective, but all require assumptions and constraints specified in advance, and can only return an approximation of field performance. Studies that work exclusively with field data have typically been used for prediction modeling only. In contrast, this study employs a novel methodology by working within the existing constraints and operational choices of SAGD field wells to infer the number of steam-oil performance relationships present, and how long they last. One by one, we eliminate parameters that cannot cause the performance differences seen in the field using only deductive reasoning and multi-parameter sensitivity analysis, and arrive at possible reasons for the performance relationships found. 1520 SAGD well pairs in Alberta, Canada are used, which is more than 90% of the wells in Alberta over the last 20 years, and to the best of our knowledge, the largest field well set of any SAGD study to date. This comprehensive data set ensures that findings are applicable across the industry. Results reveal a new and surprising finding that the EPEI ratio is set during the first few months on production, and is very difficult to change thereafter. Hence, operators have a short, time-limited opportunity to influence each well's recovery ratio (and therefore its profitability and greenhouse gas emissions). This reflects the importance of process start-up and the first few months after start-up when the near well region of the reservoir is conditioned. Finally, the stability of the EPEI ratio permits a simple, new model capable of predicting cumulative recovery months in advance.