Impact of steam trap control on performance of steam-assisted gravity drainage

Steam-assisted gravity drainage (SAGD) is a mature commercial heavy oil and bitumen recovery technology operated by over 10 operators in Athabasca and Cold Lake reservoirs in Alberta. Initially, in Athabasca reservoirs, the in situ oil phase viscosity typically exceeds 1 million cP, yet with heating to over 200 °C, the viscosity drops to between 1 and 20 cP. The process consists of two parallel horizontal wells: the top one injects steam into the reservoir while the bottom one collects mobilized oil and produces it to the surface. The process is made more thermally efficient by maintaining a liquid pool that surrounds the bottom production well and prevents escape of steam from the steam chamber. This is often referred to as steam-trap control. In field practice, the continued existence of the liquid pool is monitored by examining the temperature difference, known as the interwell subcool, between the injected steam and produced fluids. Typically, the subcool is maintained between 20 and 40 °C. Given that there are non-uniformities of the reservoir properties in the downwell direction, there is potential for loss of steam-trap control at one or more locations along the wellbore. In this study, the impact of subcool on the thermal efficiency and performance of SAGD is examined by detailed 3D reservoir simulation. The results show that there is a critical subcool value below which the steam trap fails.