Heterogeneity in the Athabasca oil sands can impede the growth of SAGD steam chambers. Here, we show how controlled-source electromagnetic (EM) methods can be used to detect growth-impeded regions and monitor changes in steam chamber growth. Our achievements are two-fold. We first generate a background resistivity model based on well logging at a field site in the Athabasca oil sands and then estimate the resistivity of the steam chambers using an empirical formulation that incorporates the effects of temperature on the surrounding rocks. Using the resulting 3D model, electromagnetic responses for any EM survey can be computed. The second, and more important, achievement illustrates that imaging SAGD chambers, as they grow in time, may be possible with cost-effective surveys. Our example uses a single transmitter loop with receivers in observation wells. In the wells, only the vertical component of the electric field is measured. Even with this limited data set, the images obtained through 3D cascaded time-lapse inversion identifies the location and extent of an impeded steam chamber. The proposed EM survey acquisition time and processing should be relatively fast and cost effective, and are expected to yield sufficient information to help make informed decisions regarding SAGD operations.
Steam Assisted Gravity Drainage (SAGD) is an in-situ recovery process used to extract bitumen from the Athabasca oil sands in northeast Alberta. In SAGD, two horizontal wells are drilled at the bottom of the reservoir (Dembicki, 2001). Steam is injected into the top well and produces a steam chamber that grows upwards and outwards. At the edge of the chamber, the heated, fluid oil and condensed water flow through the formation and are collected by the underlying horizontal production well. The chamber expands further into the bitumen reservoir as the oil drains (Butler, 1994).
The success of this technique is dependent upon steam propagation throughout the bitumen reservoir. However, reservoir heterogeneity, such as clay beds and mudstone laminations, can cause low-permeability zones that can impact the growth of the steam chambers (Strobl et al., 2013; Zhang et al., 2007). This affects the amount of produced oil and exemplifies the importance of monitoring the steam chamber growth. Successful monitoring can aid in optimizing production efforts by increasing understanding of the reservoir, decreasing the steam-to-oil ratio, locating missed pay, identifying thief zones, and more efficiently using resources (Singhai and Card, 1988).
Because the electrical conductivity of a lithologic unit is affected by steaming, electric and electromagnetic methods are promising tools to detect and image SAGD steam chambers. Additionally, these types of surveys can be much more cost-effective than seismic methods (Engelmark, 2007; Unsworth, 2005). Electric and electromagnetic surveys can also be readily installed as permanent installations. Tøndel et al. (2014) used a permanent electrical resistivity tomography (ERT) installation in the Athabasca oil sands to monitor SAGD steam chamber growth over time. From their study, electrodes can stand up to the high-temperature environment in boreholes surrounding the steam chambers while geophones can break down over time. Devriese and Oldenburg (2015) showed how the method can be extended to frequency- and time-domain EM. Permanent installations can also provide multiple data sets per year, without being limited by access to the area in wintertime only.