Short- and long-term ground deformation due to cyclic steam stimulation in Alberta, Canada, measured with interferometric radar

Shortand long-term ground deformation at the cyclic steam stimulation (CSS) enhanced oil recovery sites in the Cold Lake region of Alberta, Canada, was measured with RADARSAT-2 differential interferometric synthetic aperture radar (DInSAR) during 2011–2016. The interferometric processing of 84 highresolution spotlight images revealed extensive short-term ground uplift and subsidence greater than 15 cm over the 24-day satellite revisit cycle. Time-series analysis based on the multidimensional small baseline subset (MSBAS) methodology demonstrated that while the predominant deformation signal is cyclical in nature, a long-term subsidence and uplift with maximum deformation rates greater than 5 cm/year is also present. These measurements suggest that CSS injection and production operations result in the long-term semipermanent deformation in the area of reservoir operations, and that the response to injection and extraction can differ from the previous cycles. Introduction Enhanced oil recovery (EOR) techniques are employed to increase the amount of extremely heavy crude oil or bitumen extracted from subsurface reservoirs. The steam-assisted gravity drainage (SAGD) EOR technique consists of a pair of horizontal wells, one drilled a few meters above another (Butler, 1994; Pearse et al., 2014). High-pressure steam is continuously injected into the upper well to heat the oil and reduce its viscosity. The heated oil drains to the bottom well from which it is pumped out. The cyclic steam stimulation (CSS) or “huff-and-puff” EOR technique requires only one well, operated in three different phases (Alvarez and Han, 2013). The injection phase consists of injection of high-temperature (i.e., >200 degrees) and high-pressure (i.e., higher-than-formation fracture pressure) steam. High-temperature steam reduces the viscosity of the bitumen. High-pressure steam dilates the reservoir, which induces fracturing, resulting in increased permeability. The soaking phase allows the heat, which is mainly conveyed by the steam, to seep into the formation due to the combined effects of conduction and convection. The increased mobility of bitumen accelerates this process because of enhanced convection. During the production phase, fluid flow in the well is reversed, and the heated bitumen, along with the condensed steam, is extracted from the reservoir. The production phase can last from a few months to a few years (Vittoratos et al., 1990). Injection, soaking, and production phases comprise one cycle. A new cycle is initiated after the production rate from the well becomes too low as the reservoir is cooling down. Cycles are repeated until the reservoir operation is no longer economical. SAGD is used in thin reservoirs with high vertical permeability, and CSS is used in moderately thick reservoirs with poorer vertical permeability. CSS can also be employed in less “clean” Sergey V. Samsonov1 reservoirs, where bitumen-bearing sandstones are interbedded with thin layers of low-permeability shale, but this requires the reservoir to be “dryer” (i.e., less water content). Thin shale baffles, which would prevent efficient steam circulation in SAGD operation, are broken during CSS operation because of higher injection pressure (Richardson et al., 1978; Jiang et al., 2010). Consequently, higher confining pressure typically is required to contain the reservoir fluid in the operation zone. The Alberta, Canada, oil sands contain the third largest oil reserves in the world, after Venezuela and Saudi Arabia. As of 2015, Alberta’s proven reserves were 166 billion barrels, produced at a rate of 3 million barrels per day (Alberta Energy Regulator, 2016). Crude bitumen in Alberta is contained within three large oil sands areas: the Athabasca, centered on Fort McMurray; Peace River to the west; and Cold Lake to the southeast. The Cold Lake Oil Sands Area (CLOSA) is known for its rich bitumen deposits (about 34 billion barrels) hosted within the Lower Cretaceous Mannville Group (AER, 2016). The CSS operations have been ongoing in this region since the mid-1970s. The Clearwater Formation, which is buried at roughly 400 m depth in the Cold Lake region, is the most frequently targeted stratigraphic unit for CSS (Walters et al., 2000). The inexpensive wet steam (higher water saturation), instead of a dry steam (less water saturation) that is often used in SAGD, is the most common injection fluid in Cold Lake CSS operations. The liquid solvent addition to steam for EOR has been tested in order to enhance in-situ mobility of the bitumen and ultimately the recovery rate (Jiang et al., 2010). In recent years, there have been attempts to utilize SAGD for the recovery of bitumen from the Grand Rapids Formation in the CLOSA (Willmer and Quinn, 2015). The Grand Rapids Formation, which consists of mostly shoreline sandstones, has a high water content that limits the effectiveness of CSS (Barson et al., 2001). The overlying low permeability shales of the Upper Cretaceous Colorado Group act as a caprock that can hold the steam beneath, making SAGD even more viable (CNRL, 2010). Other production or pilot projects using techniques similar to CSS include operations targeting the Bluesky Formation in the Peace River region (Mohiddin and Koci, 2007) and Grosmont Formation in the Athabasca region (Yang et al. 2014). Pilot projects in the Athabasca region also test the combination of both cold and warm solvent injection at higher pressures to achieve reduced bitumen viscosity and enhanced formation permeability (Jiang et al., 2010). Outside Canada, CSS is employed in EOR operations in Santa Barbara in Venezuela and Liaohe in China (Alvarez and Han, 2013). The studied area (Figure 1) is one of the oldest EOR sites in Alberta (Buckles, 1979). Although recovery to date has barely exceeded half of the projected ultimate recovery, the production rate is roughly at its peak, as reported by Imperial Oil (2010). 1Natural Resources Canada. http://dx.doi.org/10.1190/tle36010036.1.