When compared with steam-assisted gravity drainage (SAGD) operations in the McMurray Formation, Athabasca Oil Sands, SAGD projects in the Clearwater Formation at Cold Lake did not perform as expected, likely because of reservoir properties. This paper will use the Orion SAGD case study to: (1) investigate the impacts of reservoir properties on the SAGD thermal efficiency by field evidences; (2) identify key geological parameters influencing each well pad; and (3) summarize major geological challenges for Orion SAGD expansion.
Wireline log data were interpreted to characterize reservoir properties, which were used to build 3D models. 3D visualizations and 2D cross sections of the reservoir revealed spatial distribution and heterogeneity of each property. SAGD production performance was analyzed using: (1) temperature profiles that monitored the growth of the steam chamber; (2) cumulative steam-oil ratios (CSORs); and (3) oil production rates (OPRates), which are direct indicators of thermal efficiency.
Results show that impermeable barriers and low-permeability zones were detrimental to steam injectivity and steam chamber growth, as observation wells in Pilot Pads 1 and 3 did not detect any steam saturation. High-permeability zones favored high steam injectivity and mobility, especially in Pad 105. Steam chambers were irregularly shaped by high shale-content zones, as two sharp spikes displayed on the temperature profile in Pad 103. Low oil-saturation zones and thin net-pays increased the CSORs, as seen in Pads 106 and 104. Impermeable barriers are almost horizontal, making no difference on well pad orientation by their dip angles. Lack of porosity variation made it difficult to identify the impact of porosity on each well pad.
The relatively extensive distribution of impermeable barriers between and above well pairs, as well as the relatively large area of low oil saturation and thin net-pay, were identified as major geological challenges.
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.
Connacher's first oil sands project, the Pod One facility at Great Divide, has been operational since 2007. The successful SAGD project has produced approximately 7 million barrels of bitumen. During the past three and a half years, the impacts of certain predicted reservoir challenges and opportunities have become apparent.
While the quality of the oil sands in this first phase of Pod One is generally good, Pad 101 South in particular has geological zones that affect SAGD operation. This includes a bitumen lean zone, and a gas cap overlying the main bitumen channel/s. Early field results matched with detailed simulations have shown positive results in maximizing well pair production. For the purposes of this paper a lean bitumen zone differs from an aquifer in two ways. The lean zone is not charged, and is limited in size. The operation is also complicated by the fact the gas bearing zone has been depleted through earlier production.
Connacher's operating practice at Great Divide attempts to achieve a pressure balance between the 3 zones (rich oil sands, lean zone, gas cap) to reduce steam loss and maximize production rates. Reducing the pressure encourages steam chamber development growth horizontally and ensures that steam contacts the highly saturated bitumen areas. How this is achieved with the highest positive impact on well productivity is illustrated with operational data and analysis including the results of simulations that recommended the optimum operating strategies.