Trace element mobility and the role of subsurface heating

Jon Fennell

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater Quality

ABSTRACT: Canada™s oil sands represent an important resource to the Canadian economy, and exist as a strategic supply of oil for the world. These hydrocarbon deposits reside beneath a vast area in northern and east-central Alberta, and are currently being accessed by two means: i) surface mining; and ii) thermal in-situ techniques such as steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). The vast majority of the oil sands deposits (>95%) beneath the Cold Lake, Athabasca and Peace River regions will require in-situ technology to recover the highly viscous bitumen to surface for processing and delivery to market. As such, a relatively large array of well pads and associated subsurface infrastructure will be required over the coming years to recover this valuable resource. The process of recovering bitumen from the oil sands deposits involves the injection of high-temperature steam (up to 300°C) in order to reduce its viscosity so it can be recovered to surface. Given these elevated temperatures, questions have been raised regarding the potential for artificial heating of adjacent aquifers, resulting in the mobilization of certain trace elements from the sediments to the adjacent groundwater. From a single well-pad perspective, the risk to groundwater quality is low given the generally localized effects. However, under the right conditions effects could extend beyond individual pad footprints and potentially affect larger areas. Therefore, an understanding of the source (or sources) of trace elements that may be released to the groundwater environment, mobility potential in response to increased temperature, and attenuating mechanisms once subsurface conditions cool is needed to properly frame the associated risk. The research program designed to answer this question employed both a field and laboratory-based approach. For the field component, an array of 21 monitoring wells (completed at depth between 75 and 90 metres below ground level) was established to determine the ambient groundwater flow and quality conditions, as well as temporal changes during the operation of a newly established production pad. With respect to the laboratory component, a series of materials testing experiments (SEM, XRD, XRF, Synchrotron Light, Sequential Soil Extraction, ICP-MS) were completed to determine sediment mineralogy, and the release of various elements under simulated geochemical scenarios. Additionally, specially designed heating experiments (ranging from 5°C to 200°C) were conducted to establish the activation energy associated with key trace elements mobilized from the sediments. Results from the field and laboratory experiments indicated the release of calcium, sodium, potassium, silicon, antimony, arsenic, boron, molybdenum, and zinc starting at temperatures as low as 30°C. Additionally, alterations to the groundwater flow field were identified due to changes in local hydraulic conductivity values in the heated zone related to a reduction in the density and viscosity of the groundwater. The process for the release of elements such as arsenic was identified as surface-related reactions likely associated with Fe-smectite mineral surfaces, and enhanced by reductive dissolution processes. The role of reductive dissolution was identified through a number of isotope measurements on dissolved and gaseous species, as well as microbial tests conducted using the BARTŽ testing approach. The activation energy (Ea) identified for release of various elements was found to be less than 10 kcal/mole, consistent with ligand-promoted reactions or mineral dissolution via surface-related reactions. The stimulus for element mobility was identified as the release of soluble organic nutrients from the natural sediments, combined with increased temperature conditions and microbial activity. Transport and fate modeling, as well as field-verified results, identified differential retardation of elements released from the sediments (e.g., Rf for arsenic = 1.6), consistent with natural chromatographic dispersion and anticipated attenuating properties of the subsurface. The presentation related to this research project will highlight key findings relating to the source(s) and mechanism(s) responsible for element release under elevated temperature conditions, the regulatory response to this phenomenon, and an approach to framing associated risks to the groundwater environment and nearby receptors.

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