From exploration to reclamation: using EM methods at SAGD sites in the Athabasca oil sandsSarah G. R. Devriese∗ and Douglas W. OldenburgGeophysical Inversion Facility, University of British Columbia

SUMMARY

In this paper, we divide the steps to explore, identify, and mon-itor a SAGD site into six stages and show how electromagneticmethods can be used at each stage. Three-dimensional inver-sion of airborne EM data provide large-scale, regional geologictrends and delineate paleo-channels and the caprock thicknessat a newly-developed property in the Athabasca oil sands. Weuse semi-synthetic models from resistivity logging and the air-borne data in conjunction with ground-based and borehole EMsurveys to characterize the oil-rich McMurray Formation andmonitor steam chamber growth over time. Periodic EM datacollection and three-dimensional time-lapse inversion allowfor high-resolution interpretations throughout the SAGD pro-cess.

INTRODUCTIONThe main economic interest in the Athabasca oil sands regionis the bitumen reservoir contained within the McMurray For-mation. Because the oil is heavy, mining or in-situ methodsare needed to extract the resource. The primary in-situ methodused is Steam Assisted Gravity Drainage (SAGD), where twohorizontal wells are drilled at the bottom of the formation (But-ler, 1994). Steam is injected and heats the oil, which becomesfluid and drains through the sands. The lower horizontal wellcollects the oil and pumps it to the surface along with con-densed steam. As oil is extracted, the steam chamber growsupwards and outwards.

Developing a site that uses SAGD to extract oil involves manystages: identifying an exploration region, gaining approval fordevelopment, constructing the necessary infrastructure, and pro-ducing the heavy oil, and finally, reclamating the region (Al-berta Energy, 2016). Figure 1 details each of the 6 distinctstages. Several types of data are collected and utilitized through-out this timeframe, including core and resistivity logging, pre-development 3D seismic, temperature and pressure monitor-ing, and repeated seismic surveys.

In this paper, we show what additional information electro-magnetics (EM) methods can provide, and further, how theycan play a role in SAGD. EM methods are sensitive to electri-cal resistivity, which varies as geologic formations change. Inaddition, as the reservoir is steamed, the resistivity decreases(Mansure et al., 1993; Tøndel et al., 2014). EM data can becollected on the surface, in boreholes, or with airborne sys-tems, providing many opportunities to add additional informa-tion to each stage in Figure 1. Zhdanov et al. (2013) invertedairborne EM data to recover the regional structure of the up-per layers over a property in the Athabasca oil sands. Tøndelet al. (2014) used permanent crosswell DC resistivity surveysto monitor the growth of SAGD chambers over time in 2D andDevriese and Oldenburg (2014) studied the feasibility of sur-face and borehole EM methods to recover a steam chamber in

3D. These methodologies can be used together to characterizea SAGD region with EM. We utilize different EM surveys foreach stage of the SAGD process and showcase them on a prop-erty currently under development in the Athabasca oil sands.

Here, we focus on the Aspen property, which is owned by Im-perial Oil, and is the future site of several SAGD well pads.The project area lies about 45 km northeast of Fort McMurrayand 25 km southeast of Fort MacKay in northeastern Alberta,Canada.

STAGE IThe first stage involves background research of the explorationregion, including analysis of existing geologic and geophysicalsurveys, core, and well-log data. Using resistivity logs, weconstruct an initial, simple resistivity model using publicly-available data from 8 wells, shown in Figure 2 (Wynne et al.,1994; Devriese and Oldenburg, 2015). Averaged values fromthe logs are assigned to each geologic formation. The geologyat the Aspen property contains the following flat-lying layers(Imperial Oil Resources Ventures Limited, 2013):

• The Quaternary consists of paleo-channels and glacialtills.

• The Grand Rapids Formation is a transgressional layerconsisting of shales and sands, and can be incised byoverlying Quaternary channels.

• The Clearwater Formation consists of shales and actsas a cap rock for SAGD operations. The WabiskawMember is a transgressional layer containing sands andshales at the bottom of the Clearwater.

• The McMurray Formation is the main oil sands reser-voir.

• A Devonian limestone unit is separated from the Mc-Murray Formation by an unconformity. Prairie Evap-orites may exist along the unconformity, either as saltor salt dissolution (Broughton, 2013).

Thicknesses and average resistivity values for each geologicunit at Aspen are given in Figure 2.

STAGE IIOnce a property is identified, mineral rights and well licensesare obtained and additional geophysical data are collected, in-cluding seismic and well logging. At this stage, airborne elec-tromagnetic data can be an additional source of informationthat can be acquired, processed, and inverted relatively quickly.Airborne EM surveys provide detailed structural informationabout the Quaternary, Grand Rapids, and Clearwater Forma-tions, and are significantly less expensive than seismic surveys.To illustrate, we invert airborne time-domain data flown overthe Aspen property.

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Figure 1: A six-stage timeline outlines the development ofSAGD sites in the Athabasca oil sands (left). EM meth-ods can supplement information at each stage, shown on theright, ranging from regional surveys to time-lapse monitoringof chamber growth. EIA = environment impact assessment,AER = Alberta Energy Regulator.

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Figure 2: Resistivity logging data from 8 wells at the Aspenproperty are used to create a simple 1D resistivity model.

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Figure 3: The figure shows a planview section from the inter-polated 3D model at an elevation of 465 m (or roughly 100m below the surface in the east and 40 m below the surfacein the west). The model shows a channel-like resistive unitin the center, with more conductive regions to the northwestand southeast. Solid line shows location of focus for SAGD;dashed line shows location of cross-sections in Figure 4.

Inversion of airborne EM dataData at the Aspen property were collected using the airborneelectromagnetic system Versatile Time-Domain Electromag-netic (VTEM) which uses a helicopter-towed transmitter andreceiver loop. The system was flow February 2014 and datawere collected at approximately 430,000 locations over a re-gion that extended 8.7 by 11.7 km, for a total of 86 north-south oriented flight lines. Twelve tie lines spaced 1000 mapart were flown in the east-west direction. The data con-sist of the z-component of the time-derivative of the magneticfield (dBz/dt) and were measured at 44 off-time time channels,ranging from 20 µs to 9 ms after transmitter shut-off.

To obtain a pseudo-3D model, we invert 5,773 soundings co-operatively in 1D (Farquharson and Oldenburg, 1993) and in-terpolate the individual 1D models (Fournier et al., 2014). Aninitial model of 20 Ωm was used. The model cells extend 200m by 200 m in the easting and northing directions and 5 mvertically, allowing each layer to be resolved but focusing onlyon regional and large-scale trends spatially. This method is ap-propriate for this region as the 1D assumption holds fairly well,given the expected layered geology at the Aspen property. Foreach inversion iteration, the reference model is updated to in-clude the influence of nearby soundings. We now have a large-scale regional resistivity model for the Aspen property.

Figure 3 shows a plan-view section through the Quaternaryat an elevation of 470 m (or roughly 30-90 m below the sur-face), indicating a paleo-channel extending from the northeastto southwest. A cross-sectional view of the 3D interpolatedmodel is shown in Figure 4(a), and delineates the Quaternary,

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Figure 4: Comparison of (a) the coarse, (b) the fine pseudo-3D recovered model, and (c) the 3D recovered model. Thefigures, at a northing of 8.4 km, are vertically exaggerated toshow variations in the conductivity with depth.

Grand Rapids, Clearwater, and top of the McMurray Forma-tions. The recovered resistivities closely match the 1D modelfrom resistivity logging (Figure 2). The Clearwater Formationis the dominant conductor, starting at an elevation of roughly400 m with a thickness between 30 and 100 m. The Quater-nary and Grand Rapids Formation exhibit varying structures,including the paleo-channel that incises into the Grand RapidsFormation. The top of the McMurray Formation is defined butthe airborne system, due to the overlying conductors and thereservoir’s depth, has limited sensitivity to the McMurray.

The recovered model shows many details but the mesh is coarsecompared to the size of SAGD chambers, which are approxi-mately 1 km long and separated by 100 m. Thus, we desire amore detailed background model. Given the size of a SAGDwell pad, we focus on a smaller region that is of interest forSAGD development (Imperial Oil Resources Ventures Lim-ited, 2013) and increase the data density used in the pseudo-3D inversion. The region is approximately 2.7 km by 2 kmand lies roughly in the middle of the survey area. The meshis now finer, using 30 m by 30 m horizontal cells. We repeatthe pseudo-3D inversion to invert 2,284 soundings and recovera detailed resistivity model of this smaller region. Figure 4(b)compares the fine pseudo-3D recovered model to the coarsemodel. The finer mesh and greater data density allowed forsignificantly more detail in the recovered model.

Using the pseudo-3D recovered model, we forward modeleddata in 3D on an ocTree mesh with the same base cell sizeto test if the pseudo-3D methodology fully represents the re-gion. The 3D forward modeled data fits the observed data inthe middle and late time channels, suggesting the deeper lay-ers, such as the Clearwater Formation, are fairly uniform andflat-lying. However, some early time channels are ill-fit. These

are sensitive to the Quaternary and Grand Rapids Formation,suggesting that there are 3D structures present, such as the in-cising paleo-channel and other smaller variations. We use thepseudo-3D recovered model as the initial model for a full 3Dinversion of the airborne data, inverting 571 soundings usingparallelization and local meshes (Yang et al., 2014). The re-covered model is compared to the coarse and fine pseudo-3Dmodels in Figure 4(c), showing small differences in the toplayers.

Using the airborne EM data, we are able to recover a highly-detailed background resistivity model. However, the data havelimited sensitivity to the McMurray Formation and the inver-sion could recover neither the bottom of the reservoir nor pro-vide an estimate of its resistivity. We use the core and log-ging data shown in Figure 2 to add a semi-synthetic reservoirto the recovered 3D model, shown in Figure 5. To recoverthe reservoir and underlying Devonian, a second survey is re-quired. Surface-based loops can be larger than airborne loops,generating greater currents in the subsurface. Receivers at thesurface can measure 3-component electric and magnetic fieldsto detect the deeper layers and recover them using 3D inver-sion.

STAGE IIIWhile waiting for approval from the Alberta Energy Regula-tor, SAGD scenarios are modeled to determine appropriate EMmonitoring surveys. Synthetic chambers are added to the resis-tivity model (Figure 5), using the formulation by Reis (1992),which was previously used in synthetic SAGD studies (Reitzet al., 2015; Devriese and Oldenburg, 2015). Possible surveyscan have inductive source transmitters, which use ungroundedwire loops. Loops at the surface can have sides ranging from10 to 1,000 m or in boreholes, coils are used. Grounded cur-rent electrodes, such as those used in DC resistivity, can beused both at the surface or in boreholes at multiple frequenciesor times to excite the earth galvanically. SAGD modeling al-lows for winnowing of feasible survey designs (Devriese andOldenburg, 2014), and determine what types of EM instrumen-tation need to be installed during the construction stage.

Synthetic work has shown that a combination of large trans-mitter loops with borehole receivers detect steam chambers atAspen while limiting survey costs (Devriese and Oldenburg,2015). The receivers, using electrodes, measure only the z-component of the electric field, which can withstand the high-heat environment and collect multi-year data (Tøndel et al.,2014). Existing observation wells are used to minimize addi-tional drilling costs. The use of two or more loops at the sur-face excite the chambers in orthogonal directions, providingdifferent information about the chambers. Such a configura-tion allows for better recovery of the chambers through inver-sion than a single transmitter alone. We use this survey config-uration in Stage V to detect and image the chambers shown inFigure 5.

STAGE IVInfrastructure is built during Stage IV, and included pipes, build-ings, roads, and power lines, which are all sources of noise and

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affect EM data. Once construction finishes and the monitoringEM surveys are installed, collecting and inverting data beforeSAGD commences allows for an updated background modelto be used as the initial model for chamber growth monitoring.Understanding how the infrastructure impacts the EM and re-sistivity model can provide key information about reliability,repeatability, and noise levels within the data.

STAGE VDuring production, the permanently-installed EM surveys areused to monitor steam chamber growth. Because of the instal-lation constructed in Stage IV, data can be collected periodi-cally in the same location. While seismic surveys are collectedevery 1-2 years, EM can provide more frequent informationabout the chamber growth. As steam growth occurs relativelyslow compared to EM data collection, multiple data sets in-crease signal-to-noise ratio and provide better data uncertaintyestimates. In addition, unlike borehole temperature and pres-sure monitoring, inversion of EM data readily provides a 3Dimage that can easily be interpreted.

Semi-synthetic steam chambers were added to the backgroundmodel (Figure 5) and EM data was forward modeled usingtwo loops on the surface and borehole receivers that measurethe z-component of the electric field. Noise is added to thedata and uncertainties are assigned prior to inversion. A back-ground model built from airborne and ground-based EM sur-veys is used as the initial and reference model in the inversion.Changes to the resistivity are limited to the region within thereservoir. The results, compared to the true model in Figure 6,show that the chambers are well-recovered in size and shape,including areas where no steam has penetrated and areas wherethe steam meanders. The data were inverted on 3 cluster nodeswith 24 processors and produces a recovered model in 19 hourslater, indicating that the feasibility of EM to monitor steamchamber growth is almost real-time.

STAGE VIOnce SAGD production stops, reclamation must return the siteto its original state. An environmental assessment determinesthe impact of SAGD on the region. Here, EM methods aid inunderstanding how the area changed due to steaming the sub-surface. Airborne surveys delineate changes in channels andaquifers within the Quaternary and identify changes to the re-sistivity of the Clearwater Formation, providing insight intothe cap rock’s integrity and effects of SAGD’s high temper-ature and pressure environment. Finally, ground-based andmonitoring surveys show how the reservoir has changed, nowthat the oil has been removed from the sand matrix. Thesepost-SAGD surveys provide greater insight into the environ-mental effects of SAGD.

CONCLUSIONUsing the development timeline for a SAGD site, we show howelectromagnetic methods can be utilized at each stage to pro-vide detailed information about the Athabasca oil sands geol-ogy, both at the regional and local scale, and how to detectand image SAGD steam chambers. Inversion of airborne EM

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Figure 5: (A semi-synthetic McMurray Formation and Devo-nian basement is added to the 3D recovered resistivity model.Synthetic steam chambers are subsequently added, generatinga realistic model which is used to detect and image chambergrowth using EM.

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data allow for detailed models to be recovered and monitoringsurveys accurately recover steam chamber locations and ex-tents while delineating blockages or areas with no steam pen-etration. This research is on-going and includes investigatingsurface EM surveys to recover greater information about theMcMurray Formation and utilizing time-lapse inversion of EMdata to monitor steam chamber growth over time.

ACKNOWLEDGMENTSWe thank Simona Costin, Brent Wheelock, Greg Josiak, andJan Schmedes at Imperial Oil for the data and discussions onthis project. Our thanks extend to Jean Legault and TimothyEadie at Geotech for aid in obtaining the necessary data files.

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http://dx.doi.org/10.1190/AM2016-13842507.1 EDITED REFERENCES Note: This reference list is a copyedited version of the reference list submitted by the author. Reference lists for the 2016

SEG Technical Program Expanded Abstracts have been copyedited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web.

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