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Energy 37 (2012) 201e206

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Energy

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Characterizing uncertainty in groundwater-source heating and cooling projects inManitoba, Canada

Grant Ferguson*

Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Canada SK S7 N 5A9

a r t i c l e i n f o

Article history:Received 2 July 2011Received in revised form17 November 2011Accepted 21 November 2011Available online 19 December 2011

Keywords:GeothermalUncertaintyRiskGroundwaterHeat pumps

* Tel.: þ1 306 966 7427; fax: þ1 306 966 5427.E-mail address: grant.ferguson@usask.ca.

0360-5442/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.11.045

a b s t r a c t

Uncertainty of performance is an obstacle for greater uptake of many renewable energy technologies thatdepend on local environmental conditions. The ability to successfully complete involving a groundwater-source heat pumps or other projects using groundwater for heating and cooling is highly dependent onlocal geological conditions. Use of geostatistical information on aquifer properties and analysis ofprevious projects in a region show some promise in assessing the risk in these projects. An example ofa possible methodology is given here for open loop heat pump and other groundwater-source heatingand cooling projects in Manitoba, Canada. Stochastic modeling of open loop heat pumps was able toexplain the number of wells unable to produce or inject sufficient water from the aquifer, demonstratingthe efficacy of this technique. This type of approach also provides insight into the viability of possiblemitigation options to successfully finish projects where conditions would not have supported theoriginal design of such systems.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Renewable energy sources face significant obstacles if they areto displace fossil fuel based energy consumption in a meaningfulway [1]. Market barriers such as high up-front costs, uncertaintysurrounding payback periods and questions on system perfor-mance [2]. Determining payback periods is difficult for nearly allenergy options due to fluctuations in the energy markets. Anadditional layer of uncertainty exists where the performance ofa system is dependent on local environmental conditions. Bothsolar and wind energy developments are dependent on localmeteorological conditions that can vary significantly at a range oftemporal and spatial scales [3e5]. Geothermal developments relyon local conditions in the subsurface, which can be highly variableand difficult to characterize due to accessibility issues [6]. Thisvariability can have drastic effects on the design of a geothermalsystem and can exclude certain design options, which in turn canhave impacts on the cost and efficiency of the developments. Therisk associated with this variability needs to be better addressed inthe planning stages of these projects, as is the case for many otherinfrastructure projects influenced by environmental factors [7,8].

Low-temperature geothermal developments generally fall intotwo types: closed loops systems, where aworking fluid is circulated

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in a pipes installed in the subsurface; and, open loop systems thatextract groundwater and usually inject it back into the samegeologic formation. Open loop systems can offer considerable costsavings if an appropriate groundwater resource is present [9]. Thisis particularly true for larger developments, where the need forhundreds ofmetres in drilling and pipe can be avoided by opting foran open loop system [10]. However, it should be noted that there isconsiderable variability in drilling costs even within individualregions that appears to be unrelated to geologic conditions [11]which can complicate comparison of open and closed systems.Open loop systems generally require more maintenance but theamount required can be quite variable, making comparisonsbetween the two types of systems even more difficult. Whether anopen loop is an option at the outset of a project is not always cleardue to the high variability in the hydraulic properties of thesubsurface. Hydraulic conductivity varies over several orders ofmagnitude [12] and is frequently the most problematic variable interms of uncertainty [13]. Even within an individual geologic unit,standard deviations of an order of magnitude or more are common[14]. These variations at the formation level make it difficult toassess the hydraulic responses at pumping and injection wells andheat transport between these wells. Most tools available foranalyzing these systems are based on the assumption that aquifersare homogeneous [15,16]. Heterogeneous situations are morecommon in reality [17,18]. Thermal properties are much less vari-able in the subsurface [19]. For this reason, along with a morestringent regulatory environment [20] and a lack of simple design

G. Ferguson / Energy 37 (2012) 201e206202

tools for open loop systems [21], closed loop systems are ofteninstalled even where an open loop system may offer significantadvantages, such as reduced drilling costs and smaller subsurfacefootprints.

Here the effect of geological heterogeneity on the ability tosuccessfully implement open loop heat pumps is examined in anaquifer in Manitoba, Canada. This setting offers the ability toconduct a prior analysis of the probability of failure using a geo-statistics of the hydraulic properties of this aquifer [22] anda posterior analysis using the provincial well database, GWDrill,which contains geological and construction details for nearly 300injection-withdrawal well pairs. These data are used to assess thelikelihood of completing two wells on the same site for open loopheat pump systems. Examination of well depths, which are chosenduring the drilling process based on a number of crude indicatorssuch as loss of fluid circulation or changes in fluid pressure duringdrilling, provides a rough estimate of the variability present in thisaquifer, assuming that the original intention is to complete twoidentical wells on the same property (Fig. 1).

2. An assessment of uncertainty in a Manitoba, Canada

2.1. The Carbonate Rock aquifer

The Carbonate Rock aquifer is a highly productive aquifer inManitoba, Canada [23,24]. This aquifer is more properly defined asan aquifer system [25], as it is a complex of several interconnected

Fig. 1. Idealized and realistic depictions of open loop heat pu

permeable rock units. The most commonly developed zone of thisaquifer system lies in the upper 15 m, where permeability isenhanced because of the presence of paleokarst features [23].However, many production wells are completed through this unitand into the less permeable underlying carbonates to allow forlarger production rates. This practice is common when productivefractures and solution features are not encountered in the upperreaches of the system.

The Carbonate Rock Aquifer has a long history of developmentfor thermal purposes that extends back more than a century [26].Prior to the advent of modern heat pump technology, coldgroundwater was directly used for air conditioning and also inrefrigeration applications. Development has intensified signifi-cantly during the past two decades, with many new buildingsopting to use open loop heat pump. In addition to these users, thereare a number of industries that use large volumes of groundwaterfor cooling in various processes. Here we examine data collectedduring the drilling of the wells used in these systems to examinethe variability encountered and the implications of this uncertainty.

2.2. Numerical modeling

2.2.1. Model setupA series of stochastic simulations of open loop systems were

created. The realizations of the transmissivity fieldwere generated inthe same fashion described by [27], where the effect of heterogeneityon heat transport was examined in the Carbonate Rock aquifer.

mps installed in the Carbonate Rock aquifer, Manitoba.

G. Ferguson / Energy 37 (2012) 201e206 203

Stochastic realizations using these geostatistics were created usingHYDROGEN [28]. HYDROGEN is a computer code for generatinga distributed attribute, which is modeled as a random space functionwith an assigned covariance function. In this case, an exponentialcovariance function was used. The geostatistics describing thedistribution of transmissivity in the aquifer come from a study con-ducted by Kennedy and Woodbury [22] where the natural log oftransmissivity in m2/s was found to have a mean value of �7.2,a variance of 2.7 and an integral scale of 25,000 m. Transmissivitiesgenerated by this programwere converted to permeabilities using anaquifer thickness of 15 m and fluid properties at 10 �C. Thesestochastic fields are used in the creation of simulations of injection-withdrawal well pairs with METRA, an integrated finite differencecode capable of solving the governing equations for groundwaterflow and energy transport in porous media [29]. Similar resultsshould be achievable with a variety of other numerical codes such asTOUGH2 [30], SEAWAT [31], FEFLOW [32], SHEMAT [33] and a rangeof others [34]. Although it could be argued that this environmentshould be treated as a fractured medium, a comparison of dualcontinuum and equivalent porous medium treatments suggestedthat there was little if any advantage to the application of the morecomplicated dual continuummodel [17]. Thewells are spaced 100 mapart and a production rate of 3.8 L/s is used. Actual spacing andinjection rates are unknown but these are reasonable values foraverage systems based on property sizes, typical uses and licensingrecords. All of thiswater is injected at a temperature 5 �C less than thebackground aquifer temperature. This change in temperature is theupper limit of that allowed by Province of Manitoba guidelines. Afixedheadof10 mabove the topof theaquifer (w5 mbgs) andafixedtemperature of 10 �C were prescribed at a distance of 2000 m fromapoint 50 m fromboth theproduction and injectionwell. These samevalues were assigned as initial conditions throughout the modeldomain. The spacing and injection rate are meant to be typical ofmoderately sized systems in Manitoba although the spacing andproduction rates are excessive for individual residences and lowerthan those at large industrial sites and commercial buildings.

2.2.2. ResultsThirty two of the 100 simulations experienced hydraulic failure

either due to creation of unsaturated conditions over much of theaquifer in the vicinity of the production well or due to excessiveincreases in head around the area of the injection well. Theremainder of the simulations produced results that avoided drasticchanges in hydraulic head, which is anticipated using the geometricmean of transmissivity for the aquifer. However, themagnitude anddistribution of these hydraulic responses were quite variable(Fig. 2).

A range of thermal responses occurred and here we examinethese responses in terms of dimensionless temperatures (Fig. 3),defined as:

T* ¼ ðT � ToÞ=ðTi � ToÞ (1)

where T is temperature, To is background temperature and Ti isinjection temperature. Thermal breakthrough, defined as a dimen-sionless temperature of 0.5, was not anticipated prior to 0.62 yearsand the mean breakthrough for the 100 simulations is 1.33 years.These results are overly conservative in this respect because thesesystemswill generally have dormant periods or periods where theyswitch from heating to cooling mode. Focusing on the first 6months, a range of temperature responses at the production wellcan be observed. Dimensionless temperature changed by as muchas 0.145 in one case and changes were less than less than 0.005 in20 of the simulations. The mean dimensionless temperatureresponse at the production well after six months was 0.025.

2.3. Database analysis

Data used in this study came from a water well databasemaintained by Manitoba Water Stewardship, GWDrill. This data-base contains information on lithologies encountered during dril-ling and well completion details. Hydraulic data, such as waterlevels and production rates are available for only a small subset ofthese wells, and are of unknown quality. Spatial coordinates ofthese wells are only approximate and here we focus on wells thatare on the same property, which are likely to be within 10e100 s ofm, due to the constraints of property boundaries and typicalspacing used in injection-withdrawal pairs [17,35]. It should also benoted that this treatment introduces some bias because wellsdrilled in areas with very low permeability are unlikely tocompleted and recorded in the database.

Of the 273 pairs of injection and withdrawal wells examined,27.9% were completed to identical depths (Fig. 4). The percentage ofidentical wells was slightly higher for commercial systems at 31.4%than residential systems at 25.7%. However, small deviations withina few metres do not represent a significant change in the design ofa system. A 15 m difference is perhaps amore useful metric becauseit is the approximate maximum thickness of the upper 15 m of theaquifer that is frequently targeted in these projects. A difference ofgreater than 15 suggests that there has been a significant deviationbetween the original design of the system. 26.7% of commercialsystems had well depths that differed by more than 15 m, whileonly 15.4% of residential systems had well depths that exceededthis amount.

There was not a strong bias between having deeper injectionwells or deeper production wells (Fig. 5). There was a slighttendency for production wells to be completed deeper than injec-tion wells. This might be strategic to allow for greater amounts ofdrawdown but there are a sufficient number of cases where theinjection wells are deeper to suggest that this discrepancy is arbi-trary. There is also no strong relationship between the difference inwell depths and the depth of the either the injection well or theproduction well.

3. Discussion

The rate of hydraulic failure predicted by the stochastic modelsis similar to the number of injection-withdrawal pairs where thedepths of the wells differs by more than 10 m, which is theapproximate thickness of the aquifer. A perfect match between thethese figures should not be expected because of the variety ofpumping rates and well spacing in actual systems and due tobecause well depths are most often determined by the judgment ofhydrogeologists, engineers and drillers in the field without thebenefit of proper hydraulic testing. Nevertheless, the similarity ofthese figures suggests that geostatistical characterization of aqui-fers and stochastic modeling can provide insight into the likelihoodof successfully installing productive wells for use in an open loopheat pump system. The simpler approach posterior of examiningwell depths in injection-withdrawal pairs may is also likely to beinstructive in this regard.

There are cases where open loop heat pump systems have beenoperating for decades with minimal change in temperature at theproduction well and other examples of thermal breakthrough afterless than a decade. However, insufficient temperature records areavailable for a meaningful posterior treatment of this issue. Ina numerical model examining several closely spaced systems, Fer-guson and Woodbury [17] noted that further investigation of local-scale heterogeneities in the fracture network present in this aquiferis required to produce more accurate heat transport models of openloop heat pump systems. Simulations of an aquifer thermal energy

Fig. 2. Four realizations of the hydraulic head field after 6 months for the stochastic models of the Carbonate Rock aquifer.

G. Ferguson / Energy 37 (2012) 201e206204

storage system situated in heterogeneous unconsolidated sedi-ments by Bridger and Allen [36] found similar issues in achievinggood fits to observations.

Risk in infrastructure projects is often defined as:

RðtÞ ¼ Pf ðtÞCf ðtÞ (2)

where R is the risk associated with failure, Pf is the probability offailure and Cf is the cost of failure. In some treatments of risk, theprobability of failure and cost are discounted by a utility function.Determining a numerical value for risk is difficult in geothermalprojects due to problems with adequately assigning a probabilityof failure due to a lack of information in many settings and alsodue to the wide range of costs that might be encountereddepending on the mitigation options available. If failure of thesystems is anticipated due to a lack of hydraulic conductivity, it isoften possible to drill deeper and produce enough water to makethe system function. This option is relatively inexpensive and isfrequently exploited as shown by the data from Manitoba.However, in some cases this is not possible because of the

presence of low permeability rocks at depth or because of the riskof connecting two hydraulically separate aquifers, which is notpermitted in some jurisdictions because of water quality concerns[37]. In these situations, drilling additional wells can sometimesallow for sufficient production and injection capacity. This can addsignificantly to the cost of the project and there are no guaranteesthat such efforts will be successful. If the results of drilling andhydraulic testing indicate that the aquifer will not support andopen loop heat pump, other heating and cooling options will needto be explored. At this point, the cost incurred will be that whichwas invested in drilling and other investigation costs. A moretroubling situation is that where thermal failure of the systemoccurs after it is commissioned. At this point, a substantialinvestment has been made in installing the system and the costcould be quite high if the system has to be replaced with anotherheating and cooling system at a point prior to reaching thepayback period. However, in some cases it will be possible tocontinue to operate the system if new wells can be installedsuccessfully. This has occurred at two large industrial facilities inWinnipeg, Manitoba. In one case, a large system installed in the

Fig. 3. Randomly selected simulated production well temperatures over time fromstochastic simulations. Relative frequency of breakthrough times is shown on insethistogram.

Fig. 5. Comparison of depths at production and injection wells in the Carbonate Rockaquifer. Solid line is 1:1 and dashed line is the best-fit line.

G. Ferguson / Energy 37 (2012) 201e206 205

late 1970s currently containing active wells has six wells that nolonger active. These wells were replaced by newer wells indifferent locations drilled between 1997 and 2004. A similarsituation exists at a nearby system installed in 1979. That systemoriginally consisted of two production wells and three injectionwells. One injection well was replaced in each of 1987 and 1998and a production well was also replaced in 1998. The two newinjection wells had sufficient capacity to replace the three originalwells, which have been decommissioned along with one of theoriginal production wells. Both of these systems continue tooperate today. However, it should be noted that it is not alwayspossible to predict the distribution of injected water in one ofthese projects [27] and the effectiveness of installing new wells isnot guaranteed. Another class of problems involves clogging ofwells due to various geochemical processes [9]. Such problems areoften difficult to predict and the costs of mitigating these issuescan be variable.

Fig. 4. Frequency of differences in depth between production and injection wells inManitoba.

Predicting the outcome of the hydraulic behaviour and associ-ated heat transport in these projects is not trivial. Examination ofprevious projects may not provide a sufficient basis for forecastingthe performance of a new project because of the variability in thesubsurface. Stochastic models could be useful in this context butthere has been reluctance to embrace these techniques in commonpractice [38,39]. One of the reasons used to avoid such analyses isa lack of sufficient data to produce meaningful geostatistics fora given geological setting. The results of this study and otherstudies in groundwater hydrology [13] suggests that collection andanalysis of such data to support risk assessment in open loop heatpump projects and other groundwater supply and protectioninitiatives is a worthwhile activity. Such initiatives are not likelyeconomically viable for individual developments and should beundertaken by government agencies. However, other options exist,such as using data from other similar settings [39,40]. Geostatisticalapproaches will not replace the need for site-specific investigationsbut will provide some guidance on the level of risk involved prior tosuch an investigation.

4. Conclusions

Uncertainty of system performance is a significant barrier tofurther uptake of groundwater-source heating and cooling tech-nologies. Assessment of this uncertainty through stochasticnumerical modeling and examination of previous projects willallow for better decisions to be made surrounding proposedprojects. There is a need to gather the appropriate data to supportboth of these styles of analysis. The Province of Manitoba’s waterwell database and the regional groundwater model of southernManitoba and associated geostatistics are examples of these datasources. Other possibilities such as geotechnical investigations,geological maps and aquifer analogs may also prove to be useful.

Low-temperature geothermal energy faces an uphill battle tobecome competitive with conventional heating and coolingoptions. In addition to the unpredictable nature of energy markets,geological heterogeneity is a substantial impediment to the uptakeand optimal development of geothermal resources. Ensuringsuccess of all geothermal projects seems unlikely but developing anappreciation of the uncertainty associated with developing thisresource will allow for better informed decisions, development ofcontingency plans and a greater percentage of successful projects.Greater success rates should ultimately lead to a better reputationfor geothermal technologies and increase their popularity.

G. Ferguson / Energy 37 (2012) 201e206206

Acknowledgements

This work was supported by an NSERC Discovery Grant to G.Ferguson.

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