AQUIFER THERMAL ENERGY STORAGE IN THE FRACTURED LONDON CHALK. A THERMAL INJECTION / WITHDRAWAL TEST AND ITS INTERPRETATION

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1 PROCEEDINGS, hirty-second Workshop on Geotheral Reservoir Engineering Stanord University, Stanord, Caliornia, January -4, 007 SGP-R-183 AQUIFER HERMAL ENERGY SORAGE IN HE FRACURED LONDON CHALK. A HERMAL INJECION / WIHDRAWAL ES AND IS INERPREAION Ryan La, Duncan Nicholson, Karen Mayo Arup Geotechnics 13 Fitzroy Street London, W1 4BQ e-ail: ryan.la@arup.co ABSRAC o coply ith current reneable energy targets substantial ne building developents in London are using the ater held ithin the Chalk aquier as a source or heating or cooling. Oing to the ractured structure o the Chalk there is the possibility o rapid transport occurring beteen the abstraction and injection boreholes at a site. his has the potential to cause theral intererence beteen the boreholes and a consequent loss o perorance and, ultiately, eventual ailure o the syste. he nature o the theral transport that occurs at a site ill priarily be dependent on the racture requency in the aquier. o help deterine the requency o the ractures beneath a proposed site in central London a theral injection and abstraction test as designed and undertaken. he test as the irst o its kind in the United Kingdo and its interpretation ill be used to assist ith the design o the proposed heating and cooling syste. he test consisted o heating and storing ater on site, olloed by injection into a packered section o a borehole. Once the hot ater as injected the pup as reversed and the ater abstracted. he teperature o the packered section o the borehole as onitored ith theristors throughout the test. he results ere then interpreted ith the United States Geological Survey (USGS) SURA 3D code. he results suggest that the racture requency beneath the site is suiciently high to liit the possibility o rapid theral transport occurring beteen the to boreholes. he results have been used in urther nuerical odels that predict the long ter perorance o the proposed syste in this lo regie. INRODUCION here has been an increasing interest in using the ater contained ithin the Chalk aquier underneath central London as a source or heating or cooling a building, in cobination ith either ground source heat pups or heat exchangers (Arup, 006). his is in direct response to a change in the planning raeork, instigated by the Mayor o London (Mayor o London, 004). he Mayor s strategic policy on reneable energy states that substantial ne developents should eet 10% o their total energy requireents ro reneable sources. Indeed, it is anticipated that these targets ay go urther and require 0% ro reneables and a 35% reduction in carbon eissions. Using the ater ithin the Chalk aquier is currently the principal ethod that developers have identiied or eeting these targets. Under the abstraction licensing legislation the Environent Agency currently requires that in order to aintain current ater levels in the Chalk aquier ne abstractions ust re-inject the ajority o the ater abstracted (Environent Agency, 005). Reinjection o ater at a dierent teperature to that at hich it has been abstracted presents a nuber o probles including: 1. he possibility o heating or cooling the aquier beneath the site during the lietie o the building.. he possibility o rapid theral transport beteen the injection and abstraction boreholes beneath the site due to the racturing ithin the Chalk. o better understand the long ter perorance o proposed ground source systes (proble 1 above) Arup has used a nuber o nuerical odels based on the USGS SURA 3D code (La, 005). he SURA code is ell proven and is capable o coupling groundater lo ith convective and conductive heat transport. hese odels have to

2 date treated the Chalk as a hoogenous ediu. he lo ithin the Chalk aquier actually occurs through a nuber o ractures, ith the Chalk atrix acting as an ipereable aterial (Price 1987, Blooield, 1996). Although the racturing is oten dense (Moench, 1995) there is the possibility that a sall nuber o distinct ractures ay link the abstraction boreholes ith the injection boreholes (Figure 1.) Interconnecting ractures As the operational lo rates ill be ixed at a constant rate or a proposed syste it is only the requency o the ractures that ill alter the theral breakthrough ties. he racture aperture ill only aect the head required to drive the lo. he requency o the ractures should theoretically deterine the response o the aquier to the injection and abstraction o heated ater. he racture requency ill aect both the luid velocity and the dissipation o energy into the Chalk atrix. his theory as tested in a nuber o dierent nuerical odels previous to the actual test to prove that the response or dierent racture requencies ould be suiciently dierent to be easurable during the test. his ork is in the process o being reported. Abstraction borehole Injection borehole he longer the test, the greater the luid penetration and thereore the ore accurate the interpretation o the aquier structure beneath the site. Hoever, this has to be balanced ith both the an hours required to undertake the test and the ability to heat suicient quantities o ater on site. Figure 1. Illustration o the potential theral intererence caused by ractures linking abstraction and injection boreholes he lo beteen the boreholes ould, in such conditions, be channeled through a saller volue or cross-sectional area o racture. Flo rates ould thereore be uch aster than or a hoogenous ediu. he saller the nuber o ractures that carry the lo, the aster the lo rates ill be. Faster lo rates are likely to cause aster theral breakthrough ties beteen the injection and abstraction boreholes. Conversely, as the nuber o ractures carrying the lo increases, so does the volue and surace area through hich the ater los, decreasing the velocity. In addition, the surace area o the Chalk atrix exposed to the hot / cold re-injected ater is increased, causing greater energy dissipation ro the racture into the atrix. hese to actors are the properties o the aquier that need to be deterined beore a realistic appraisal o the proposed heating and cooling syste can be ade. HE HERMAL ES PROCEDURE he theral test consisted o heating a total o 6000 litres o ater (by eans o iersion heaters) in a nuber o therally insulated containers (Figure ). HERMAL ES ASSUMPIONS he theral test itsel is based on the assuption that luid transport ithin the Chalk can be represented by a nuber o parallel horizontal ractures. his is not an unreasonable assuption or the Chalk in hich ractures, hen observed, are oten parallel and broadly ollo bedding plane structures (Blooield, 1996). Figure. Plastic insulated containers or heated ater storage Once the ater as suiciently heated to a teperature close to the likely peak injection teperature or the proposed syste, the ater as puped into a 3.66 packered section o the

3 borehole. he pup as placed ithin the packered section along ith three theristors; one at the base, one at the iddle and one at the top o the section (Figure 3). he location o the packered section corresponded to the section o the borehole here peak lo rates had been observed in previous lo etre testing, in this case 144 belo ground level. Figure 4 shos the equipent and packer being loered into the borehole. he lo rate ro the heated containers to the packered section o the borehole as kept at a constant 5 l/s. Once the heated ater had been exhausted the lo rate as reversed and the ater abstracted at a constant 5 l /s. heristor 3.66 Figure 4. Equipent being loered into the borehole Pup -144 Chalk Borehole Packer Figure 3. Scheatic section o theral test equipent ES INERPREAION he principal ethod used to interpret the test as nuerical odeling. Analytical solutions have been developed or an injection / abstraction test or the oil industry (Kocabas and Horne, 1990) or a constant injection rate and teperature olloed by a constant abstraction rate. Hoever, the injection rate or the test itsel, although close to constant, did vary throughout the test. In addition, the teperature o the ater ithin the packered section o the borehole as not constant throughout the test. Analytical solutions also becoe increasingly coplex hen dealing ith radial lo (Moench, 1995). o represent the actual conditions that occurred during the test nuerical odeling ith tie variable inputs as considered to be the ost appropriate approach. ANALYICAL COMPARISON o test the validity o the nuerical odel construction and the USGS SURA code or this type o proble a coparison as ade beteen an analytical solution or theral injection through a single racture and the results derived ro a nuerical odel. he analytical solution or theral transport in a single racture is ell knon and is based on the ork by Lauerier, he equations that need to be solved to deterine the teperature at any point ithin the racture are as ollos:

4 Eq.1 Eq. here: δ δ λ δ ( Cρ ) + ρcv = t h δz δ δ ( Cρ ) + ρcv = ( λ) t ρ = density o ater C = speciic heat capacity o ater φ = porosity o the racture δ ( C ρ ) = total voluetric heat capacity o the racture (includes any aterial in the racture) q V = velocity o the ater V = ρ φ q = ater injection rate δ ( ) A = teperature gradient in the ater in the racture (λ) = bulk theral conductivity o the racture λ = bulk theral conductivity o the aterial around the racture δ = rate o change o teperature gradient in the racture h = the vertical depth o the racture δ δz = the teperature gradient in the aterial surrounding the racture. Equations 1 and can be solved (Zabarnay, 1998) ith the olloing boundary conditions: = teperature ithin the racture at any point = teperature o the ipereable z, surrounding aterial. = initial teperature o the aquier = injection teperature 1 t = (,, ) ( x, y,0) = 1 ( x,0) = 1 ( o, t ) = z,0) = 1 = 1; x z, z, he solution then becoes: θ hen Eq. 3 And = er h ( Cρ) x t > C ρ V λx C ρv λ ( Cρ) x t Cρ CρV ( Cρ) x θ = 1 hen t < C ρ V Where θ = ( ( x, t ) ) /( 1 ) ( x, t ) Equation 3 as solved in an Excel spreadsheet and the teperature at a nuber o dierent points along the racture plotted against tie. he paraeters used are those shon in able 1. Paraeter Value Units C 418 J/kgºC ρ 1000 kg/³ ρ r 800 kg/³ λ.8 W/ C r 890 J/kgºC q 0.5 l/s φ 0.3 diensionless φ 0.98 diensionless t variable s h able 1. Paraeters used in the analytical solution

5 he nuerical odel in SURA consisted o a single racture split horizontally by syetry to save on odel running tie. In order or the nuerical odel to treat the racture as loing ater the dispersivity as set to zero. he odel grid diensions ere set to coply ith the Courant nuber hich represents the relationship beteen the velocity o the luid and the grid size o the odel. A Courant nuber less than 1 is preerred or nuerical accuracy: t C = v l through the nodes is constant this paraeter ill only aect the pressure required to achieve this lo. he results ro the coparison beteen the analytical odeling and the nuerical odeling can be seen in Figure 6. his shos that the correlation beteen the to results is extreely good or all distances ro the injection point. he ethod used to represent the racture and the odel construction and running (tie stepping, grid deinition) are clearly acceptable. Where: C = Courant nuber v = average linear velocity o the luid t = nuerical tie step l = diension o the largest grid cell in the direction o lo Fluid enters SURA through individual nodes and is then applied across a cell (Figure 5) Injection ace eperature (C) ie (seconds) a) Fracture plane represented by to planes o eleents Injection nodes or the racture eperature (C) Eleent Rate A Rate B Injection node Cell Rate A ie (seconds) b) Figure 6. Coparison o results or a) analytical and b) nuerical odeling or a single racture Figure 5. Representation o racture plane in the nuerical odel A single injection cell thereore spans our eleents. he racture plane itsel as represented by to ros o eleents to ensure that the luid entered the odel through the eleents ith the correct hydraulic and theral paraeters. In addition, nodes on the outer edge o the odel ere allocated hal the injection rate o the other injection nodes (Rate A and Rate B respectively). he racture hydraulic conductivity applied to the odel as that derived ro the cubic la or a 1 racture, although as the injection rate HERMAL ES MODEL CONSRUCION As ith all nuerical odels a balance has to be aintained beteen the odel resolution and the odel running tie. o reduce the odel grid size and thus the running tie, horizontal and vertical syetry as used to split the packered section o the borehole (Figure 7). Ater this alteration, the odel running tie as reasonably ast.

6 Z X Y X Injection point Figure 7. Lines o syetry and odel layout MODEL OPERAION he data ro the injection period o the actual test as input as a tie dependent variable into the SURA code. he average teperature recorded in the borehole during injection as used as the teperature input. he code as then recopiled using the Copaq Fortran copiler beore being run. he diension o the atrix above the racture (X in Figure 7) as varied or each odel run and the lo rate altered accordingly. he odeling coenced ith a racture requency o one or the 3.66 packered section and as progressively increased by reducing the distance X. RESULS X Fracture Lines o syetry Matrix he results o the theral test can be seen in Figure 8. he results sho that a near constant lo rate as achieved during the test. he oent hen the lo is reversed can also be seen. he teperature recorded at the theristors varied by up to a axiu o 3 C during the test ro hich it is inerred that the ater in the packered section o the borehole as not ell ixed during the injection process. he teperature rose to a peak o 3 C beore the abstraction occurs and then dropped o during abstraction. Once the abstraction has inished there as a slight teperature rise in the borehole. eperature ( o C) ie (seconds) Figure 8. he results o one o the theral tests Flo (l/s) he results o the average teperature recorded during the test and those generated by the nuerical odel or dierent racture requencies can be seen in Figure 9. Fracture requencies ithin the packered section o, and 3 are shon to highlight the dierent predicted responses. ep (C) ie (seconds) -1 - Actual est 3 Figure 9. heral test results copared ith nuerical odeling results or dierent racture requencies It can be seen ro Figure 9 that the actual result ro the test could not be atched exactly to a particular racture requency. Hoever, it is clear that the response obtained ro the test is ore siilar to the curve generated or a higher racture requency than or a loer requency. Flo is clearly not occurring through a sall nuber o isolated ractures (the response curve generated or a racture requency o over the packered section, particularly in the early stages o abstraction, is arkedly dierent to that obtained ro the tes. Even a racture requency o still gives a ore rapid drop ransducer Botto Middle op Average Flo

7 in teperature during abstraction than that achieved during the test. A closer representation o the test is achieved hen the racture requency rises to approxiately 3 (one every 0.11). Although this result is still not identical to that observed during the test it is uch closer than those obtained ith loer racture requencies. he dierence in the heat dissipation pattern or dierent racture requencies can be seen in Figures 10 and 11. Figure 10 shos the extent and nature o the theral penetration or a single racture. In this case, the penetration distance is approxiately 7 ro the borehole during the injection period. Conversely, or a racture requency o 3 (one every 0.11) the penetration is closer to 3 and the theral penetration pattern is arkedly dierent. Injection point 10 approxiately represented as a series o horizontal ractures ith a requency o one every 0.`1 (approxiately 3 ractures over the packered section o the borehole). his result can be used in subsequent odels that predict the perorance o the syste over the lietie o the building. DISCUSSION here are soe issues that arise ro this sort o test and its interpretation. O priary iportance is the penetration that is gained during the test. With a penetration distance o approxiately 3 it is likely that the test is, to a degree, representing the acidised portion o the Chalk aquier. he lo in this acidised section o the aquier ill occur through preerentially enlarged channels. As the distance ro the borehole increases the channels are likely to becoe less enlarged and the lo ill travel through a greater nuber o ractures. he test ill thereore be conservative and under estiate the racture requency. REFERENCES Figure 10. heral dissipation pattern or a single racture 10 Figure 10. heral dissipation pattern or a racture requency o one every 0.1 CONCLUSIONS A theral test as designed and ipleented at a central London site to investigate the nature o the theral transport. he results o the test indicate that the lo as not carried by a single or sall nuber o ractures. he ractures that carry the lo can be Arup. (006), Internal project lists. Mayor o London (004). Green Light to Green Poer he Mayor s energy strategy. Environent Agency (005). Rising Groundater Levels in the Chalk-Basal Sands Aquier o the Central London Basin. La, R., Nicholson, D (005). A 3 diensional odel o the potential ipacts on an aquier o to energy storage systes. Proceedings, Consoil. Price, M., (1987). Fluid lo in the Chalk o England. In: Go, J.C., Willias, B.P.J. (Eds.), Fluid Flo in Sedientary Basins and Aquiers Geological Society Special Publication No. 34. he Geological Society, London, pp Blooield, J., (1996). Characterisation o hydrogeologically signiicant racture distributions in the Chalk: an exaple ro the Upper Chalk o southern England. Journal o Hydrology. McDonald, A., Allen, D., (001). Aquier properties o the Chalk o England. Quarterly journal o Engineering Geology and Hydrology. Kocabas, I., Horne, R.N., (1990). A ne ethod o orecasting the theral breakthrough tie during reinjection in Geotheral reservoirs. Proceedings, 15 th orkshop on Geotheral Reservoir Engineering, Lauerier, H.A., (1955). he transport o heat in an oil layer caused by the injection o hot luid. Applied Science Resources 5, -3, Zabarnay, G.M., Shurchkov, A.V., Chetveryk, H.O., (1998). he coputer progras or calculation o soe paraeters o geotheral systes. Institute o

8 Engineering herophysics, Ukranian National Acadey o sciences, Kiev,. Moench, A.F, (1995). Convergent radial dispersion in a double-porosity aquier ith racture skin: analytical solution and application to a ield experient in ractured Chalk.