BOREHOLE SEALING IN A COAXIAL HEAT EXCHANGER BY BENTONITE TREATMENT Frank.B.Cruickshanks, Olof Andersson, James Bardsley Climate Change Division, Meteorological Service of Canada, 45 Alderney Drive, Dartmouth, Nova Scotia, B2Y 2N6, Canada SWECO VIAK AB, P.O. Box 286, SE-201 22; Malmö, SWEDEN Earth Source Energy Systems Inc PO Box 531, Halifax Central, Halifax NS B3J 2R7 E-mail: Frank.Cruickshanks@ec.gc.ca; olof.andersson@sweco.se; palmer@ns.sympatico.ca ABSTRACT To store cold only for direct cooling using Borehole Thermal Energy Storage technology requires more efficient Borehole Heat Exchangers (BHE) than the single or double U-pipes that are normally applied in these types of storage systems. A more cost effective (low thermal resistance) coaxial (concentric) borehole heat exchanger suitable for cooling without chiller machines has been tested in Halifax, Nova Scotia, Canada. Field tests were conducted to determine whether a borehole could be sealed with bentonite to make it water tight and suitable for coaxial type borehole heat exchanger i.e. no groundwater flux. The experiment is part of pre investigations needed for a full-scale pilot BTES cooling plant expected to be realised by the Department of National Defence, dockyard naval base on Halifax Harbour. INTRODUCTION In October of 2004, a borehole sealing experiment aimed at demonstrating that a borehole could be sealed tightly by using bentonite as a sealing medium (Bentonite is a natural clay mineral, derived from volcanic ash that will swell and gel in the borehole). The experiment also involved evaluating a given procedure for the bentonite treatment and to monitor and evaluate the efficiency of treatment. The experiment was part of the pre investigations needed for a full-scale demonstration BTES cooling plant expected to be realised for the Department of National Defence on the Halifax Harbour. BACKGROUND To store cold only using Borehole Thermal Energy Storage (BTES) technology requires more efficient Borehole Heat Exchangers (BHE) than the single or double U-pipes that are normally applied in these types of storage systems (Figure 1). Figure 1 Traditional U-Tube and Concentric Tube Heat Exchangers
A single U-pipe in a grouted borehole will typically have a thermal resistance that corresponds to 5-6 o C temperature differences between the rock and the heat carrier fluid in the U-pipe. A double U-pipe will reduce this loss of temperature quality to 3-4 o C. However, a coaxial pipe or tube will have an optimal thermal efficiency and cut the difference to 1-2 o C, since it allows the fluid to have a direct contact to the borehole wall. The efficiency differences are illustrated Figure 2. Figure 2 Thermal resistances for different kinds of BHE systems (2) In addition to the higher efficiency, potential advantages with a coaxial type BHE includes the possibly of having water without antifreeze as the heat carrier fluid, which will increase the capacity of the store in cooling and decrease the pumping cost, as water with antifreeze mixed in is more viscous than water alone. As compared to a U-Tube type configuration, the length of the borehole required to cover the same amount of cooling is greatly reduced. The boreholes can be drilled in a smaller diameter and to greater depths, which reduces drilling costs. To further improve the efficiency the inner part of the coaxial tube could be insulated. There are some draw-backs to using the coaxial BHE; most obvious is that the boreholes must be almost perfectly sealed to not allow the heat carrier to infiltrate into the rock. Some other disadvantages may include an increased cost for the coaxial tube (compared to single U-pipe), difficulties inserting the tube in a centralized position, a current lack of standard well top equipment and the limited availability of smaller diameter drilling equipment in Canada, say 114mm. EXPERIMENTAL OBJECTIVES The main objective with the sealing experiment was to demonstrate that a borehole could be sealed properly by using bentonite as a sealing medium (bentonite is a natural clay mineral, derived from volcanic ash that will swell and gel once it has been inserted into the borehole fractures).the experiment also involved evaluating a given procedure for the treatment and to
monitor and evaluate the efficiency of treatment. The experiment is regarded as part of pre investigations needed for a full-scale demonstration BTES cooling plant expected to be realised for the Department of National Defence in Halifax, Nova Scotia, Canada THE BOREHOLE AND GEOLOGY An existing borehole located in Armdale, Halifax was used to perform the bentonite sealing experiment. The borehole is 100 m deep and has a 171.4 mm steel casing down to 6 m. The diameter of the open hole is 155.6 mm. The borehole is drilled into an anticline in the Halifax Formation. A TV inspection performed prior to the experiment showed that the rock consists of fine-grained thin-bedded slate. The bedding is clearly of metamorphic origin and shows a low water yield, Figure 3. Figure 3 Geology of the Halifax Area and the Armdale and Dockyard Sites. EXPERIMENTAL SET-UP Arrangements were made to elongate the existing steel casing. A plastic coaxial borehole heat exchanger (BHE) was installed and a specially constructed high pressure well-top with inlet and outlet tubing and couplings was also installed (Figures 4a and 4b). For the treatment, a unit for grouting was used as well as a water tank with a volume of 1 m 3. A truck with back up water was available during the treatment and a vacuum-pump truck was brought to site for the disposal of bentonite slurry after treatment.
Figure 4a. Picture of principal experimental borehole configuration Figure 4b. Diagram of principal experimental borehole configuration TREATMENT PROCEDURE The grouting unit was first connected to the water tank and to the borehole inlet. The outlet from the borehole was connected to the water tank. Whereas the water tank was connected to the mixer to blend the slurry and the water that was replaced by slurry was disposed in the water tank (Figure 5). Figure 5 Experimental Set-up, the intake of the well is being connected to the grout pump
In the second step bentonite slurry was pumped down through the coaxial pipe to the bottom of hole, gradually replacing the water in the borehole with slurry. This procedure continued until returning water from the well started to show traces of slurry as a sign that it was filled to the surface. The slurry was mixed in a portion of 1/25 (1 kg bentonite to 25 l of water). All together 55 kg of bentonite were mixed with 1375 l of water. A high yield type of bentonite was used (Figure 6). The approximate properties of the mixture were 4% bentonite solids by weight and a viscosity of 8-9 centipoises. The slurry was pumped into the hole at a rate of some 50 l/minute and it took approximately 35 minutes to fill the borehole. Figure 6 A light bentonite slurry was blended in a mixer and then pumped into the borehole In the third step the slurry was circulated in the borehole for approximately 20 minutes with the highest flow rate possible. During this phase the slurry became homogenously mixed. In this circulation the smaller tanks on the grouting unit were used (Figure 7). Figure 7 Mixing of the slurry in the grout pump
During the process of pumping the borehole, pressure was noted on the supply pipe manometer (Figure 8). Borehole pressure went from being approximately 2.8 bars, at the end of pumping down the slurry, to 2.0 bars during the circulation process. In the fourth step the valve on the return pipe was gradually closed in order to pressurise the borehole. This stage lasted for 5 minutes and a maximum pressure of 6 bars was kept for another 5 minutes. Figure 8 A manometer on the inlet tube monitored the borehole pressure during the treatment process In the fifth step of the treatment, the slurry was replaced by water. The water was pumped through the borehole inlet and the slurry disposed of in a simple plastic container from where it was sucked to the tank on the vacuum truck (Figure 9). Figure 9 Thickened slurry being vacuum
The replacement of slurry with water took some 20 minutes of pumping and some 1500 l of water was used. After this time the "thicker" slurry was out of hole but another 2000 l of water was flushed through the system to clean the hole to a greater extent. At that time the return water was purposely slightly grey due to still remaining colloidal of clay minerals. In the sixth step of the procedure the borehole was placed under hydrostatic pressure in order to set the bentonite and to let residual bentonite provide further sealing. The inlet tube was connected to the city water (Figure10) and by a flow meter on the supply line the infiltration rate could be monitored. Figure10 The well is hooked up with the city water in order to finalise the sealing and to measure the tightness The hydrostatic pressure was set to 4 bars and was kept for 48 hours, but allowing a flow of water to the borehole to keep pressure up. Hence, the infiltration rate was measured. Readings of infiltration rate and pressure were taken at five occasions during this final stage of the experiment. TREATMENT EFFECT The injection capacity of the well before treatment was measured at 4 l/min, with an injection pressure of 0.2 bars. This corresponds to a specific capacity of 2 l/s x m. Immediately after the treatment, the specific capacity had dropped to 0.02 l/min showing an initial treatment factor of approximately 100. After three hours of injection, the injection capacity was down to 0.013 (factor 160) and after 24 hours to 0.006 (factor 330). At the final reading, 48 hours after treatment, the capacity was down to 0.005 l/min x m representing a treatment factor of 400. This means that the well has a more than 400 times lower groundwater flow capacity after treatment than before.
CONCLUSIONS AND OBSERVATIONS The concept of using bentonite for sealing the borehole wall to an acceptable degree worked out as predicted. The sealing factor is in the order of 400, which makes the borehole water tight in practical terms. A learning lesson, partly unexpected, was that the borehole had to be under injection pressure for a longer period of time to reach the final clogging effect. However, this final setting of the bentonite is not labour intensive. The procedure for sealing using a grouting unit and a couple of water tanks also worked out as predicted. The operational time for the procedure with the equipment at the site was approximately 4 hours. By increasing the grout pump capacity, and by treating several boreholes in series, the time required to seal them could be reduced by a considerable amount. Theoretically, three to four times fewer boreholes i.e. less drilling will be required to store and retrieve energy from a cold store in the Halifax Formation slates using the concentric borehole design. ACKNOWLEDGEMENTS The Panel on Energy Research and Development (PERD) and Environment Canada, Atlantic Region, Climate Change Division, for funding and support. Bluenose Well Drilling Ltd. represented by Ralph Jacobs, for performance of sealing. REFERENCES 1. Andersson, Olof. Coaxial BHE Experiment in Halifax Canada (Borehole Sealing by Bentonite Treatment), Report Prepared for Environment Canada, Climate Change Division (Atlantic Region), Halifax, October 2004. 2. Hellström, G, and Kjellsson, E. (2000). Laboratory measurements of heat transfer properties of different types of borehole heat exchangers. Proc. of Terrastock 2000, Stuttgart, Germany, August 28 September 1, 2000.