LABORATORY INVESTIGATION OF SUPERCRITICAL CO 2 USE IN GEOTHERMAL SYSTEMS

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1 PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198 LABORATORY INVESTIGATION OF SUPERCRITICAL CO 2 USE IN GEOTHERMAL SYSTEMS Mario Magliocco 1,2, Steven Glaser 1,2, and Timothy J. Kneafsey 2 1 Department Civil and Environmental Engineering University of California, Berkeley, CA Lawrence Berkeley National Laboratory Berkeley, CA mag@berkeley.edu ABSTRACT The use of carbon dioxide (CO 2 ) as a heat transfer fluid has been proposed as an alternative to water in enhanced geothermal systems (EGS). Numerical simulations have shown that under expected EGS operating conditions, CO 2 could achieve more efficient heat extraction performance than water. The simulations indicate that the performance advantage of CO 2 over water is a function of operating parameters, reservoir temperature, and flow geometry. Since the pore space in formations that would be likely utilized for EGS are initially saturated with water or brine, the creation of a CO 2 based EGS reservoir would involve a development period where the resident water would be removed over time as CO 2 is injected into the formation. Using a specially constructed laboratory apparatus that is capable of flowing temperature-controlled supercritical CO 2 through a heated porous sample, we have investigated the behavior of CO 2 as a working fluid in EGS system. In our laboratory based experiments we have investigated the injection of CO 2 into a heated water saturated sample and have found that preferential flow paths develop which bypassed much of the sample pore space. Additionally we have also explored the behavior of dry CO 2 based heat extraction from a heated sample and found that the behavior is highly dependent on the operating conditions. Finally we have explored the relative performance of dry CO 2 versus singlephase water based heat extraction and found that under the operating conditions studied that water and CO 2 exhibit similar heat extraction rates. INTRODUCTION The novel concept of using supercritical CO 2 (scco 2 ) as the working fluid in an enhanced geothermal system (EGS) for both reservoir creation and heat extraction was first proposed by Brown (2000). The advantages of using CO 2 instead of water as the process fluid in a closed loop EGS system include a much lower viscosity of CO 2 resulting in substantially larger mass flow rates for a given pressure drop between injection and production points, and a much larger density difference between cold fluid in the injection well and hot fluid in the producer providing increased buoyancy forces for CO 2. As an ancillary benefit, practical operation of a CO 2 system would result in some de facto carbon sequestration due to fluid loss into the surrounding formations (Brown, 2000, Pruess 2006). Numerical simulations of a five-spot well pattern in a hot dry rock system indicate that (1) CO 2 achieves larger heat extraction rates than water, and (2) the relative advantage of CO 2 increases with decreasing reservoir temperature, ranging from approximately 50% larger heat extraction rates at T = 240 o C to about 80% larger rates at T = 120 o C (Pruess personal communication 2009). The increasing heat extraction efficiency of CO 2 at lower reservoir temperatures can be better understood by considering the effects of pressure and temperature on the fluid mobility m = ρ/μ (ρ is the fluid density, μ is the fluid viscosity) (Figure 1, Press 2006). These experiments are intended create a data set that can then be used to validate numerical modeling results and provide insight into the planning, development, and operation of a CO 2 based EGS reservoir.

2 water or brine. The creation of a CO 2 based EGS reservoir in an initially brine-saturated reservoir would involve a development period where the resident water would be removed over time as CO 2 is injected into the formation. Simulations have shown that water removal would occur through two principal processes: (1) immiscible displacement of aqueous phase by the CO 2 -rich phase, and (2) dissolution of water into the flowing CO 2 stream (Pruess 2010). During the operation of a CO 2 based EGS reservoir, it is expected that the initial production fluid would be composed of the native brine until the breakthrough of CO 2 occurs, after which a brine-co 2 mixture would be produced. Figure 1 - Fluid mobility of CO 2 (Preuss 2006) The contour lines in Figure 1 indicate the fluid mobility for the pressure and temperature shown on the axes. According to Darcy s law, for a given pressure gradient, fluid flux is proportional to the fluid mobility. CO 2 mobility exhibits a much more dynamic dependence on temperature and pressure than water, and under the conditions expected to be present in a geothermal field, the CO 2 mobility is much higher than that of water. In a water-based 5-spot EGS system the majority of the driving head loss occurs near the injection well. This is because the lower mobility resulting from the colder conditions coupled with the high Darcy flux due to the radial flow pattern around the well require a high driving pressure gradient. This is in contrast to the higher mobility CO 2 -based system where the head loss is much more evenly distributed across the entire flow path. Typically, the pore space in formations that would be likely utilized for EGS is initially saturated with In order to verify the results of the numerical simulations, we have performed laboratory experiments that have injected cooled CO 2 into a heated water saturated sample to study the reservoir development stage. We have also injected cooled CO 2 into a heated CO 2 -saturated sample to study the behavior of a fully developed, dry reservoir. We have also injected cooled water into a heated water saturated sample in order to compare the behavior of a water-based system to a CO 2 based system. EXPERIMENT DESCRIPTION Experimental Apparatus The apparatus consists of a temperature-controlled pressure vessel filled with porous media through which temperature controlled fluid can be introduced by means of high-pressure, high-flow rate pumps (Figure 2). The pumps can be operated to provide a constant fluid injection rate, or a constant differential pressure. The fluid was delivered by a pair of Quizix C K pumps, capable of 5,000psi (345 bar) and 400 ml/min fluid delivery rate. The pumps are capable of precisely controlled continuous and pulse- Figure 2 - Simplified schematic of the experimental apparatus.

3 free flow with a resolution of 27.2 nanoliters. To ensure that the pumps are filled with high-density liquid CO 2, the injection fluid is passed through a chiller before entering the pumps. After leaving the pumps, the fluid can be either chilled by a second fluid chiller, or heated depending on the desired experimental parameters. Before entering the vessel, the injection fluid passes through a Siemens coriolisstyle mass flow meter. The meter could also be placed at the outlet and used in conjunction with the pump flow readings to measure fluid accumulation in the vessel. The pressure vessel is a hollow stainless steel cylinder with an inside diameter of 9.1 cm, outside diameter of 12.7 cm, 50.8 cm distance between the end caps, and a pressure safety rating of 34.5 MPa (345 bar, 5000 psi). Instrumentation access to the interior of the vessel is through three axial passages through one end cap, and one passage through the other. The central passages through the end caps are used as the injection and production ports and the remaining two passages are used exclusively to pass thermocouples through. The vessel is oriented vertically such that the central axis is in line with gravity. It has been shown that even for small length scales, buoyant effects of scco 2 can have a large effect on the dynamics of a scco 2 -based system (Liao and Zhao 2002). For a horizontal flow arrangement, buoyant forces can result in pressure gradients that are oriented perpendicular to the vessel axis, complicating the dynamics. For modeling and comparison purposes, the experiments were operated such that the flow path was in the same orientation as the gravity-induced pressure gradient. Temperature measurements within the sample were made with 23 stainless-steel clad type-t thermocouples, which have a small diameter (0.79mm) in order to increase the sensor response time and to minimize disturbance to fluid flow. The thermocouples are arranged at various elevations and radii in the sample such that each successive vertical level is offset angularly to minimize vertical sensor shadowing (Figure 3). The offset angle we used is based on the Golden Angle (137.5 degrees), found in plant phyllotaxis that has been shown to minimize shadowing (King 2004). At one elevation in the porous medium, two thermocouples were mirrored so that they were both at the same radial distance from the central axis of the vessel. The radially mirrored temperature measurement was designed to test our assumption of a radial symmetry in the heat transfer process. Figure 3 - Orthographic diagram of thermocouple placement inside the vessel. Axis units are in meters. Colored dots index the different elevations and correspond to the colors in Figures 5, 6, & 8 Because the end caps are large, the injection port of the vessel was lined with a length of nylon tubing through the end cap in order to provide thermal insulation for the injected fluid as it passed through the relatively massive end cap. The injection port was also fitted with a single thermocouple mounted where the injected fluid enters the sample space (not shown in Figure 2). The sand used in the test sample was prepared from F95 Ottawa silica sand (U.S. Silica). Sieving and washing resulted in a narrow grain size distribution (Figure 4). The mean grain size falls between 147 and 105 microns with no measurable portion below a grain size of 45 microns.

4 % Retained Porous Media Grain Size Distribution Mesh Opening Size (mm) Figure 4- Porous medium grain size distribution The sand was dry placed in the vessel in multiple lifts with vibratory compaction between lifts. This method produced a relative density of 84 percent. The porous core sample properties are listed in Table 1. Table 1: Design case system properties Porous Core Properties total core length L = 50.8 cm cross sectional area A = 6.54x10-3 m 2 grain density ρ R = 2650 kg/m 3 grain specific heat C R = 20 J/ g/ C rock thermal K = 2.51 /m/ C conductivity permeability k = 9.3x10-13 m 2 porosity ϕ = 41% mean grain size d 50 d mm The vessel was wrapped with heat tape that extended around the exterior of the cylinder and both end caps. The heat tape thermal output was controlled by either a PID controller using a thermocouple secured on the vessel exterior or with a pulse width modulated signal. This allows the vessel boundary to maintain a constant temperature or a constant heat flux. Finally the vessel was wrapped in an aerogel insulation jacket and sealed. The current supplied to the heat tape was monitored with a true RMS current sensor. The pressure at the outlet of the vessel was controlled by a pair of digital backpressure regulators in series. The fluid exiting the backpressure regulators was vented to the atmosphere at a safe location outside of the building. A differential pressure sensor connecting the inlet and outlet of the vessel was located at the base of the vessel. The tubing that connected the differential pressure sensor to the inlet is encased in a constant temperature water bath so that the state of the fluid column in the tube could be determined. We developed software that incorporates experimental control and data acquisition. All sensor readings were collected by a single Labview-based program that allows for accurate time synchronization of experimental data. The program is capable of controlling the pumps, vessel heat input, and the backpressure regulators. Combining these functions allows for a tightly integrated experimental setup, faster data processing, faster experimental turnaround time, and less chance of experimental errors. Table 2 lists the range of operating parameters that we have used in our experiments. Our current apparatus is capable of achieving temperatures up to 200C and operating pressure of up to 275 bar. Table 2: Range of operating parameters Parameter Min Value Max Value back pressure 87 bar 138 bar initial core temperature 50 C 100C injection rate 50 ml/min 200 ml/min Experimental Procedure The sand-filled vessel was filled with fluid and pressurized to the experiment pressure, and the vessel was heated. For the single-phase CO 2 experiments, the vessel was filled with CO 2, for the two phase CO 2 /water and the single phase water experiment the vessel was initially filled with water. The pumps and tubing were then emptied of water if present, filled with CO 2, and pressurized to the vessel pressure. The backpressure regulators were set to the desired pressure, and CO 2 injected into the bottom of the vessel at a prescribed volumetric flow rate. The pumps maintain a constant flow over multiple injector volumes through computer controlled pumps switching. Experimental Challenges During the course of our experiments we have met with several practical challenges that are related to CO 2. The density of CO 2 can change significantly with relatively small changes in pressure or temperature. For example, the density of the CO 2 in the vessel could change by more than a factor of two within the conditions of a single experimental run. Practically, this behavior made it difficult to measure the mass of the CO 2 without knowledge of the current state of the CO 2. When using the first iteration of our apparatus we inferred the mass flow rate of fluid entering the vessel by recording the volumetric flow rate of the

5 pump, measuring the pressure and temperature of the CO 2 exiting the pump, using a lookup table to find the density of CO 2, and finally calculating the mass flow rate. This method proved difficult as the pressure and temperature of the fluid in the pump changed throughout the course of a single experimental run. To solve this problem we employed a Siemens corriolis based mass flow meter near the inlet of the pressure vessel. This device was capable of measuring the mass flow of the fluid without requiring knowledge of the current state of the fluid being measured. The dynamic density of the CO 2 also made measuring the pressure differential across the vessel difficult. The differential-pressure sensor we used in our apparatus required a tubing connection between the inlet and outlet of the vessel so that the relative pressure difference could be measured. Since our vessel was mounted vertically, in line with gravity, the weight of the fluid in the tubing connecting the delta-pressure sensor with the vessel outlet resulted in a hydrostatic pressure component on one side of the delta pressure sensor. With fluids such as water that don t exhibit drastic density changes, the hydrostatic pressure due to the fluid column in the connection tube is relatively constant and can simply be subtracted from the reading. With CO 2 we found that small changes in pressure and temperature in the column produced relatively large changes in density that caused the hydrostatic pressure to change depending on the state of the CO 2 in the connection tube. In the second iteration of our experimental apparatus we attempted to impose a constant temperature on the connection tube by using a small diameter tube and placing it in a relatively large temperature controlled water bath. The temperature of the bath was recorded by a thermocouple throughout the experiment, and a pressure sensor was located at the top end of the tube. Using these measurements, the state of the fluid in the column could be determined and used to calculate the hydrostatic pressure component and subtract it from the differential pressure measurement. The final difficulty we experienced due to the behavior of CO 2 was in our backpressure regulation. We used a pair of backpressure regulators driven by a PID control system. As the state of the CO 2 exiting the vessel changed, the density and the viscosity of the CO 2 passing through the backpressure regulator valve changed. This made it very difficult to identify PID coefficient values that would provide a sufficiently stable control tune that would work over the course of a single experiment. To alleviate this problem on our final apparatus iteration we placed the back pressure regulator downstream of a heat exchangers in order to keep the CO 2 passing through the regulator valve at a more constant temperature. RESULTS Single-Phase CO 2 The temperature data from twenty-two thermocouples from a representative single phase CO 2 experimental run is shown in Figure 5. The thermocouples are numbered primarily in order increasing radii and secondarily by increasing elevation in the vessel. Thermocouple one for example is located on the central axis at the bottom of the vessel, while thermocouple number twenty-two is located near the vessel wall at the top of the vessel. It can be seen from the plot that there is an initial temperature gradient present in the saturated medium with a lower temperature at the base. This gradient is most likely due to gravity and thermal convection. The temperature front can be seen in the plot as it passes axially through the sample past the measurement locations. After the initial sharp temperature drop, the temperatures gradually approach equilibrium, and a radial temperature gradient then develops, indicated by the grouped lines spreading out. The exterior thermocouple locations trend towards a higher temperature than those that are more central (solid lines). The large spikes can be seen in temperature data for TC1 at the vessel inlet are due to short interruptions in flow as the pumps stop during the pump switch over event. During the brief time when flow was stopped, heat from the steel end cap increased the temperature of the cold fluid that was located within the injection passage The interplay between convective and conductive transport can be seen in the shape of the temperature vs. time curves. A purely convective process would feature sharp thermal fronts and a near-vertical slope at the time when the cold fluid slug reached the thermocouple. A purely conductive process would generate a gentler slope with smooth transitions. The experimental run shown in Figure 5 was at a relatively high flow rate with a calculated bulk Peclet number of around The steep temperature front corresponds to convectively dominated behavior. Figure 6 shows temperature data from a lower flow rate experimental run that was less convectively dominated (only temperature data from the central axis of the vessel are shown for clarity). The calculated bulk Peclet number of this experimental

6 Temperature (C) Temperature (C) 60 Temperature History, 200 (ml/min) Flow Rate, 60C Vessel Temperature, 83 bar Back Pressure Time (s) TC1 TC2 TC3 TC4 TC5 TC6 TC7 TC8 TC9 TC10 TC11 TC12 TC13 TC14 TC15 TC16 TC17 TC18 TC19 TC20 TC21 TC22 Figure 5 - Temperature vs. time data from twenty-two thermocouples from a representative single phase CO 2 experimental run. Plot line colors indicate the thermocouple elevation and correspond to the colored markers shown in the thermocouple diagram (Figure 2) run was approximately 550, or one third that of the run in Figure Temperature History, 100 (ml/min) Flow Rate, 100C Vessel Temperature, 138 bar Back Pressure Time (s) Figure 6 - Temperature vs time from a experimental run with a CO 2 flow rate of 100ml/min, and a bulk Peclet number of 556. TC1 TC4 TC8 TC12 TC13 TC17 TC21 Water & CO 2 Compared In order to compare the performance of CO 2 heat extraction with that of water, we performed a single phase water run at the same temperature and pressure as a previous CO 2 run (Figure 7). Both experiments were run with a volumetric fluid injection rate of 150 ml/min, an initial core temperature of 75C, and a backpressure of approximately 1450 psi. The CO2 and water experiments had approximately the same pressure differential across the sample for both water and CO 2 (2 bar), and a similar mass flow rate (2.51 g/s for water, and a range of 2.3 to 2.04 g/s for CO 2 ) which allowed a more straightforward comparison of the performance of the two fluids. The injected water and CO2 increased in temperature after the first two pump volumes due to the fact that we were recycling the injection fluid. The CO 2 plot shows a much steeper temperature front indicating a more advection dominated flow than the water experiment.

7 Heat Extraction Rate (Watts) Figure 7 - Comparison of temperature history data of water and CO 2 as working fluids. Top plot shows a water run and the bottom plot is a CO 2 run. (Thermocouple labeling is not consistent with other figures in this paper) In order to compare the heat extraction of the two fluids, we used the experimental data to calculate the heat extraction rates of the two fluids. Figure 8 shows the heat extraction rate of water (blue line) and the heat extraction rate of CO 2 (red line). Initially the heat extraction rates of the two fluids are similar, with a higher rate for water at the start of the experiment and CO 2 overtaking it in as the experiment progresses. The performance of CO 2 stays somewhat stable despite the fact that the mass 300 Heat Extraction Rate 250 H2O CO Time (s) Figure 8 - Comparison of the heat extraction rate of water and CO 2 under similar experimental conditions.

8 Temperature (C) flow rate is decreasing during the experiment due to the increase in injection temperature. The water heat extraction rate decreases after the first two pump volumes due to the increased injection temperature. Since both the water and CO 2 were operated at the similar pressure and volumetric flow rate, it can be assumed that the work performed by the pumps in both experiments was comparable. A complete thermodynamic characterization of the runs would require measurements of the fluid accumulation inside the vessel which was not recorded for these experiments. Two Phase Experiments We performed an experimental run in which cold CO 2 was injected into a heated water-saturated sample, analogous to the initial development phase of a CO 2 geothermal reservoir. Figure 9 shows the temperature history. The temperature drop at the bottom of the vessel, where the cold CO 2 was injected, was very pronounced, but temperature trends at the other thermocouple locations were very smooth and gradual. The temperature drop at the top of the vessel was not significantly greater than the temperature drop that would be expected due to passive cooling to the lab atmosphere. The temperature spikes seen at the injection location (e.g. at 1200 seconds) were due to a temporary cessation of injection flow during the pump switchovers. The overall temperature change shown in this experiment is not as drastic as the single phase experiments. The presence of viscous fingering was indicated by multiple pieces of experimental evidence. After the CO 2 was vented to the atmosphere, it was found that approximately 74 percent of the pore volume still contained the original water. More than one pore volume of CO 2 was injected into the vessel, and if no fingering occurred the CO 2 would act as a solid slug displacing all of the water originally present in the vessel. Further evidence of fingering is indicated by the CO 2 breakthrough occurring at approximately 500 seconds after injection began. Breakthrough occurred after approximately 251 ml of CO 2 was injected into the vessel, corresponding to approximately 20 percent of the sample pore space. The volume of CO 2 injected at breakthrough, and the remaining water in the pore space indicate that very little water was lost after the initial water was displaced during the CO 2 flow path formation. Compared to our previous results with an initially CO 2 saturated sample, our new results show much less change in temperature in the vessel during the course of the experiment. Besides the effects of Time (s) TC1 TC2 TC3 TC4 TC5 TC6 TC7 TC8 TC9 TC10 TC11 TC12 TC13 TC14 TC15 TC16 TC17 TC18 TC19 TC20 TC21 TC22 Figure 9 - Temperature history of cold CO2 injection into a heated water-saturated sample, 45 C initial temperature, 25 ml/min flowrate, 17 MPa (2500 psi) outlet pressure.

9 viscous fingering, this smaller temperature drop is most likely the result of the higher heat capacity of water compared to CO 2, the lower flow rate of the injected CO 2, and the higher temperature of the injected CO 2 when compared to our earlier single phase experiments. At the initial conditions, the water-saturated sample contained 234 kj of heat energy in the pore space compared to 123 kj for CO 2 under the same conditions. The thermal energy of the water in the vessel was so great and the cooling capacity of our injected fluid was so low, that we only saw evidence of the cooling effects of the CO 2 flow at the injection end of the sample. DISCUSSION The design of our experiment was primarily intended to produce a data set that could then be used to validate numerical modeling of the use of CO 2 as the working fluid in an EGS reservoir, and was not intended to be directly applicable to full-scale EGS systems. Specifically, our porous medium sample was not designed to replicate the characteristics of the flow paths that would be expected in a field-scale geothermal system. Despite this, our results can be used to gain insights into the behavior of CO 2 as an EGS working fluid. Our single-phase CO 2 experiments have shown that the heat extraction behavior of CO 2 is sensitive to the initial and operating conditions; injection rate, operating pressure, and initial reservoir temperature (Magliocco 2011). The Peclet number and the shape of the temperature histories vary greatly depending on the initial and operating conditions. We have also shown experimentally that CO 2 and water have comparable behavior under one particular set of initial and operating conditions. This result could be viewed as discouraging in the context of CO 2 based EGS, but was in line with previous modeling results (Preuss 2007). The greater benefits of CO 2 over water are expected to occur in the 5-spot well geometry that is dominated by radial flow. In our apparatus, the flow is predominately linear along the length of the vessel with small portions of radial flow near the inlet and outlet boundaries. In addition, the thermal energy stored in the large end caps used in our vessel may retard the development of cold zones near the fluid injection point. Experimental results for the more realistic conditions of CO2 injection into a water-saturated sample show convincing evidence of preferential flow of CO 2 into the saturated sample. Much of the sample pore space appears to have been bypassed by the flowing CO 2, as indicated by the temperature data, the early CO 2 breakthrough, and the remaining water in the sample at the conclusion of the experiment. The effect of CO 2 bypassing large regions of the sample (or reservoir) will reduce the effective heat transfer to the working fluid, thus this is an important consideration warranting further investigation. In future experiments, a much clearer picture of the system could be realized by analyzing the produced fluid over the course of the experiment to determine the ratio of CO2 to water. CONCLUSION Using our laboratory apparatus, we have now successfully explored three key aspects of using CO 2 as an alternative to water in geothermal systems. During the initial injection of CO2 into a previously water-saturated sample, we found that preferential flow patterns strongly affect the heat production of the system. Our single-phase experiments have shown that heat production and the temperature profile of the system is strongly affected by the injection rate, back pressure, and initial system temperature. We have also demonstrated that under a particular set of operation conditions that water and CO 2 have comparable heat extraction rates. We are currently working on expanding the temperature and pressure range of our experiments, in order to create a well curated body of data that can be used to validate numerical modeling software. REFERENCES Brown D.. (2000), A Hot Dry Roc Geothermal Energy Concept Utilizing Supercritical CO 2 Instead of ater. In Proceedings of the Twenty- Fifth Workshop on Geothermal Reservoir Engineering, Stanford Univ, King, S., F. Beck, and U. Lüttge. "On the mystery of the golden angle in phyllotaxis." Plant, Cell & Environment 27.6 (2004): Liao S.M. and Zhao T.S. (2002), An experimental investigation of convection heat transfer to supercritical carbon dioxide in miniature tubes, Int. J. Heat Mass Transfer, 45, Magliocco, M., Kneafsey, T., Pruess, K., Glaser S., (2011) Laboratory experimental study of heat extraction from porous media by means of CO 2 Proceedings of the Thirty-sixth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 31 February 2 (2011) SGP-TR-191 Magliocco, M., Kneafsey, T., Preuss, K., Glaser, S. (2012), Laboratory and Numerical Studies of Heat Extraction from Hot Porous Media by Means of Supercritical CO 2, Proceedings of

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