IMPACT OF GROUNDWATER FLOW ON THE STOCKTON GEOTHERMAL WELL FIELD

Transcription

1 ABSTRACT IMPACT OF GROUNDWATER FLOW ON THE STOCKTON GEOTHERMAL WELL FIELD Dr. Claude M. Epstein Richard Stockton College of New Jersey Environmental Studies PO Box 195 Pomona, New Jersey (609) FAX (609) Heat generated by the Stockton College Geothermal Facility has tended to accumulate within its well field during the first three seasons of operation. Heat flow within and around the well field is controlled by two factors. First, conductive-convective heat generated by the 400 geothermal heat exchangers flows horizontally and radially heating some layers more than others based on those layers thermal properties. Second, as the regional groundwater flow in the Upper and Lower Cohansey aquifer flows across the well field is gains heat which it passes beyond the well field downflow. Consequently, the Upper and Lower Cohansey aquifers within the well field show temperature troughs within the temperature profile caused by unheated groundwater drawn into the field from one side. Groundwater, heated as it passes through the well field, results in temperature peaks in the Upper and Lower Cohansey aquifers downflow of the well field. 1. INTRODUCTION The Stockton College Geothermal Facility, the largest single seasonal underground thermal energy facility in the world (B. Sanner & others, 1996) serves as the source of air conditioning and heating of the college s buildings. The well field, which serves as the heat source and sink, occupies approximately 5.7 acres and is situated beneath one of the college s parking lots. It consists 400 geothermal heat exchangers that extend to depths of 425 feet. For more details on well field construction see C. Epstein & others (1996). The cooling cycle of this facility is typically greater than the heating cycle. Consequently, the ground gets warmer from one year to the next. In addition, the geothermal well field, which serves as the heat source and sink, is not homogeneous. It is made up of layers of saturated sand, saturated clay and mixtures of the two. The thermal transmissivity and hydraulic conductivity of sand is greater than that of clay. Consequently, the rates of heat transport differ with sediment type. The regional groundwater

2 flow, accentuated by well water withdrawals, adds further complexity to the manner of heat transport within and beyond the geothermal well field. 2. HYDROGEOLOGIC SETTING The well field contains three aquifers (i.e., the saturated sand layers) separated by confining beds (i.e., saturated clay and silt layers). These were identified by means of three gamma radiation well logs (Figure 1). Aside from slight differences in surface elevation, the logs are similar. The first, or shallowest peak, marks a confining bed between approximately 75 and 115 feet. This confining bed that separates the Upper Cohansey aquifer from the Lower Cohansey aquifer. The Upper Cohansey aquifer lies between the surface and the top of the confining bed, while the Lower Cohansey lies beneath the base of the confining bed to approximately 150 to 180 feet. A thick confining bed lies beneath the Lower Cohansey. Beneath this confining bed lies the Rio Grande water-bearing zone, the third aquifer, from 315 feet to 350 feet. This is followed by another confining bed beneath 350 feet. Figure 1. Geophysical Well Logs of the Geothermal Well Field Groundwater flow in the region as a whole this from west to east (C. Epstein, ). However, groundwater flow within the Cohansey aquifers is accelerated by the operation of the college s water supply wells. These wells are downflow of the well field, tapping the Lower Cohansey aquifer. The radius of influence of these supply wells extends through the well field and beyond, resulting in steeper hydraulic gradients across and downflow of the well field. Moreover, the confining bed between the Upper and Lower Cohansey aquifers is thought to be discontinuous or leaky based on well tests conducted in nearby locations. Thus water withdrawn by the college not only affects the Lower Cohansey directly but the Upper Cohansey as well by leakage into the Lower Cohansey aquifer.

3 3. TEMPERATURE PROFILES Under Natural Conditions A consistent temperature profile exists in areas unaffected by the well field. This profile is fairly constant except for the shallowest 45 feet. Between the surface and 30 feet, temperature decreases with depth in the summer and increases with depth in winter, reflecting seasonal heat transfer with the atmosphere (Figure 3). Between 30 and 45 feet temperatures decline with depth but fluctuate with the seasons. Below 45 feet, temperatures remain fairly constant throughout the year and show a three fold pattern. First, temperatures decrease from 15 to nearly 13 degrees Celsius between 45 and 145 feet. Then, temperatures remain at approximately 13 degrees Celsius between 145 and 235 feet. Finally, temperatures increase to over 14 degrees Celsius from 235 feet to 350 feet. Figure 2. Location of Monitoring Wells in and around Well Field In the Geothermal Heat Exchangers The water flowing through the geothermal heat exchanger flows should generates a constant temperature from top to bottom. This was observed in monitoring well B17 and to a lesser extent in I40. (These monitoring wells are in the same boring as the heat exchangers.) Temperature profiles of both wells rise and fall with the season, but distinct layers of reduced heating are observed in I40. Though several temperature troughs developed in the geothermal profiles of I40 from February, 1994 to April, 1997, two are especially noteworthy (Figure 3). The largest temperature trough occurred at a depth of approximately 125 feet and was present since April of The decrease in temperature for this trough was as much as 4.20 degrees Celsius during one summer. The second, weaker temperature trough occurred at approximately

4 60 feet and was present since October of The temperature decrease for this trough was as much as 1.40 degrees Celsius also in the summer. The deeper temperature trough is in the Lower Cohansey aquifer while the lesser trough is in the Upper Cohansey aquifer. Temperature troughs seen occasionally in B17 were far more ephemeral, involving small changes in temperature. The reason for the difference in the profiles of I40 and B17 has Figure 3. Geothermal Profiles of Monitoring Wells in & around Well Field to do with their position in the well field. I40 is on the upflow side of the well field where unheated groundwater is drawn into the well field while B17 is downflow where groundwater has been heated by many geothermal heat exchangers as it moved across the well field. Within the Well Field Heat flow within the well field during the operation of the well field is complex, involving changes in temperature across the field, with depth, and during the annual heating/cooling cycle. These temperature changes were determined by reviewing the temperature profiles in six wells are located within the well field (oriented from upflow within the well field to downflow within the well field, I30, GH3334, GH13, EF22, CD3334, and CD1213) and another two (B26 and B1) situated just downflow from the well field (Figure 3). The mean temperature of these wells increases across the well field from the groundwater upflow side to the groundwater downflow side. This is due to the increased number of geothermal heat exchangers encountered as groundwater passes through the well field (Table 1).

5 However, the increase in temperature is not uniform with depth (Figure 3). The most consistent and dramatic temperature change occurs in the Lower Cohansey aquifer. The temperature decrease from the overlying confining bed to the Lower Cohansey aquifer under pre-operation conditions was approximately 0.7 degrees Celsius. But the temperature decrease under operation conditions was from 1.0 to 3.2 degrees. This temperature decrease was greatest on the upflow side of the well field and smallest on the downflow side. Table 1. Mean Temperatures Across the Well Field Another temperature decrease occurs in the Upper Cohansey aquifer. Before the operation of the well field, the temperature in the Upper Cohansey aquifer was approximately 0.7 degrees warmer than that of the underlying confining bed. But with the operation of the well field, the temperature in the Upper Cohansey aquifer dropped from 1 to 3.2 degrees cooler that of the underlying confining bed. This temperature inversion spread through the well field through several heating/cooling cycles. Temperature in the Rio Grande water-bearing zone was approximately 0.4 degrees cooler than its underlying confining bed. But unlike the two overlying aquifers, temperatures at these depths actually increased from 0.4 to 1.5 degrees with respect to its underlying confining bed. Downflow from the Well Field Groundwater flowing through the well field picks up heat generated by the geothermal heat exchangers, passing it further downflow of the well field. Three wells are situated downflow of the well field. These are B26, B1 and the college drive wells (Figure 3). B26 is 16 feet downflow of the well field while the college drive well is 475 feet downflow. Both show a similar profile. B1 is 21 feet along the northern margin of the well field and shows a different profile. The average temperature of B26 before the operation of the well field was 13.4 degrees Celsius. At first, the temperatures remained constant. But through the summer and into the winter, mean temperatures increased to reach a maximum of 15.3 degrees in January, From then on the

6 mean temperature ranged between 15 and 16.5 degrees. By July 27th, 1994, zones of high temperatures developed at depths of 80 and 140 feet with an intervening trough at 105 feet (Figure 3). The two peaks correlate with the Upper and Lower Cohansey aquifers while the trough correlates with the intervening confining bed. These peaks developed each year through the summer into the fall, then declined through the winter and spring. (During one interval, May through September of 1996, the geothermal profile reversed with what were peaks being trough and what was a trough becoming a peak.) But for the most part, the Cohansey aquifers remained warmer than the intervening confining bed. The College Drive well shows the same pattern as B26 (Figure 3). Two peaks with an intervening trough established themselves by September 23rd, The peaks correlate with the Upper and Lower Cohansey aquifers while the trough correlates with the confining bed between them. The B1 wells developed an inverted profile by September 15th, The Cohansey aquifers correlates with troughs instead of peaks while a peak developed, correlating with the confining bed that separates the two aquifers. This resembles the profile of the wells within the well field. A second peak formed at a depths between 300 and 320 feet from November 12th, 1994 on. This peak correlates with the Rio Grande water-bearing zone. 4. THE WARMING OF THE STOCKTON COLLEGE UNDERGROUND THERMAL ENERGY FACILITY Warming Within the Well Field The operation of Stockton College s geothermal heating/cooling facility has elevated the temperature of the underlying sediments from approximately 13.3 degrees Celsius to between 14.1 and 27.5 degrees Celsius depending on the season and the proximity to its geothermal heat exchangers. The temperature of the heat exchangers varied most, between 14.1 and 27.5 degrees. They reach their temperature extremes at the times of greatest use, August for cooling and February for heating. The temperature of the sediments between the heat exchangers was less varied, between 14.5 and 19.0 degrees. They reached their temperature maxima between two to five months after the heat exchangers reached their maxima and their minima two to three months after the heat exchangers reached their minima (Table 1). Warming Downflow The equation for one dimensional heat flow is made up of two heat transport mechanisms. These mechanisms are conductive heat transport and convective heat transport (P.A. Domenico & F.W. Schwartz, 1998). The direction of heat flow by either mechanism is horizontal and radial. But regional groundwater flow is horizontal flows in one direction, in this case from the south to the north across the well field (Figure 3).

7 If groundwater flow was not a factor, heat transport should raise the temperatures of all the wells surrounding the well field. It did not. Wells L23 (south of the field) and F46 (west of the field), showed no temperature change at any depth throughout the three years of study. But the other wells surrounding the field- B1 (north of the field), B26 (east of the field)- showed elevated temperatures by the eighth month of operation. Even the college drive well, 475 feet northeast of the well field, showed elevated temperatures after a little over two years of well field operation. However, the magnitude of heat flow varied with depth. Troughs (i.e., depths at which temperatures were lower than overlying and underlying depths) in the temperature profiles occurred in monitoring wells at the levels of the Upper and Lower Cohansey aquifers. The magnitude of these temperature troughs increases across the well field. Even the monitoring wells in one of the heat exchangers (i.e., I40) showed troughs in the Cohansey aquifers. These temperature troughs gave way to peaks in the two wells downflow of the well field (i.e., B26 and College Drive). Groundwater flow within the Upper and Lower Cohansey aquifers was sufficient to transport cooler, unheated groundwater into the well field from the upflow side of the field creating the troughs observed in temperature profiles. It transported heated groundwater downflow thereby creating the a 0.7 degree peak in the Upper Cohansey aquifer and a 1.35 degree peak in the Lower Cohansey aquifer. The time when these temperature peaks formed is consistent with calculated rates of groundwater flow. The groundwater flow velocity in the Cohansey aquifers accelerates due to the operation of the college s water supply wells. The drop in hydraulic head between EF22 in the center of the field and B26 at the downflow margin of the field are used here to calculate hydraulic gradient across the well field. The drop in hydraulic head between B26 and the College Drive well were used to calculate the hydraulic gradient downflow of the well field. Hydraulic gradient steepens by an order of magnitude downflow of the well field (Table 2). The velocity of groundwater flow across and downflow of the field was calculated from these gradients, the mean hydraulic conductivity determined from slug tests, and porosity. The number of days it would take groundwater to flow from the downflow margin of the well field to the college drive well is twice that of groundwater flow within the well field. The number of days it took groundwater in the Upper and Lower Cohansey aquifers to reach the college drive well is 632 and 680 days respectively. These values are in agreement with the number of days (i.e., 677days) its took for the first heat maximum from the well field to reach the college drive wells based on the temperature profiles.

8 Table 2. Groundwater Flow Properties In and Downflow of the Well Field 5. ACKNOWLEDGMENTS The author is grateful for the financial support of Richard Stockton College, the Sandia National Laboratory, the Electric Power Research Institute and the South Jersey Transportation Authority. He is also grateful to the considerable logistical support of Dr. Charles Tantillo, Tom Lang, Marvin Witmer and Fred Burk of Richard Stockton College and the intellectual support of Drs. Lynn Stiles, Sipra Pal, Louise Sowers, Harold Taylor, Karen York, Bradley van Guilder, Sarwar Jahangir, Alan Steinberg and Kenneth Harrison of Richard Stockton College. He is especially grateful for all the efforts of his students who did much of the field work and were a constant source of support and criticism. 6. REFERENCES D.A. Domenico & Schwartz, F.W. (1998). Physical and Chemical Hydrogeology. John Wiley & Sons, Inc pp Epstein, C., (1994). Hydrogeology of the Stockton College Geothermal Well Field. In Monitoring and Analysis of the Geothermal Heat Pump Installation at Stockton College (NJ), edited by L. Stiles, 1994 Report to Atlantic Electric, EPRI, SNL, and SJTA. pp.6-1 to Epstein, C., (1995). Temperature Changes in the Geothermal Well Field. In Monitoring and Analysis of the Geothermal Heat Pump Installation at Stockton College (NJ). edited by L. Stiles, 1995 Report to Atlantic Electric, EPRI, SNL, and SJTA, pp. 3-1 to Epstein, C., W. Skinner, W., L. Stiles, H. Taylor, L. Sowers, & S. Pal, (1996). Geothermal Heating on a Very Large Scale : the Stockton College Facility. Well Water Journal, 9(3):

9 B. Sanner, Klugescheid, M., Knoblich, K., & Gonka, T. (1996). Saisonale Kaltespeicherung im Erdreich. Geissener Geologische Schriften. Nr 59, 181S., p.16.