Technical Report NTB 87-21
Analysis of long-term pressure recovery data from the Böttstein, Weiach, Riniken and Leuggern boreholes
The results of the long-term monitoring program, using available data up to 01 September, 1987 for the Böttstein, Weiach, Riniken, and Leuggern boreholes are presented as a summary of the interpreted hydraulic conductivity, formation pressure, and equivalent freshwater head. A total of 22 test zones are examined, 12 of which are located in crystalline rock. Formation pressures and equivalent freshwater heads are reported for 20 zones. Also included are estimates of the uncertainty associated with the freshwater head values due to measurement errors of the various monitoring devices used. For seven zones, representative hydraulic conductivities were determined.
The Böttstein borehole was drilled between October 1982 and June 1983. The observation phase began in January 1984 with the installation of a multipacker system consisting of eight packers and eight monitoring zones. The uppermost zone (Zone 1) is open to the Buntsandstein via perforated casing. The remaining seven zones are all open to various depths in the crystalline. Pressure gauges are installed on observation tubes to Zones 2, 3, 5, 6, 7, and 8 and to the tubing string for Zone 4. Interpreted formation pressures at the wellhead increase from 170 kPa for Zone 1 to 740 kPa for Zone 8. Reported at the center of the zone, the pressures range from 3344 kPa to 14597 kPa. Freshwater heads range from 364.4 m above sea level (m asl) to 416.5 m asl, all of which are above ground surface datum, 347.46 m asl. The standard deviation of the freshwater heads lies between 0.19 m (Zone 2) and 0.44 m (Zone 8). It depends strongly on the uncertainty in fluid temperature downhole and generally increases with depth. Representative hydraulic conductivities are interpreted for Zones 2, 3, 4, 5, and 7 and range from 2.0E-14 m s-1 to 3.0E-7 m s-1.
The Weiach borehole was drilled between January 1983 and November 1983. The observation phase began in April 1985 with the installation of a multipacker system consisting of six packers and seven monitoring zones. Zones 1 and 2 are open to the Muschelkalk and Buntsandstein, respectively, through perforated casing. Zones 3 and 4 are similarly open to the upper and lower Rotliegendes. Zones 5, 6, and 7 are in the uncased crystalline. Pressure gauges are connected to the monitoring tubes to Zones 2, 3, 5, 6, and 7. Zone 4 is monitored by a gauge connected to the tubing string. Zone 1 is monitored with a water level tube into the annular region between the wellhead and the uppermost packer. Formation pressures were estimated for Zones 1, 2, 3, 5, 6, and 7. They range from 455 kPa to 850 kPa at surface or from 7970 kPa to 23270 kPa at the center of the monitored zones. Equivalent freshwater heads range from 344.0 m asl to 452.2 m asl. Only the lowest value, which is the freshwater head value for Zone 1, is below the ground surface datum, 368.66 m asl. The standard deviation of the freshwater heads is the largest of the four boreholes and increases from 0.48 m for Zone 1 to 2.60 m for Zone 7. Two representative hydraulic conductivities are interpreted, 3.0E-10 m s-1 and 2.0E-12 m s-1, corresponding to Zones 3 and 5, respectively.
The Riniken borehole was drilled between June 1983 and January 1984. The observation phase began in July 1985 with the installation of a multipacker system consisting of five packers and five monitoring zones. The five zones are open to the Schilfsandstein, Muschelkalk, Buntsandstein, and to two intervals in the Rotliegendes. The top four are zones in perforated casing and Zone 5 is in open borehole. There is a pressure gauge attached to the monitoring tube to Zone 1. The other 4 zones are monitored by water level tubes. Formation pressure at surface is 745 kPa for the Schilfsandstein. Formation pressures at the center of the zones range from 5867 kPa to 16558 kPa which are calculated from both gauge readings and water level readings. Freshwater heads range from 353.5 m asl to 459.7 m asl. Only the maximum value, which is the freshwater head value for Zone 1, is above ground surface datum, 385.07 m asl. The standard deviation of the freshwater heads lies between 0.20 m (Zone 2) and 0.90 m (Zone 5). There are no representative hydraulic conductivities interpreted for Riniken.
The Leuggern borehole was drilled between June 1984 and February 1985. The observation phase began in May 1985 with the installation of two bridge plugs and a Production Injection Packer defining three zones. All three zones are open to crystalline rock. Zones 1 and 2 are monitored by gauges at the surface. The data for Zone 3 were not available at the time of data interpretation. Formation pressures are 52 kPa and 80 kPa at surface or 8257 kPa and 12231 kPa at zone center for Zones 1 and 2, respectively. The corresponding equivalent freshwater head values are 363.1 m asl and 364.7 m asl, both above ground surface datum, 358.80 m asl. The standard deviation of the freshwater heads is 0.41 m for Zone 1 and 0.95 m for Zone 2. No hydraulic conductivities are reported as representative for Leuggern.
The uncertainty of equivalent freshwater head due to instrumentation uncertainty is expressed as two standard deviations. For normal distributions, ± 2σ corresponds to a 95 percent confidence interval. For uniform distributions, ± 2σ corresponds to a confidence interval greater than 95 percent. The minimum 2σ value is ± 0.38 m at Böttstein Zone 2 and the maximum value is ± 5.20 m at Weiach Zone 7. The primary cause for the uncertainty is the uncertainty surrounding fluid density. Formation pressures at the center of a zone are derived from surface readings using a density that is depth-integrated over varying temperature, pressure, and salinity. Freshwater head is then computed using a uniform density of 1000 kg m-3. Uncertainty in salinity for density measurements contributes to uncertainty in density for three of the four boreholes. However, the major cause for density uncertainty is temperature uncertainty. This is particularly true for Weiach where the assumed ± 4°C uncertainty band can propagate to an uncertainty as much as ± 4.5 m for freshwater head.