1 - Alaska Energy Data Inventory

Air 1 eakage through the rock is governed by the permeab il ity of therock and the head difference between the internal air pressure and theextern a 1 groundwater pressure. Since groundwater 1 eve 1 sin the area arenot accurately known, the estimation of air leakage through the rock isapproximate. Po1arconsu1t has stated in their November 1982 report:"Judging from the observed rock quality and experience from similarconstruction in Norway, it seems to be quite realistic to find a site forthe chamber in the proper area giving low or practically no air leakage."Air 1 eakage through the rock was est imated by two methods:1) calculations using rock permeabil ity values in the Darcy Equation, and2) extrapolations from experience at the Juk1a and Driva projects where airchamber surge tanks are used.Permeability values measured in drill hole DH 115 range from7.06 X 10- 4 ft/min to 2.17 X 10- 6 ft/min. A common permeability valuefQr' granite is 10- 8 cm/s or 1.97 X 10- 7 ft/min.Based on the fact thatthe drillers, when performing permeability tests (Lugeon Tests), werelooking for areas of high permeability, and the fact that testing will bedone during construction to locate an area of low permeability, the valueof K = 2.17 X 10- 6 ft/min was used for this calculation. Modification ofthe Darcy Equation for air as opposed to water loss, is related to therelative viscosities of air and water. Using the ratio of water viscosityto air viscosity yields a factor of 90.5. Studies done for the Juk1aproject indicate that the ratio of air loss to water loss could be as highas 200 to 500. For conservatism, a factor of 500 was used for thiscalculation. After DH 115 was drilled, water flowed out of the top of thehole at elevation 775 ft. At a later date, no water was observed flowingfrom the hole. For lack of more specific information, a groundwaterelevation of 775 ft was assumed.For conservatism, the minimum distance of720 ft to the face of the mounta"in was used. The Darcy Equation was usedas follows:Q = 500 KA (H l- H 2)LB4-19