Water Curtains in Gas Storage. An Experimental Study

Sammanfattning: Water curtains have been used for more than 20 years to prevent the escape of gas from caverns excavated in rock. Today, the use of water curtains appears to be increasing, with applications in various forms of storage caverns.

Although the effect of injecting water into a fractured rock is well known, the understanding of the processes involved have been limited. In order to clarify the functioning and limitations of a water curtain system around a cavern with compressed gas, a series of tests were conducted in a pilot-scale rock cavern (Röda Sten). During the tests it was possible to study the geometrical boundary conditions required for successful functioning of the water curtain, the effect of operational errors and limitations of the gas pressure with different water curtain layouts and pressures.

The research revealed several important factors, such as:

- No gas migration into the rock mass occurred, provided the water curtain pressure, at all points, was higher than the gas pressure. The gas pressure in the cavern ranged from approximately 0.4 MPa to 1.0 MPa.

- With a reduced water curtain, which only consisted of the boreholes above the cavern, the gas migration into the rock mass could not be controlled, even at a gas pressure close to the natural ground water pressure (approx. 0.4 MPa).

- Major air leakages, due to a simulated failure of the water supply system, were completely stopped by pressurising the water curtain. The air containment was not significantly affected by the presence of air in the rock mass after an air leakage.

- The maximum water curtain pressure, with the gas containment completely maintained, was 2.5 - 3 times the natural ground water pressure. Further increase in the water curtain pressure resulted in an increased hydromechanical breakdown of the rock mass and loss of the gas containment at gas pressures above 1.0 MPa. A maximum pressure of 1.7 MPa was reached in the water curtain and microseismic events, due to shear failure of pre-existing joints, were successfully registered and their locations calculated.

- The well test method which best decribed the geohydrological conditions of the rock mass was found to be cross-hole tests. The hydraulic conductivity from these tests was found to be log-normal distributed and the calculated geometric mean agreed well with the equivalent, large-scale, hydraulic conductivity of the rock mass. Straddle-packer tests and single well tests resulted in significantly smaller values of the hydraulic conductivity.