Gas Threshold Pressure Tests in Deep Boreholes for Determining Two-Phase Flow Properties of the Host Rock at the Proposed L/ILW Repository, Switzerland

Abstract

ABSTRACTSafety assessment of potential gas release from the L/ILW requires information on two-phase flow behavior of the host rock. To determine the relevant formation properties for gas flow through the host rock, a combination of hydrotest and gas-injection sequences were designed and subsequently implemented in the Nagra field testing program at Wellenberg. The results of the design caculations indicate that the gas threshold pressure can be estimated from a gas injection test followed by a shut-in test using the log-log diagnostic plot of the gas injection sequence and the Homer plot of the recovery sequence. For a composite well-aquifer model, the gas threshold pressure of the outer zone, when estimated from the gas injection sequence, is increased, because water is displaced from the inner zone into the outer zone as gas is injected into the inner zone. This results in the overestimation the gas threshold pressure of the outer zone, and in turn, of the air-entry pressures in the corresponding capillary pressure models. The pressure response during the gas injection sequence in a composite well-aquifer model can be drastically different for different relative permeability models. For a Brooks-Corey model, the pressure indicates a distinct increase when gas flow starts into the outer zone. For a Grant model, the pressures can actually decrease during a constant-rate gas injection sequence. A decrease in interval pressure during gas injection was observed in two different field tests in the Palfris Formation at Wellenberg. In borehole WLB-SB4as, the gas injection test in interval VM16 was analyzed with the inverse code ITOUGH2 to determine the two-phase flow parameters for two different conceptual models. The simulations indicated that the particular pressure response, characterized by a sudden decrease during gas injection, could not be reproduced with a Brooks-Corey relative permeability model. However, assuming a Grant model, a sudden pressure decrease during the gas injection sequence could be simulated. The results indicate the importance in determining the hydraulic properties and the well-aquifer model from a combined analysis of hydro- and gas injection tests, in order to accurately determine the two-phase flow parameters.