Degassing and two-phase flow pilot hole test report

Abstract

A pilot hole test was conducted to support the design of the Degassing of Groundwater and Two-Phase Flow experiments planned for the Hard Rock Laboratory, Aespoe, Sweden. The test consisted of a sequence of constant pressure borehole inflow tests (CPTs) and pressure recovery tests (PRTs) in borehole KA2512A. The test sequence was designed to detect degassing effects from the change in transmissivity, or hydraulic conductivity, and storativity when the borehole pressure is lowered below the groundwater bubble pressure. The entire 37.3m of the borehole section was tested without packers. Flow response to pressure changes in CPTs occurred rapidly. Flowrates fluctuated before attaining a steady trend, probably due to effective stress changes when borehole pressure was reduced for the first time. These factors decreased the sensitivity of type-curve fits to values of specific storage. The relationship between borehole pressure and steady-state flowrates was linear over borehole pressures of 1500 kPa (abs) down to 120 kPa (abs) during testing in December 1994, indicating that processes that may change hydraulic conductivity at low borehole pressures, such as degassing, calcite precipitation or turbulence, did not occur to a measurable degree. Test results during January and February of 1995 suggest that degassing may have occurred. The hydraulic conductivity measured at a borehole pressure equal to 120 kPa (abs) was 20% lower than the hydraulic conductivity measured at a borehole pressure of 1500 kPa (abs); the latter value was 10% lower than the hydraulic conductivity measured in December, 1994. The volumetric gas content measured during this time was 1% v/v. Pressures in monitoring well KA2511A responded to the testing in KA2512A. Step-changes in flowrates coincided with blasting at 3300-3400 m tunnel length. The magnitude of these changes was greater at the lower borehole pressures. Step increases in pressures in KA2511A also coincided with the blasts.