OR/17/067 End summary

Daniels, K A, and Harrington, J F. 2017. The response of compact bentonite during a 1-D gas flow test. British Geological Survey. (OR/17/067).

A 1-D gas injection test on compact Mx80 bentonite has been conducted and the results have been presented. The test has comprised two stages: an initial hydration of the sample with DI water from the backpressure end, and a gas-testing phase. The hydration was conducted at 1 MPa, with a helium pressure of the same magnitude applied to the injection end of the sample to prevent the injection filter from becoming wet. Over the duration of the testing period, the gas pressure applied to the injection end of the clay sample and a pressure ramp was created to steadily increase the gas pressure, until the gas entry pressure was exceeded and gas entered the sample. The gas migrated through the clay and changes in porewater pressure, swelling pressure and flowrate were observed by the instrumentation around the sample. The porewater pressure showed a marked and almost instantaneous increase in pressure from about 1 MPa to about 10 MPa at 63.8 days, coinciding with a sudden spike in the outflow from the sample.

The data presented in this study shows that different recording instruments are required to provide a fuller picture of gas migration through bentonite because they show different features of the test at different times. The continued increase in the stresses measured by the axial and radial load cells after gas breakthrough mirrored the increase in injection pressure; this suggested that, at the start of the test, the sample was not in complete hydraulic equilibrium and a redistribution of the fluid in the sample caused the clay to continue to expand through the test

At day 71, the injection pump was stopped and the injection pressure was allowed to slowly decay. The outflow dropped to zero at this point, and no outflow was observed until day 81. After this point, the outflow was sporadic, occurring in fluctuations and spikes and suggesting that new gas pathways continued to open and close as the test progressed. The test has shown that there are dynamic processes were operating within the clay that will govern the spatial and temporal development of permeability within the sample.