Gas Migration in Shear Zones (GAM) - In-situ Tests
The initial phase included a series of hydraulic cross hole tests to characterise the water flow through the shear zone.
A series of dipole tracer tests were carried out using different tracers:
- Solute tracers (non-reactive tracers e.g. fluorescent dyes).
- Particle tracers (microspheres / nanospheres and pharges).
- Gas tracers (helium, xenon, argon and SF6 (sulphur hexafluoride
Solute and Particle Tracer Tests
Five successful tracer tests were carried out with non reactive solute tracers.
Tracer recovery was high, but particle tracer tests showed a strong dependency of tracer recovery on the orientation of dipole field.
GAM : In-situ tracer tests
A series of Radar Reflection surveys were also carried out during gas injection tests. Radar surveys were conducted before the start of the gas injection and then 1, 5 and 22 hours after the start of the injection. The reflectograms obtained from these survey indicate that the gas propagated through the shear zone by displacing the pore water.
The difference in the reflectograms is shown after 1hour, 5 hours and 22 hours of gas injection. This technique allows the visualisation of the gas flow path.
After 5 hours significant changes in the reflectograms could be seen, and after 22 hours the flow field seemed to stabilise. The gas distribution agreed with the results of the hydraulic tests which suggested these were the most likely flow paths in the shear zone.
Gas Tracer Tests.
11 gas injection tests in total took place at the GTS. The dipole distances were between 1 and 2 meters and a wide range of gas injection rates were applied.
The retardation of the more soluble gases was surprisingly low and the breakthrough times were short (breakthrough time refers to the time elapsed between addition of tracer and its first arrival at the extraction end).
GAM : Injection Test Setup
The diagram above represents the experimental set up for the gas tracer tests of the in situ test. A controlled flow of gas is injected into the injection interval. The shear zone is isolated via hydraulic packers at both the injection and extraction boreholes. The extracted water/gas mixture is separated and the gaseous portion is sent to a mass spectrometer for analysis of the gas fraction (see photo below).
GAM : Mass Spectrometer for analysis of the gas fraction
For each experimental run graphs were produced of the gas concentrations measured at the extraction side of the experiment. The graph below shows that the injected gas He, Ar, SF6 and Xe behave similarly in the shear zone and recovery is around 100 % for each of these gases. H2S had a low recovery rate and this is thought to be due to the reactivity of the H2S within the injection and extraction equipment of the test (H2S gas was in a N2 carrier gas).
GAM : Gas Injection Results
Gas Migration in Shear Zones (GAM)