Evaluation of common hydrological tracers in porous media PRATHAP MOOLA BERGUR SIGFÚSSON, ANDRI STEFÁNSSON

Research motivation  The aim is to study water-rock interaction and H 2 S mineralization in porous media  One effect of mineralization is the change in porosity  Tracer tests can be used to study the in situ porosity changes

Why tracer tests?  To study the groundwater flow path from injection well to monitoring well in aqueous systems with the aid of recovery upon time  To examine the characteristic behavior of the subsurface layers and to calculate effective porosity

What is an “ideal“ tracer? An ideal tracer:  Should have a low detection limit  Should not react chemically or be absorbed  Should be easily available and inexpensive

Experimental tracer tests  The chemical behavior and reactivity of some tracers were studied in the laboratory  Laboratory experiments have similar setup as tracer tests in nature Tracer Injection point Sampling point Teflon column Injection well Ground water system Monitoring well

Tracers 6 common geohydrological tracers were studied  Amino G acid  Amino Rhodamine G  Fluorescein sodium salt  Napthionic acid  Pyranine  Rhodamine B Amino G Acid Amino Rhodamine G Fluorescein söddum Nap thionic acid Pyranine Rhodamine B

Experimental setup  Flow-through column experiments were conducted at room temperature  The teflon column was preloaded with rock  The tracer was injected and the recovery monitored at the outlet PTFE column Spectrophotometer Tracer Pump Blank

Experimental setup  Various conditions  pH = 3, 6.5 &9  Basaltic glass, rhyolite and quartz(45-125µm) PTFE column Spectrophotometer Tracer Pump Blank

How is pore volume measured with the help of breakthrough curves?

Results  Three types of tracer trends were observed :  Type 1 – always non-reactive in all studied rock types and pH (100% recovery)  Type 2 – always reactive in all studied rock types and pH (70-90% recovery)  Type 3 – sometimes reactive, sometimes non-reactive depending on rock type and pH

What does it mean? Some common hydrological tracers react upon water-rock interaction BUT an ideal tracer should NOT REACT chemically or be absorbed!

Next steps  Water-rock interaction involves rock dissolution followed by secondary mineral formation  This process may result in a decrease in porosity as the minerals formed occupy more space than the minerals dissolved  Recent experiments of H 2 S mineralization demonstrate this (talk by Snorri Guðbrandsson)

Pore volume changes upon H 2 S mineralization  The water-rock interaction and H 2 S mineralization in porous media are currently being studied in our group  It is planned to follow the mineralization as a function of time using laboratory tracer tests to measure the pore volume changes Sampling point Intlet Outlet Reactor

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