Meso-Scale Field Testing of Reactive Tracers in a Model Geothermal Reservoir

Meso-scale field testing of thermally reactive tracers was conducted at the Altona field site in a well-characterized, single subhorizontal bedding plane fracture roughly 100 m2 in active area located 8 meters below ground surface. The spatial distribution of subsurface groundwater flow was previously characterized using ground penetrating radar (GPR) measurements. The reservoir rock, initially at 11.7 °C, was heated using 74 °C hot water injection in a two-spot pattern using an injection to production well separation of 14 m. During the heating process, a series of thermally degrading tracer experiments were used to characterize the progressive in situ heating of the fracture. In addition, a conservative, carbon-cored engineered nanoparticle tracer was used to measure the residence time distribution (RTD) of fluid flowing from injector to producer. Fiber Optic Distributed Temperature Sensing (FODTS) was used to continuously measure the spatial distribution of heat exchange at ten locations spread out between the injection and production well. The experiments revealed reduced recovery of the thermally degrading tracer as the reservoir was progressively heated indicating that the advancement of the thermal front was proportional to the mass fraction recovered of the thermally degrading tracer. Both GPR imaging and FODTS measurements reveal that flow was reduced to a narrow channel which directly connected the two flowing wells and led to early and rapid thermal breakthrough. Computational modeling of conservative/reactive tracer and heat transport in a two-dimensional discrete fracture demonstrate that subsurface characterization using conservative tracers alone could not uniquely characterize the Altona field site. The inclusion of the thermally reactive tracer, however, provided improved resolution of the spatial distribution of flow after 1 day of hot water injection.

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