Injection related issues of a doublet system in a sandstone aquifer - A generalized concept to understand and avoid problem sources in geothermal systems

This study proposes a concept and presents a workflow to examine potential reasons for low injectivity of
sandstone aquifers. Injection related problems are a major challenge for the sustainable utilization of geothermal
waters. In order to completely understand and avoid the geothermal reinjection problems, potential problem
sources acting on different scales should be taken into consideration. Thus, in the workflow, possible problem
sources are considered on regional, reservoir and local scale and categorized into 1) effect of regional hydraulics
(potential presence of overpressure and upward flow) 2) inadequate reservoir performance (limited extent, low
permeability and performance) and 3) local clogging processes (particle migration, mineral precipitation, microbial
activity). Hydraulic conditions are characterized by defining the pressure regime and the direction of
vertical driving forces. The reservoir properties are given by determining the grain size and the size of the
reservoir layers, as well as the permeability and the transmissivity of the reservoir and the capacity of the
injector. Physical, chemical, and biological clogging processes are investigated by specifying the rock properties
and determining particle content of the fluid; by analysing the type, probability and amount of the scaling and
estimating the potential for corrosion; and by evaluating the possibility of biofilm formation. The concept and the
workflow were first tested on a geothermal site (Mez˝ober´eny, SE Hungary, installed in 2012) that had to stop
operation because of unsuccessful reinjection. The low injectivity of the well is a consequence of several separate
problems and their interaction: Reservoir properties are insufficient due to low permeability and transmissivity
of the reservoir and the limited vertical and horizontal extension of the sandstone bodies. Precipitation of carbonates,
iron and manganese minerals is predicted in hydrogeochemical models and observed in solid phase
analysis. Microbial material is produced from the particularly high organic content of the produced thermal
water. Injection problems due to hydraulic effects are not expected since the regional pressure regime is slightly
subhydrostatic. In summary, reservoir properties determine a low injectivity, which is further decreased to a
critical level by the clogging processes. The proposed generalized concept guides a detailed reservoir and
geothermal system analysis which is essential for a sustainable geothermal operation.