Abstract
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 (Mezőberény, 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.
Highlights
Introduction and objectivesReinjection of cooled geothermal brines into porous reservoirs can ensure pressure stabilization of reservoirs in order to avoid pressure decline and to mitigate subsidence (Gringarten, 1978; Axelsson, 2012)
The objective of this paper is to introduce the potential problem sources, propose a concept and present a workflow to understand and prevent problems related to geothermal reinjection in porous reservoirs
We analysed the hydraulic conditions of the aquifer on a regional (~10 km) scale through characterizing the pressure regime
Summary
Reinjection of cooled geothermal brines into porous reservoirs can ensure pressure stabilization of reservoirs in order to avoid pressure decline and to mitigate subsidence (Gringarten, 1978; Axelsson, 2012). Reinjection is mandatory due to negative impacts of surface discharge as an alternative to dispose the produced water. Negative consequences of surface discharge would be reservoir depletion, thermal or chemical pollution of rivers or lakes and contamination of the atmosphere by e.g. methane, hydrogen sulphide or carbon-dioxide (Axelsson, 2012). The injection technology of oil industry with high pressure injection is often unsustainable and uneconomical for geothermal systems in the long-term because of the high pumping costs required to generate the extra injection pressure (Szanyi et al, 2014).
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