| Abstract |
Fluid injection into overpressured regimes is only possible with an injection pressure higher than the aquifer pressure. The elevated aquifer pressure eventually prevents fluid inflow through reinjection. Elevated pressure (much higher than hydrostatic) in an aquifer under exploitation, however, can also evolve naturally due to overpressure dissipation from underlying units. The suggested basin-scale hydraulic evaluation complements the traditional geothermal potential and risk analysis. A pressure-regime analysis can be also used to estimate the injection capacity and reinjection problems. As a case study, we evaluated the hydraulic conditions surrounding the Mezőberény doublet system (40x50 km in SE Hungary). This site faces reinjection problems since 2012, which resulted in stopping geothermal production. The overpressure was assumed to be among the potential reasons for reinjection problems at that site. The analysis was carried out by 1) analyzing the properties of the underlying aquitard unit, 2) defining the pressure regime and the vertical driving forces, 3) modelling the influence of the overpressure on the hydraulic conditions of the reservoir, 4) evaluating the temperature state, 5) characterizing the water chemistry. Results show that overpressure is present in the underlying units beneath the reservoir with extreme pressure values, dynamic pressure increments (e.g. Δp=11.21 MPa) and superhydrostatic pressure-gradient (indicating ascending flow). Moreover, in the wider study area, the overpressure is likely dissipating into the aquifer indicated by positive dynamic pressure increments (Δp=0.77 MPa and Δp=0.14 MPa) and superhydrostatic pressure gradient (y=11.43 MPa/km). However, at the closer surrounding of the study system, this effect could not be observed: neither the pressure regime nor is the vertical pressure gradient (9.7 MPa/km) superhydrostatic within the reservoir. The reason can be that – based on seismic and well data – the underlying aquitard units are thick and continuous below the study site. Thus, the dissipation of overpressure by vertical leakage through aquitards is likely blocked by them. This is supported by the numerical modelling which suggests hydrostatic conditions at the study site. The numerlical modelling also showed the possible presence of elevated hydraulic heads in the reservoir due to overpressure beneath the aquifer in case of thinned aquitard and/or conduit fault. Consequently, injection problems due to an overpressured regime is not expected close to the well of Mezőberény. However, based on the results, this effect is proposed to be considered in the farther part of the study area as well as in other overpressured regions. We also propose to take hydraulic conditions into consideration and to apply the approaches of this study during geothermal exploration, risk and problem analyses. This work is part of the ENeRAG project, which has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 810980. |