Dr. Judit Mádl-Szőnyi and Dr. Szilvia Simon from Department of Physical and Applied Geology at Eötvös Loránd University in Hungary demonstrated hydrodynamic approach based on regional pore pressure evaluation considering Duna-Tisza Interfluve part of the Pannonian Basin, Hungary characterized by overpressured and superimposed gravity-driven flow regimes. The work published in journal, Geothermics, compared the results of evaluation with experiences of thermal water utilization of the region.
Reinjection has been known to be a key factor in course of preliminary phase of thermal water exploration because of sustainability and economic reasons. Its previous understanding on influence of subsurface pore pressure regime is known to be a factor in preliminary resource evaluation and planning. This research emphasized on regional pore pressure regimes as an essential factor in planning, in addition to provide a workflow for its evaluation.
The approach gave the first priori estimation of pressure conditions which does not substitute the detailed evaluation of recharge conditions which later, during the planning phase of geothermal investment.
Fluid pressure conditions of the sedimentary basins described the nature of the preliminary hydrodynamic regions of different parts of the basin. Unconfined basins were characterized by close to hydrostatic pressure conditions representing hydraulic pressure communication flow systems where continuous meteoric water exchange can be supposed while confined sedimentary basins can be characterized hydrodynamically by underpressure or overpressure due to many geological processes such as erosion, deposition, tectonic compression.
When analyzing the potential geothermal reservoirs of the Duna-Tisza Interfluve, its Pre-Neogene basement comprised of brittle flysch, carbonate and metamorphic rocks. Relief variability of the basement is in excess of 3000m as relatively shallow (-500) to (-800)m asl in the west while deep (-2000) or even (-3500)m asl in the east. Algyő and Endrőd aquitards were unproductive ones as they are dissected by faults and structures which disturb their integrity. The Pre-Pannonian and Szolnok aquifers were characterized by limited vertical and horizontal extent. Neogene part of the Great Plain AF is handled as thermal water reservoir for this research and noted by Great Plain AF (N).
Average geothermal gradient of the study area was found to be 4.770C/100m higher than the average Hungarian gradient of 4.50C/100m. Pre-Neogene Aquifer (basement reservoir) and Great Plain Aquifer (N) had 4.830C/100m and 4.240C/100m respectively. However, the Quaternary part of the Great Plain Aquifer displayed elevated value of 6.60C/100m. Geothermal gradients increased from Algyő aquitard to Szolnok aquifer and Endrőd aquitard with 6.410C/100m, 8.310C/100m and 11.10C/100m respectively. The best reservoirs of the Pre-Neogene basement and Great Plain Aquifer (N) were characterized by relatively lower temperature gradients values in study area.
The pressure-elevation profile compiled for the whole study area displayed overpressure below (-1500)m asl. Algyő aquitard and Szolnok aquifer were characterized by normal hydrostatic pressure to slightly overpressure for the former and severely overpressure for the latter with (-2500)m asl respectively. Endrőd aquitard showed hydrostatic pressure at (-2400)m asl as severely overpressure value existed at an elevation of (-2100)m asl. Pre-Pannonian aquifer became overpressure between the range of (-1000)m asl to (-1600)m asl and highest overpressure occurred at (2500)m asl. Pre-Neogene basement aquifer at (-3400)m asl can be characterized by normal to slightly overpressured values at (-2200, -2800, -3500)m asl.
When interpreting flow regimes for an area pressure increment, pressure increment in the Great Plain aquifer varied in range of +/- 0.5MPa for the study area. Great Plain aquifer (N) of the study area is characterized by less than 0.5MPa positive pressure increments independent of the depth, except from some overpressured values in the East.
Evaluation of the local study areas showed negative pressure increments in Great Plain aquifer (N) and the Pre-Neogene basement. Pressure-elevation profile also showed that vertical pressure gradient is less than hydrostatic (ϒ=9.74MPa/Km) between Great Plain AF (Q-N) and basement carbonate reservoirs. The pressure-elevation profiles of local study area 2 (Fülöpjakab at 107m asl) and study area 3 (Kistelek at 88m asl) gave near hydrostatic pressure conditional of 9.55MPa/Km and 9.81MPa/Km. The hydraulic situation in local study area 1 and 2 (Kecskemét and Fülöpjakab) implies such type of favorable reinjection possibilities into the Great Plain Aquifer (N) without energy investment. Field experiences could prove these proposed conditions.
Mádl-Szőnyi and Simon (2016) analysis when applied could also be helpful in aligned planning of drinking water, thermal water and hydrocarbon utilization in a sedimentary basin.
Mádl-Szőnyi, J. Simon, S. Involvement of Preliminary Regional Fluid Pressure Evaluation into the Reconnaissance Geothermal Exploration—Example of an Overpressured and Gravity-Driven Basin. Geothermics, 2016, Volume 60, pp 156-174.
Department of Physical and Applied Geology, Eötvös Loránd University, Budapest, Hungary 1/C Pázmány P. stny., Budapest 1117, Hungary.
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