In geothermal systems with matrix-dominated flow, fluid circulation may involve multiple porous reservoirs, with heat being extracted from two or more reservoirs separated from one another by layers of low permeability.
In the Dutch geothermal industry, these fluid flows within the wells are not separated by using multiple injection and production annuli, but are instead mixed. In this type of production, it is assumed that the volume ratio between the reservoirs for the extracted fluid is equal to the volume ratio between the reservoirs for the injected fluid. In other words: the flow distribution within each reservoir is in equilibrium, with the volumes of water extracted from one reservoir and reinjected into it being equal. It is expected that the average reservoir pressures in such a system will be close to or equal to the initial conditions. Differences in reservoir characteristics between the injection and production wells within a single reservoir, such as differences in thickness or permeability, may potentially influence fluid circulation and cause an unbalanced flow. Unbalanced flow may also result from differences in well completion between the injection and production wells, and/or differences in the interface between the well and the reservoir (‘skin factor¹’) for the injection and production wells. Finally, differences in the properties of the fluids between the reservoirs, such as salinity or temperature, may also cause an unbalanced flow. The latter may result from the fact that the injection fluid is a homogenised fluid in which the produced fluids with different properties are no longer separated but mixed. It is expected that the potential effects of unbalanced flow will include, amongst other things, a change in reservoir pressures. The effects of unbalanced flow may, in turn, lead to hazards such as seismic activity or ground uplift/subsidence.
Geothermal systems involving production from multiple reservoirs have not yet been systematically investigated. Nor have potential measures to mitigate the effects of unbalanced flow – for example, through well design – been investigated. The aim of this proposed study is to provide this information by determining the effect of reservoir and fluid properties on fluid circulation in geothermal operations involving production from multiple reservoirs, as well as the implications, associated hazards and mitigating measures.
The study was carried out by VITO, with Sproule ERCE as a subcontractor, bringing together strong expertise in the fields of geothermal reservoir engineering, geomechanics and the modelling of thermal-hydraulic-mechanical (THM) reservoir processes.
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In geothermal systems with matrix-dominated flow, fluid circulation may involve multiple reservoirs, with heat being extracted from two or more reservoirs separated from one another by low-permeability layers.
In the Dutch geothermal industry, these fluid flows within the wells are not separated by the use of multiple injection/production strings, but are mixed. In this type of production, it is assumed that the volume ratio between the reservoirs for the extracted fluid is equal to the volume ratio between the reservoirs for the injected fluid. In other words: the distribution of flow within each reservoir is in equilibrium, with the volumes of water extracted from a reservoir and reinjected into it being equal. It is expected that the average reservoir pressures in such a system will be close to or equal to the initial conditions. Differences in reservoir characteristics between the injection and production wells within a single reservoir, such as differences in thickness or permeability, may potentially influence fluid circulation and cause an unbalanced flow. Unbalanced flow may also result from differences in the completion of the injection and production wells, and/or differences in the connection between the well and the reservoir (‘skin factor1’) for the injection and production wells. Finally, differences in the properties of the fluids between the reservoirs, such as salinity or temperature, may also cause an unbalanced flow. The latter may be due to the fact that the injection fluid is a homogenised fluid in which the produced fluids, with their different properties, are no longer separated but mixed. It is expected that the potential effects of unbalanced flow will include, amongst other things, a change in reservoir pressures. The effects of unbalanced flow may, in turn, lead to hazards such as seismic activity or uplift/subsidence.
Thermo-hydro-mechanical (THM) models to better understand geothermal systems, such as their lifespan (e.g. Crooijmans et al., 2016; Daniilidis et al., 2021) or mechanical issues (see the KEM-15 project, Pandey et al., 2018), have so far been applied to cases involving a single reservoir. Geothermal systems involving production from multiple reservoirs have not yet been systematically investigated. Nor have potential measures to mitigate the effects of unbalanced flow, for example through well design, been explored. The aim of this proposed research is to provide this information by determining the effect of reservoir and fluid properties on fluid circulation in geothermal operations involving production from multiple reservoirs, as well as the implications, associated risks and measures to mitigate them.
The project focused on the analysis of a representative production scenario involving two zones for a geothermal dual in a Dutch geothermal environment, corresponding to siliciclastic sedimentary aquifers.
The three research questions defined as the starting point for the project were addressed in a thorough and technically sound manner.
Effect of reservoir, well and fluid properties on flow and pressure (RQ1)
The report demonstrates that differences in reservoir permeability, thickness and well-related parameters are the dominant factors governing flow distribution and pressure development during mixed geothermal production. The base scenario consisted of two highly permeable and laterally contiguous sandstone reservoirs, each 50 m thick, separated by a 200 m-thick impermeable seal, representative of Dutch low-enthalpy geothermal systems at depths of approximately 1,500–2,500 m. A single producer–injector pair, spaced 1,500 m apart, was drilled vertically into both reservoirs, with no hydraulic connection outside the wells. Extreme cases, such as laterally discontinuous reservoirs or wells with a large negative skin effect and relatively low permeability, show that these cause long-term flow imbalances and the greatest pressure deviations from the base case scenario.
Effects of unbalanced flow and associated risks (RQ2)
The study identifies unbalanced flow as an inherent but usually transient characteristic of mixed production. Potentially associated risks are discussed, and ground subsidence and the reactivation of faults are explicitly evaluated. The modelling results demonstrate how geothermal activities induce pressure and temperature changes in the producing reservoirs. As explained in the project report, the analytical and semi-analytical assessments of mechanical effects are based on different assumptions and therefore yield divergent results. The analytical approach is more conservative and predicts reactivation even in the base case scenario (near the injection well). The semi-analytical assessment of mechanical effects, which is based on the ‘nucleus-of-strain’ approach – whereby spatial stress variations are smoothed out and differential stresses may be underestimated – does not predict a shear failure for the base case or the simulated cases with a discontinuous reservoir. Overall, the report shows that even within the typical Dutch operational constraints, certain geological configurations that promote unbalanced flow can lead to increased risks. These are related to (i) the reactivation of nearby faults as a result of over-injection into one of the reservoirs, combined with the cooling effects of the injected fluid, and (ii) land subsidence resulting from the accelerated depletion of one of the reservoirs. The study does not address the occurrence of depletion-related fault reactivation.
Mitigation measures (RQ3)
The report sets out a range of mitigating and preventive measures against the effects of unbalanced fluid circulation. These include well design options (e.g. zonal isolation, expandable packers), adaptive operational strategies, continuous monitoring (pressure, temperature, flow, ground subsidence) and the use of bow-tie analyses. Under conditions of unbalanced fluid circulation, the report states that the upper pressure limit for geothermal activities must be restricted to values close to the hydrostatic gradient.
The report focuses on a representative Dutch context, taking into account production from two zones within a clastic reservoir, and provides a comprehensive investigation into the exploitation of mixed geothermal reservoirs, including an analysis of reservoir pressure and temperature across a range of relevant scenarios. Particular attention is paid to conditions that may lead to unbalanced fluid circulation. The effects of unbalanced fluid circulation are evaluated, with an emphasis on the reactivation of faults and ground subsidence.
The report proposes relevant measures to mitigate the effects of unbalanced fluid circulation. The study emphasises the importance of:
• site-specific characterisation of the reservoir, including an assessment of the reservoir’s continuity and connectivity,
• a cautious approach to reservoirs that are thin, laterally discontinuous or poorly connected,
• monitoring strategies for the early detection of unbalanced flow and anomalies in pressure and temperature,
• adaptive management of operations.
‘Co-mingled production’ is defined as the simultaneous exploitation of multiple hydraulically isolated reservoirs via a single borehole. This study focused on the Dutch context and provides an assessment of mixed geothermal production and the conditions that may lead to unbalanced fluid circulation between reservoirs. To this end, the project analysed a representative production scenario involving two zones for a geothermal dual in a Dutch geothermal environment, corresponding to siliciclastic sedimentary aquifers.
It highlights how changes in pressure and temperature can influence subsurface behaviour, including the reactivation of faults and ground subsidence. The findings underscore the importance of clearly defined operational limits, a thorough characterisation of the reservoirs and continuous monitoring to support operational decisions. The report also proposes relevant measures to mitigate the effects of unbalanced fluid circulation.
According to the KEM panel, the consortium employed a clear, well-structured and appropriate methodology to answer the defined research questions.
Based on the report’s findings and the recognised limitations of the models used, further research using more advanced, fully coupled THM models would be a next step towards obtaining more accurate answers to the research questions. Such models would provide a more comprehensive picture of the interactions between pressure, temperature and rock stresses and enable a more refined assessment of fracture reactivation for selected, relevant scenarios.
The practical implications of the results of the KEM-52 project have been interpreted by the State Supervision of Mines.