This paper evaluates the impact of reduction of doublet well spacing, below the current West Netherlands Basin standard of 1000 to 1500 m, on the Net Present Value (NPV) and the life time of fluvial Hot Sedimentary Aquifer (HSA) doublets. First, a sensitivity analysis is used to show the possible advantage of such reduction on the NPV. The parameter value ranges are derived from West Netherlands Basin HSA doublet examples. The results indicate that a reduction of well spacing from 1400 to 1000 m could already influence NPV by up to 15%. This effect would be larger in more marginally economic HSA doublets compared to the West Netherlands Basin base case scenario. The possibility to reduce well spacing is supported by finite element production simulations, utilizing detailed facies architecture models. Furthermore, our results underline the necessity of detailed facies architecture models to assess the potential and risks of HSA doublets. This factor significantly affects doublet life time and net energy production of the doublet.@en

The primary challenge for efficient geothermal doublet design and deployment is the adequate prediction of the size, shape, lateral extent and thickness (or aquifer architecture) of aquifers. In the West Netherlands Basin, fluvial Lower Cretaceous sandstone-rich successions form the main aquifers for geothermal heat exploitation. Large variations in the thickness of these successions are recognised in currently active doublet systems that cannot be explained. This creates an uncertainty in aquifer thickness prediction, which increases the uncertainty in doublet lifetime prediction as it has an impact on net aquifer volume. The goal of this study was to improve our understanding of the thickness variations and regional aquifer architecture of the Nieuwerkerk Formation geothermal aquifers. For this purpose, new palynological data were evaluated to correlate aquifers in currently active doublet systems based on their chronostratigraphic position and regional Maximum Flooding Surfaces. Based on the palynological cuttings analysis, the fluvial interval of the Nieuwerkerk Formation was subdivided into two successions: a Late Ryazanian to Early Valanginian succession and a Valanginian succession. Within these successions trends were identified in sandstone content. In combination with seismic interpretation, maps were constructed that predict aquifer thickness and their lateral extent in the basin. The study emphasises the value of palynological analyses to reduce the uncertainty of fluvial hot sedimentary aquifer exploitation.

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This study finds that the geothermal doublet layout with respect to the paleo flow direction in fluvial sedimentary reservoirs could significantly affect pump energy losses. These losses can be reduced by up to 10% if a doublet well pair is oriented parallel to the paleo flow trend compared to perpendicular. The chance that flow paths are formed perpendicular to this trend strongly depends on the net sandstone volume in the reservoir. Detailed fluvial facies architecture realisations which are used in this study, are generated with a process-based approach utilizing geological data from the Lower Cretaceous Nieuwerkerk Formation in the West Netherlands Basin. Finally, this study emphasizes the importance of detailed facies architecture modelling for the assessment of both risks and production strategies in Hot Sedimentary Aquifers.@en

Required distance between doublet systems in low enthalpy geothermal heat exploitation is often not fully elucidated. The required distance aims to prevent negative interference influencing the utilisation efficiency of doublet systems. Currently production licence areas are often issued based on the expected extent of the reinjected cold water plume on the moment of thermal breakthrough. The production temperature, however, may not immediately drop to non-economic values after this moment. Consequently, heat production could continue increasing the extent of the cold water plume. Furthermore, the area influenced by pressure because of injection and production spreads beyond the cold water plume extent, influencing not only the productivity of adjacent doublet systems but also the shape of cold water plumes. This affects doublet life time, especially if adjacent doublets have different production rates. In this modelling based study a multi parameter analysis is carried out to derive dimensionless relations between basic doublet design parameters and required doublet distance. These parameters include the spacing between injector and producer of the same doublet, different production rates, aquifer thickness and minimal required production temperature. The results of this study can be used to minimize negative interference or optimise positive interference aiming at improving geothermal doublet deployment efficiency.

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Huge amounts of heat are stored in sedimentary aquifers in the Dutch subsurface. The amount of heat would be sufficient to provide our national heat demand for decades without any greenhouse gas emissions. Exploitation of this type of resource started some 10 years ago in the Netherlands. In 2016, 16 geothermal doublet systems had been installed that produce geothermal heat, and each year 2 to 3 new systems are realised. A doublet system consists of a production well that extracts hot formation water from kilometer deep aquifers. After the heat is extracted from the water in heat exchangers, the cooled water is reinjected into the same aquifer at approximately 1 to 1.5 km distance from the production well. Most of the current Dutch doublet systems provide heat for the horticulture sector. These systems have an average net energy production of approximately 10 MWth and therefore hundreds of additional systems are required to significant amounts of our heat consumption with geothermal energy. This PhD thesis investigated doublet system design and deployment strategies to optimise exploitation and increase the possible number of doublet systems exploiting the same aquifer. Based on detailed geological models, subsurface flow simulations are used to evaluate parameters such as required injector-producer distance, the preferred orientation of a well pair with respect to geological trends and required doublet distance to avoid negative interference. Based on the results, regional doublet deployment strategies can be developed to make optimal use of geothermal heat from sedimentary resources.@en

A three-dimensional model is used to study the influence of facies heterogeneity on energy production under different operational conditions of low-enthalpy geothermal doublet systems. Process-based facies modelling is utilised for the Nieuwerkerk sedimentary formation in the West Netherlands Basin to construct realistic reservoir models honouring geological heterogeneity. A finite element based reservoir simulator is used to model the fluid flow and heat transfer over time. A series of simulations is carried out to examine the effects of reservoir heterogeneity (Net-to-Gross ratio, N/G) on the life time and the energy recovery rate for different discharge rates and the production temperature (T_min) above which the doublet is working. With respect to the results, we propose a design model to estimate the life time and energy recovery rate of the geothermal doublet. The life time is estimated as a function of N/G, T_min and discharge rate, while the design model for the energy recovery rate is only a function of N/G and T_min. Both life time and recovery show a positive relation with an increasing N/G. Further our results suggest that neglecting details of process-based facies modelling may lead to significant errors in predicting the life time of low-enthalpy geothermal systems for N/G values below 70%.

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This paper analyzes the relation between well spacing and Net Present Value of a Hot Sedimentary Aquifer geothermal doublet. First, a sensitivity analysis is carried out to evaluate the effect of uncertainty of geological and production parameters on the Net present Value. Second a finite-element approach is utilized to study the effect of fluvial facies architecture on geothermal energy production. For this purpose detailed fluvial facies architecture models are created utilizing a process-based facies modelling approach. These models and reservoir properties are based on a geological dataset of the Lower Cretaceous Nieuwerkerk Formation in the West Netherlands Basin (WNB). Results of the sensitivity analysis show that a 10% variation in well spacing from a 1000m base case scenario could vary the NPV by 10%. The minimal required well spacing is dependent on the reservoir thickness, flow rate and the allowed production temperature drop. The simulations results show that the theoretical advantage of a reduction in well spacing could be balanced by a poor well connectivity between the wells because of the characteristic of fluvial reservoir architecture. @en

Fluvial sandstone reservoirs composed of stacked meander belts are considered as potential geothermal resources in the Netherlands. Net-to-gross, orientation and stacking pattern of the channel belts is of major importance for the connectivity between the injection and production well in such reservoirs. Understanding the influence of the fluvial sandstone architecture on the heat recovery is necessary for designing geothermal doublet projects in fluvial reservoirs. A detailed outcrop study of the Huesca fluvial fan is used as an analogue for fluvial reservoir architecture. The study shows meandering fluvial sandstone bodies, laterally amalgamated and vertically stacked within the meander belts. Meander belts in the outcrop have a width of 1 to 1.5 km and are up to 4 km long. Sandstone bodies are surrounded by impermeable floodplain deposits and poorly permeable crevasse splay sandstones (Donselaar & Overeem, 2008). The floodplain deposits decrease the net-to-gross and connectivity in the meander belt. In order to identify the most influential geological features controlling the flow and heat transfer in geothermal aquifers, several reservoir models of fluvial sandstones are constructed with increasing geological detail. These models are based on the results of geological fieldwork on the Huesca fluvial fan in Miocene, Ebro Basin, Spain. Several models with different geometry of the fluvial sandstone bodies, orientation of the channel belts and stacking patterns of the channel bodies are considered. A finite-element approach is utilized to study the geothermal energy production. The effects of different levels of architectural complexity on the geothermal energy production, by conducting several accurate numerical simulations, are discussed. The results show that utilizing simplified reservoir models can lead to a significant error in predictability of the heat recovery from deep fluvial sandstone formations. @en

Currently six geothermal doublets are realized in the WNB. Five of these doublets target the same Lower Cretaceous fluvial sandstone interval, the Nieuwerkerk Formation. About 40 exploration licences are granted. Many of them also have sandstones in the same fluvial interval, the Nieuwerkerk Formation as a target. To successfully and efficiently produce geothermal energy from this fluvial sandstone interval, the distribution and internal architecture of the sandstone bodies must be better understood. Fluvial sandstone reservoirs are composed of stacked meander belts and floodplain fines. Net-to-gross, orientation and stacking pattern of the channel belts is of major importance for the connectivity between the injection and production wells in fluvial sandstone reservoirs@en