The creation of expanded diameter gravel wells in unconsolidated formations for High-Temperature Aquifer Thermal Energy Storage Systems
Theoretical and numerical evaluation of borehole stability
Tessel M. Grubben (TU Delft - Water Systems Engineering)
Martin Bloemendal (TU Delft - Water Systems Engineering, KWR Water Research Institute)
Martin L. van der Schans (Geo-engineering, KWR Water Research Institute)
Niels Hartog (KWR Water Research Institute, Universiteit Utrecht)
Philip J. Vardon (Geo-engineering)
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Abstract
High-Temperature Aquifer Thermal Energy Storage (HT-ATES) systems have the potential to cost-effectively store large volumes of thermal energy, bridging the supply-demand gap for variable renewable heat sources, such as solar thermal or power-2-heat conversion. These systems involve the injection and extraction of heated and cooled groundwater in aquifers via tube wells. A HT-ATES system will be showcased at TU Delft, which involves the use of an Expanded Diameter Gravel Well (EDGW) to increase well capacity and reduce mechanical clogging compared to conventional wells. This has the potential to reduce the number of wells needed and lower the costs of the HT-ATES system. An EDGW has previously been constructed at depth in unconsolidated formations using a jetting technique for borehole expansion. The missing explanation for the collapse of the second well highlights a knowledge gap regarding the stability of an expanded diameter borehole in unconsolidated formations. To prevent collapse of future expanded boreholes and to better manage the drilling process, this study aims to investigate the effects of an enlarged diameter on well stability through a theoretical analysis. The stability of the EDGW borehole is evaluated in two ways. Firstly, the effects of an enlarged diameter on the stability of the well are evaluated analytically using a poroelastic framework. Different conditions are taken into account regarding the stress state, mud pressure, and hydraulic conductivity of the aquifer. Secondly, field test conditions for the anticipated EDGW in the HT-ATES system are simulated numerically using the two and three-dimensional finite element software. The final results of this study are presented in the form of critical conditions regarding stress state, required mud pressure, and hydraulic conductivity for enlarged diameter boreholes in unconsolidated formations. Additionally, a design for the EDGW field test as part of the HT-ATES system in Delft is proposed, taking into account uncertainties such as the in-situ stress state and strength parameters of the formation.
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