Comparison of the Modelled Hydraulic Impact of Multi Legs and One Leg Geothermal Wells in the Dogger Reservoir in the Paris Area

Abstract

The Dogger reservoir is a deep carbonate reservoir (top at more than 1500m depth vertical) used extensively for district heating in the Paris area. The heat of the reservoir is mined using the doublet design, with a production well pumping the hot geothermal water (60-80°C) which is then reinjected colder (25-40°C) in an injection well after releasing its heat to the surface network via a heat exhanger. Few geothermal sites use the triplet design with two injection wells and one production well before transitioning to a fully new doublet when the wells productivity gets too low. New projects are being carried out in less productive areas of the reservoir using new well designs to ensure a better productivity/injectivity of the wells despite the lower permeability. One of these design uses oil&gas multi-legs design with one slanted leg and two U-shaped legs to produce fluid from the most productive layers of the reservoir. Modelling the hydraulic impact of this new doublet design can be challenging given the small scale of these legs (400m with a 7 inch diameter) compared to the extension of the reservoir that needs to be taken into account (100m thickness and 100 km² horizontal extension). Different approaches are used by the different consultancies working on this reservoir, one approach represents the legs as linear drains in a permeable layer of a multi-layers model. Comparison of this multi-legs design with classical one leg design show that the multi-legs design improves the productivity/injectivity index of the wells but the hydraulic impact of this multi-leg design is more important on nearby (2 km) other geothermal operations. Given the current 1 bar hydraulic impact accepted by the regulators for new geothermal operations on existing geothermal operations, this might mean that the geothermal projects targeting these less productive areas of the reservoir will need to be more spread out to limit hydraulic interference. A comparison of the results of the different modelling approaches might help reduce the uncertainty on the hydraulic impact of this multi-legs design and thus help the development of the mining scheme in these new development areas. This comparison will be done in an upcoming project lead by the BRGM in 2025.