# Reservoir connectivity concerns in the dogger aquifer? an assessment through a geothermal doublet

Daniel Otoo, Emmanuel Mouche, Benjamin Brigaud

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To cite this version:

Daniel Otoo, Emmanuel Mouche, Benjamin Brigaud. Reservoir connectivity concerns in the dogger aquifer? an assessment through a geothermal doublet. 10th UK Geothermal Symposium, The Geological Society London, Nov 2023, Londres, United Kingdom. hal-05295006

HAL Id: hal-05295006

https://hal.science/hal-05295006v1

Submitted on 2 Oct 2025

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# Reservoir Connectivity Concerns in the Dogger Aquifer? An Assessment through a Geothermal Doublet.

Daniel Otoo, Emmanuel Mouche, and Benjamin Brigaud.

Le Laboratoire des Sciences du Climat et de l'Environnement

email: daniel.otoo@lsce.ipsl.fr

## Abstract

Previous studies identify high permeable intervals of Ooid-dominated and Peloid-dominated lithofacies in the Dogger aquifer as significant controls on thermal movement between doublets. This study evaluates the extent and distribution of these units to ascertain their impact on reservoir connectivity on geothermal production in the limestone reservoir. The study involves (1) geobody geometry delineation from inversed 2D seismic using statistical analysis of petrophysical logs. (2) thermal simulation over thirty years in the Eclipse and PumaFlow simulators. Notable results from seventy simulations are as follows; (i) The initial 0.5 °C drop in scenarios with GL < 800 m (Case 1) is realized two years after production and about five years with GL > 1200 m (Case 2). High geobody connectivity generates wide transmissivity paths for injected water, thus a higher energy production rate of ~ 9.72 x 108 KJ/d in case 2 compared to ~ 8.70 x 108 KJ/d in case 1. (ii) Reservoir temperature decreases with decreasing NG with a constant rate of water injection and production. In a scenario with N/G of 44.12%, the temperature reads 69.78 °C, while another scenario with N/G of 10.8% records 63.45 °C. Considering the correlation between geobody geometry, connectivity, and thermal transmissivity, comparing results to historical temperature records from a geothermal doublet indicates a well-connected aquifer with a mean geobody length > 1200 m.

## Introduction

After thirty years of geothermal production, we investigate concerns of reservoir connectivity or otherwise in the Dogger aquifer to enhance understanding of thermal exchange between geothermal doublets. Studies by Brigaud et al. (2014), and flowmetry records from 78 wells, identify high permeable peloid-dominated and Ooid-dominated units (lognormal mean of +1000 mD) above the mean permeability of bounding lithofacies (lognormal mean of <200 mD) as controls of thermal movement in the reservoir. The work evaluates the extent and distribution of these high permeable units (i.e. geobodies) and their implications on reservoir connectivity, therefore, fluid migration. The work relies on existing 2D seismic inversions (Allo et al. 2021) to estimate the geometry of geobodies and geothermal simulation for a thirty-year duration in Eclipse and PumaFlow software (Beicip-Franlab).

## Material and methods

Models covering 9 km2, 25 km2, and 100 km2 are extracted from an existing 3D facies model by Thomas et al. (2023) to build petrophysical and aquifer models around the geothermal doublet (i.e. GAL1/GAL2). The model domains are to ascertain the effects of boundary conditions on thermal migration. Through sensitivity testing, the 100 km2 model domain eliminates such boundary controls around the doublet. Inversed 2D seismic lines by Allo et al. (2021) are converted from time to depth using the velocity model tool in Petrel (Schlumberger, 2022). With a porosity and permeability cut-off of (0.15 – 0.2) and (1000 mD – 10000 mD), we use the most-occurring range of geobody lengths for multiple reservoir models for the geothermal simulation, like the theoretical studies by Sauty et al. (1982). Water at 49 °C is injected and produced at 4200 sm3/day from an initial reservoir condition of 75 °C and 170 bars. The fluid model (PVT), rock physics model, and rock thermal conductivity parameters are from the BRGM report by Gringarten & Sauty (1977).
## Results

The results are in five parts (1) Lithofacies geometry effect on thermal transport. Scenarios where geobody length (GL) is < 800 m realizes the commencing 0.5 °C drop within two years of production, which is about half the average duration of the same event in scenarios where GL > 1200 m. Geobody length > 1200 m produced high connectivity and transmissivity paths leading to an energy production rate of ~ 9.72 x 108 KJ/d compared to GL < 800 m scenarios with ~ 8.70 x 108 KJ/d (2) Constant porosity and permeability of 0.17 and 5000 mD in Ooid-dominated and Peloid-dominated units, with no flow in bounding facies. Initial 0.5 °C drop after production averages between 3 to 5 years with a production temperature of 59 °C to 65 °C at the end of the simulation. (3) Constant porosity and permeability of 0.20 and 700 mD in Ooid-dominated and Peloid-dominated units, with average porosity and permeability of 0.1 and 25 mD. Noticeably, there is a rise to 76 °C before the temperature decrease during production. Likewise, the initial 0.5 °C decrease from reservoir condition averages 3 to 4 years with a production temperature of 60 °C to 65 °C after 30 years of simulation. (4) Variable porosity and permeability from upscaled wells. Again, there is an initial temperature rise to 76 °C, attributable to the porosity and permeability of bounding lithofacies. The inceptive 0.5 °C drop after production averages 1 to 2 years, with a temperature of 56 °C to 68 °C after 30 years. (5) Net-to-Gross (NG) effect on thermal transport in the aquifer. The reservoir temperature decreases with decreasing NG under constant water injection and production rate. For example, scenarios with N/G of 44.12% attain an average of 69.78 °C, and N/G of 32.9% realizes 67.5 °C, while N/G of <=11.0 record 63.45 °C after 30 years of geothermal production.

## Conclusion

High permeability zones consisting of ooid-dominated and Peloid-dominated facies and their control on geothermal production are the focus of our assessment of reservoir connectivity or otherwise in the Dogger aquifer. Comparing results to historical temperature records from a doublet supports simulation scenarios with geobody length > 1200 m, therefore, high connectivity in geothermal production from the Limestone reservoir.

<figure><img src="image_3.png" /><figcaption>Figure 1. Isosurface temperature profile for a model with N/G of 32.9% after 30 years of geothermal production.</figcaption></figure>