Kanton Genf

Applying High-Resolution 3D Modelling to Unlock Geothermal Resources in Geneva

30.01.2019

In the global search for renewable or low-emissions energies, emphasis is also given to exploration of geothermal potential as well as best possible characterization of known resources. High-resolution numerical model proves to be a key toolto successfully and robustly resolve sub-surface structure, fractures and lateral lithofacies variation controlling subsurface fluid circulation.

G. FABRE, Sr. Geomodelling Expert, Geneva Geo Energy S.A. (GGE)

As integral part of SIG’s strategic GEothermie 2020 work programme, GGE leads a pilot study on high-resolution geological modelling of the prolific Mesozoic interval of the Geneva Basin. It shows that digital 3D subsurface modelling constitutes an effective and efficient approach to quantitively describe and assess subsurface resource potential. It integrates large scale basin architecture with key geological features and hence, greatly improves understanding of subsurface water circulation systems. As such, high-resolution 3D geomodelling proves to be a valuable project management tool, too.  It delivers best possible valuation of the geothermal target resource and allows to decide with a high degree of confidence on drilling locations, target interval description and well depth definitions. Equally, it allows for best possible well engineering and operations planning including cost contributing quantified input to the planning and development of future cantonal heat networks.

Systematic Data Management and Data Integration
Comprehensive gathering, quality reviewing, eventual reprocessing and reliable data interpretation are at the very core and essential for accurate geological model building at highest possible confidence levels. Equally important are accurate identification and management of uncertainties inherently embedded in the data including adequate consideration of aliasing and boundary effects.

For the objective Cretaceous interval all available 2D seismic was carefully reviewed and so called “confidence maps” were established for all main seismic markers. This allows to identify areas where the data quality might impact on the robustness of the geo-cellular model. A similar review and uncertainty management approach has been conducted for all relevant well data including also a systematic review of corresponding petrophysical well log interpretations, core measurements and mechanical rock properties.

The integration and cross-validation of well and seismic information merged with the regional geological evolution of the Geneva Basin, provided a solid basis for elaborating a viable structural and sedimentological conceptual model capturing all critical elements characterizing the deposition, deformation and preservation of the objective interval.

This robust but analogue conceptual model in turn, forms the base for building a reliable digital geological model in 3D, the tool to deliver best possible lateral and vertical prediction of reservoir geometry, quality and continuity. The translation of the analogue conceptual model into a digital model is achieved by applying geostatistical techniques and mathematical tools which are very sensitive to input data quality, sampling or repartition. These methods, with their own strengths and weaknesses, are developed and applied to represent specific geological rules. Deep familiarity and competence with the process is essential, as the choice of an inadequate method influences the quality of and the confidence in the resulting model. The integrity of both, the conceptual model elaboration and the parameterization of the digital model, respectively, constitute the key success elements to achieve an adequate representation and understanding of the objective reservoir and consequently, the circulation mechanisms of subsurface water flow.

Well to Basin Scale Extension
In the specific case of Geneva Cretaceous pilot, the task was to provide a high-resolution model (25m of lateral resolution; 1m of vertical resolution) in the vicinity of the recently drilled (2018) and successful GEo-01 geothermal exploration well. The objective included capturing and integrating large-scale geological and structural features (5 to 20 km lateral resolution).

To bridge the well to basin scale gap, a hybrid tailor-made workflow has been developed in three steps:

• Construction of a structural framework for the whole Mesozoic interval which adequately reflects the structural architecture of the Geneva Basin.
• Extraction from the basin scale structural framework a low-resolution regional model, where relevant geological, petrophysical and mechanical properties are represented and aligned with well observations, regional trends and main geological concepts.
• Production of a high-resolution model in the area around a well location merging regional trends from the low-resolution grid with local well scale properties controlling well productivity. This model is strengthened by an uncertainty analysis able to predict the stability and robustness of all parameters previously simulated.

The resulting high-resolution 3D model is an essential decision tool for geothermal exploration and development operators and their associated project management teams. It delivers through a hybrid workflow approach and is coupled with an uncertainty analysis and management process, that exploits the strength of combining both, regional and local scales, respectively.

This pilot study provides the base for a more comprehensive regional and systematic review of the geothermal potential and resources present in the Geneva’s Basin.

Geneva Geo Energy ist Sponsor von Geothermie-Schweiz.

Newsübersicht