Evaluating the influence of cyclical versus non-cyclical fluvial sedimentation on geothermal flow
J.M. Doff (TU Delft - Civil Engineering & Geosciences)
H.A. Abels – Mentor (TU Delft - Applied Geology)
Denis V. Voskov – Mentor (TU Delft - Reservoir Engineering)
Joep E.A. Storms – Mentor (TU Delft - Applied Geology)
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Abstract
To produce accurate 3D fluvial architecture models, it is important to understand the influence of internally generated autogenic controls versus externally generated allogenic controls. When studying a fluvial system, autogenic controls such as river meandering and avulsions generally dominate over 103-104 years. Allogenic controls such as climate change and tectonics dominate more at time scales of 105-106 years. A dominant control is astronomical climate change that may produce cyclical fluvial sedimentary successions. The resulting alluvial architecture of autogenic and of allogenic forcing is highly different and it is expected to largely influence geothermal flow through fluvial reservoirs. The aim of this research is to depict the impact of allogenic (cyclical) versus autogenic (non-cyclical) fluvial sedimentation on geothermal flow. Process-imitating and stochastic based modelling software Flumy is used to generate fluvial facies models where either hypothetical cyclical forcing or hypothetical non-cyclical forcing was the dominant force. These models are subsequently tested for geothermal flow using DARTS (Delft Advanced Terra Simulator). We find that the Flumy numerical model can be used to produce hypothetical cyclical and hypothetical non-cyclical alluvial architecture. The main difference between the architectures is the shape of the sand bodies. The cyclical model has overall thicker, laterally wider sand bodies. The non-cyclical model has thinner, less wide, but more often connecting sand bodies. Geothermal flow modelling shows that matching N/G in the cyclical and non-cyclical model in the 20-40 % N/G range gives similar pressures at a constant, fixed water rate. This points to the hypothesis that, at comparable N/G, the well connectivity must also be similar. The non-cyclical model breakthrough times in the 20-30 % N/G range are generally equal or slower, compared to the cyclical model. This gives credence to the hypothesis that the cyclical model has overall equal or slower flow paths between the well connections in the geothermal doublet. The difference in flow path hypothesis can be properly tested through visualizing 3D streamlines and is a recommendation for the future.