How Can Temperature Logs Help Identify Permeable Fractures and Define a Conceptual Model of Fluid Circulation? An Example from Deep Geothermal Wells in the Upper Rhine Graben

Jeanne Vidal,Albert Genter,Carole Glaas,Régis Hehn

doi:10.1155/2019/3978364

Abstract

Identifying fluid circulation in fracture zones (FZs) is a key challenge in the extraction of deep geothermal heat from natural reservoirs in the Upper Rhine Graben. This study focuses on permeable FZs present within the granitic basement penetrated by deep geothermal well GPK-1 at Soultz and GRT-1 and GRT-2 at Rittershoffen (France). The various temperature (T) log datasets acquired from these wells during production and at equilibrium, with the associated flow logs, allow for the unique opportunity to interpret fluid circulation at the borehole scale. All permeable FZs identified by permeability indicators measured during drilling operations and from image logs spatially coincide with positive or negative T anomalies observed in the T logs during production and/or at equilibrium. However, within the FZs, partially open fractures act as narrower paths for circulation at different temperatures. These temperatures can even be estimated with confidence if the associated flow log is available. The polarity of the T anomalies correlates with the state of equilibrium of the well and thus can change over the well history. During production, the temperature of the water inflow through the fractures can be estimated relative to the mixture of water circulating below the fractures. At thermal equilibrium, the water temperature is estimated with respect to the temperature of the surrounding rock formation. Because temperature fluxes and geothermal fluids are intimately linked, T logs are a useful, reliable, and very sensitive tool to localize the inflow of geothermal water through FZs.

Highlights

In the Upper Rhine Graben (URG), the underground temperature distribution is spatially heterogeneous, and a series of local anomalies with temperatures above 140°C at a 2 km depth are observed at the regional scale [1,2,3,4]
Four permeable fracture zones (FZs) are identified in GPK-1, and they are all associated with negative thermal anomalies at thermal equilibrium (Table 1 and Figure 2)
A network of permeable FZs was identified in well GPK-1 at Soultz and wells GRT-1 and GRT-2 at Rittershoffen based on the reinterpretation of T logs obtained during production and at thermal equilibrium; these T logs were correlated with permeability indicators measured during drilling operations, acoustic image logs, and flow logs obtained during hydraulic tests

Summary

Introduction

In the Upper Rhine Graben (URG), the underground temperature distribution is spatially heterogeneous, and a series of local anomalies with temperatures above 140°C at a 2 km depth are observed at the regional scale [1,2,3,4]. These temperature anomalies are interpreted as convective cells circulating along vertical local faults (Figure 1) [2, 5, 6]. The top of the granitic basement, which is highly fractured and affected by hydrothermal alteration, presents a low geothermal gradient associated with the vertical flow of brine and is locally affected by temperature anomalies interpreted as the thermal signatures of the FZs (Figures 2–4) [14,15,16]

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Discussion

Conclusion