Session 5B: Geophysics

The formation of clay cap and its expected temperature are closely related to the geological setting of each geothermal field. In Sumatra Island, the overall geological setting is highly influenced by the interplay between a field's distance to the Great Sumatran Fault Zone and its pre-existing geological conditions. This interplay shapes both geological structures and hydrothermal fluid flow patterns, complicating the correlation of the base of clay cap with temperature.

To characterize the base of clay cap temperature in Sumatra Island, a comprehensive assessment was conducted of magnetotelluric (MT) data and borehole data such as the methylene blue (MeB) measurement of smectite clay in cuttings and temperature logs from five explored and developed geothermal fields of Pertamina Geothermal Energy (PGE). First, the consistency of the 3D resistivity model from MT surveys was evaluated by comparing it with the 1D model. Subsequently, the 1D model was refined using MeB data from wells to identify the base of the clay cap. We then examined the correlation between the base of the clay cap and temperature by analyzing nearby wells selected close to MT station locations.

The findings reveal a strong correlation between the base of the clay cap and the associated temperature. The details of this correlation vary depending on the geological setting of the geothermal system and especially the geothermal system evolution. This characterization offers a preliminary method for estimating the temperature of geothermal systems known or assumed to exist below the base of clay cap in Sumatra and, potentially, globally, tailored to specific geological settings and geothermal system evolution.

Elevated temperatures in volcanic geothermal systems result from high heat flow caused by shallow intrusion of magma. Magmatic intrusion acts as the major heat source of the system and drives hydrothermal convection. Understanding the nature and location of magmatic heat sources is therefore relevant for characterizing the geothermal system. Magnetotelluric studies around the world have revealed numerous occurrences of deep electrical conductors below volcanic geothermal prospects. Interpretations of such deep conductors often invoke the presence of magmatic melt and magma derived fluids thereby arguing in favor of interpreting deep conductors as related to the magmatic heat source. However, the architecture of magmatic systems is rather complex and strongly depends on the degree of solidification, residual melt, and volatile content. Electrical resistivity alone does not provide conclusive evidence about the nature of deep conductors. We describe a selection of scenarios and their impact on geothermal reservoir formation: Under favorable conditions magmatic mush reservoirs in the upper crust can remain stable over long geological timescales thereby providing a constant heat supply for a geothermal system. Undesirable situations are encountered when H2S from degassing magma combines with water to form sulfuric acid brine, a fluid with exceptional high electrical conductivities. Such acidic hydrothermal plumes can form above largely cooled intrusions and make geothermal drilling unattractive. In cases where volcanic eruption happened through sedimentary overburden, deep conductors may also represent buried fluid-bearing sediments, hence being unrelated to magmatic systems and potential heat sources. Therefore, conclusive interpretations of deep conductors require a comprehensive geoscientific approach. First, a detailed understanding of the geological setting, eruptive history and magma evolution is required and, second, integration of various types of geophysical observables is key to understand the internal structure and fundamental petrophysical properties of deep electrical conductors under volcanic geothermal prospects. Here we present a surface to depth hypothetical model of a volcanic high-temperature geothermal system and a classification or grouping for deep conductors based on the geological setting, volcanic history and experience from well data. As representative examples we show seven case studies from geothermal prospects in East Africa and Indonesia where deep conductors have been encountered in 3D models.

Geothermal energy development in Montserrat, West Indies, can help to improve energy security and reduce the island’s heavy reliance on fossil fuels. A more detailed understanding of the spatial variation in physical properties across the lithologies which host the reservoir can greatly increase the ability to harness this renewable resource. Previous work was largely based on macro-scale characterization of the geothermal reservoir using geophysical techniques including seismic tomography. To enhance our understanding of the large-scale geophysical models we measured P- and S-wave velocities on three cores retrieved from the Montserrat’s third geothermal well using a servo-controlled triaxial apparatus. Our results showed that P-wave velocity decreased by 9 % when temperature was increased to 150 Degrees Celsius. Our data also assessed the relationship between seismic properties and parameters such as porosity, lithology and alteration. These important relationships improve our ability to interpret geophysical data and target productive geothermal wells.

This review paper focuses on applying the Traffic Light System (TLS) in managing induced seismicity, particularly in geothermal energy development. Induced seismicity, a consequence of human activities such as geothermal energy production, poses significant challenges to the environment and surrounding communities. Inspired by its conventional use in road traffic management, the TLS offers a promising approach to mitigate these risks by categorizing seismic events based on their magnitudes and dictating operational responses.

The paper presents a comprehensive workflow for implementing the TLS in geothermal projects, starting from the initial seismic monitoring to the application of operational controls based on predefined seismic thresholds. This workflow emphasizes the importance of continuous monitoring, real-time data analysis, and the adaptive management of geothermal operations to minimize the risk of significant seismic events.

Further, the paper reviews several case studies from around the world, showcasing the effectiveness of the TLS in diverse geological settings. These case studies highlight the system's flexibility and its potential to be tailored to specific project needs and seismic risk profiles.

The review argues that the TLS not only enhances the safety and sustainability of geothermal energy production but also serves as a crucial tool for gaining public trust and regulatory approval. By systematically managing the risk of induced seismicity, the TLS contributes to the responsible development of geothermal resources, aligning with global efforts to transition to renewable energy sources.

The Cape Modern project, a next generation Enhanced Geothermal System (EGS) field, is being developed in Southwest Utah. The initial phase of the project involved the stimulation of three horizontal wells using a plug-and-perf hydraulic stimulation technique. To ensure comprehensive monitoring, an extensive microseismic network was deployed, comprising shallow borehole sensors, a surface nodal array, deep borehole fiber optic sensors, and 3-component (3C) passive sensors. A key observation from this setup was that the high-precision double-difference-based locations derived from the sparse regional shallow borehole array provided spatial resolution comparable to that of the deep borehole sensors, achieving accuracy down to a few hundred feet in real-time during injection operations. The high-resolution microseismic data from the deep borehole fiber optic-based Distributed Acoustic Sensing (DAS) revealed extensive linear features reactivated during the stimulation process. Interestingly, the orientation of these linear features was misaligned with the maximum horizontal stress (SHmax) orientation, suggesting the reactivation of preexisting natural fractures, faults, or lithological boundaries within the host rock and intrusive bodies. As part of the induced seismicity mitigation plan, a Traffic Light System (TLS) was implemented, relying on real-time monitoring from the shallow borehole array. Throughout the operations, five amber-level alerts were triggered, necessitating temporary pauses in stimulation. Following each alert, seismicity levels returned to normal, indicating the effectiveness of the TLS in managing induced seismicity.