Seismic Velocity/Temperature Correlations and a Possible New Geothermometer: Insights from Exploration of a High-Temperature Geothermal System on Montserrat, West Indies

Abstract

Abstract:In 2013, two production wells were drilled into a geothermal reservoir on Montserrat, W.I. (West Indies) Drilling results confirmed the main features of a previously developed conceptual model. The results confirm that below ~220 °C there is a negative correlation between reservoir temperature and seismic velocity anomaly. However, above ~220 °C there is a positive correlation. We hypothesise that anomalous variations in seismic velocity within the reservoir are controlled to first order by the hydrothermal mineral assemblage. This study suggests a new geophysical thermometer which can be used to estimate temperatures in three dimensions with unprecedented resolution and to indicate the subsurface fluid pathways which are the target of geothermal exploitation.

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“…The Montserrat samples are from core recovered from depths of 1480 m from the MON-1 well, one of two exploratory geothermal holes drilled into the western flank of Soufrière Hills volcano [95,96]. MON-1 well was drilled through a 530 m thick sequence of andesite lava flows and breccias related to the young Soufrière Hills volcanic complex overlying a sequence of sandstones, mudstones and clays (530–1210 m depth).…”

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“…Furthermore, as the database of seismic events (natural and induced) in and around the reservoir grows, the identification of important geological structures and features (such as the zone of brittle/plastic transition) becomes clearer [30,43]. Indeed, as shown by Ryan and Shalev [31], correlations between seismic velocity and in situ temperatures could even make remote sensing of reservoir temperatures feasible. Thus an overall geothermal reservoir model comprises an assemblage of mutually consistent geological [43], geophysical [30,31] and hydrogeological models [43] should be developed in parallel, and iteratively updated to achieve harmonious coupling between them.…”

“…Indeed, as shown by Ryan and Shalev [31], correlations between seismic velocity and in situ temperatures could even make remote sensing of reservoir temperatures feasible. Thus an overall geothermal reservoir model comprises an assemblage of mutually consistent geological [43], geophysical [30,31] and hydrogeological models [43] should be developed in parallel, and iteratively updated to achieve harmonious coupling between them. The overall aim is a robust and constantly-evolving conceptual model of each geothermal reservoir, which is carefully adjusted to ensure that as much consistency as possible is achieved between concepts and available data.…”

“…Within the zone of hydrothermal circulation, seismic processes may provide valuable insights into reservoir functioning. For instance, variations in mineral assemblages correlated with hydrothermal alteration are such that there is a negative correlation between reservoir temperature and seismic velocity anomalies at temperatures less than ~220 °C, whereas at higher temperatures the correlation is positive [31]. Hence interpretation of natural seismic data may provide direct estimates of reservoir temperatures, in addition to its more orthodox applications in delineating spatial boundaries and internal structures in reservoirs [31].…”

“…Increasingly, it is combinations of tools with differing physical basis that is proving most useful in achieving less equivocal signatures of reservoir geometries and hydrogeothermal properties [23,30,31]. Hence advances in joint inversion of different types of geophysical data must remain a property in geothermal research.…”