David Dempsey

University of Canterbury | UC · Department of Civil and Natural Resources Engineering

Groundwater is present at shallow depth under many coastal low-lying cities. Despite the importance of protecting coastal urbanised areas from flooding and climate-change-induced sea-level rise, the effects of shallow groundwater fluctuations are rarely investigated. The aim of this study was to determine characteristics of shallow groundwater, inc...

We analysed seismic data from Ruapehu volcano (New Zealand) to improve volcanic event catalogues by locating signatures of rapid hydrothermal seal formation and failure. Events are detected temporally within the seismic record using an empirical model ofsealing/pressurization/eruption proposed for Whakaari volcano in New Zealand. The approach allow...

Limiting global temperature rise to between 1.5 and 2°C will likely require widespread deployment of carbon dioxide removal (CDR) technologies for sectors with hard-to-abate emissions. As financial resources for decarbonization are finite, strategic deployment of CDR technologies is essential for maximizing atmospheric CO2 reductions. Carbon captur...

Geothermal electricity generation has low carbon emissions compared to hydrocarbon alternatives. Nevertheless, recent attention on emissions of magmatic CO2 and other non-condensable gases (NCG) has prompted interest in their capture and reinjection. The geothermal industry is uniquely placed to effect CO2 sequestration due to existing reinjection...

Plain Language Summary To better understand the small earthquakes triggered in geothermal fields, we used two kinds of data‐driven models: time series feature engineering (a machine learning method) and the non‐negative linear regression method. We applied these techniques to microearthquakes recorded in the Rotokawa geothermal field, New Zealand,...

We present an algorithm based on Self‐Organizing Maps (SOM) and k‐means clustering to recognize patterns in a continuous 12.5‐year tremor time series recorded at Whakaari/White Island volcano, New Zealand (hereafter referred to as Whakaari). The approach is extendable to a variety of volcanic settings through systematic tuning of the classifier. Hy...

Induced seismicity due to natural gas extraction from the Groningen reservoir, The Netherlands, has been occurring since the early 1990s. Prospective forecasts of the possible maximum earthquake magnitude for different gas extraction scenarios have previously been published. However, their accuracy and continued relevance are unknown. Here, we eval...

Braided river systems are continuously changing environments, comprising heterogeneous gravel-forming meandering channels and bars, and dynamic surface water–groundwater (SW–GW) interactions. In alluvial aquifers associated with braided rivers, river leakage is volumetrically the most important source of groundwater recharge, yet neither the leakag...

Phreatic explosions at volcanoes are difficult to forecast but can be locally devastating, as illustrated by the deadly 2019 Whakaari (New Zealand) eruption. Quantifying eruption likelihood is essential for risk calculations that underpin volcano access decisions and disaster response. But estimating eruption probabilities is notoriously difficult...

Volcanic eruptions that occur without warning can be deadly in touristic and populated areas. Even with real-time geophysical monitoring, forecasting sudden eruptions is difficult, because their precursors are hard to recognize and can vary between volcanoes. Here, we describe a general seismic precursor signal for gas-driven eruptions, identified...

Here, we investigate the techno-economic feasibility of an enhanced geothermal system (EGS) for direct heating, using a new horizontal-well with partially-bridging fractures design. To do this, we developed numerical models to evaluate reservoir dynamics and output, economic performance and environmental benefits. This model was used to determine o...

Volcanic eruptions that occur without warning can be deadly in touristic and populated areas. Even with real-time geophysical monitoring, forecasting sudden eruptions is difficult because their precursors are hard to recognize and can vary between volcanoes. Here, we describe a general seismic precursor signal for gas-driven eruptions, identified t...

Plain Language Summary Previous research has shown that eruptible magma bodies in the Taupō Volcanic Zone (TVZ), New Zealand may only have been stored in the crust for less than 100 years. These magma bodies have contributed to highly explosive, caldera‐forming eruptions, so understanding the timescales over which heat is lost from these magma bodi...

Enhanced Geothermal Systems (EGS), engineered deep rock heat exchangers, are often touted for their massive untapped renewable energy potential. However, numerous technical and financial challenges must be overcome before the technology is widely deployed. To be viable, EGS must engineer a heat sweep by circulating fluid through a large volume of r...

To obtain the fullest picture of geothermal systems, it is necessary to integrate different types of data, for example, surface electromagnetic surveys, lithology, geochemistry, and temperature logs. Here, by joint modeling a multichannel data set we quantify the spatial distribution of heat transfer through the hydrothermally altered, impermeable...

Novel technologies to store hydrogen in geological formations can substantially enhance New Zealand’s renewable energy market and help mitigate climate change impacts. New Zealand already supplies about 80% of its electricity demands from renewable sources, mostly geothermal, hydro and wind power. However, over 60% of the country’s net energy consu...

In geothermal exploration, magnetotelluric (MT) data and inversion models are commonly used to image shallow conductors typically associated with the presence of an electrically conductive clay cap that overlies the main reservoir. However, these inversion models suffer from non-uniqueness and uncertainty, and the inclusion of useful geological inf...

There is a fast-growing inventory of studies on borehole image logs acquired in geothermal reservoirs as more operators elect to deploy this technology. Our contribution to this inventory is to illustrate how judicious use of these data may reveal the geologic controls on permeability. We also provide an open source Python library that enables othe...

The upper North Island of New Zealand has large concentrations of population and infrastructure that make it vulnerable to earthquakes on the Kerepehi and Wairoa North faults. Using a physics-based simulator, we modelled 0–50 Hz ground motions for Mw 7.3 and Mw 6.6 characteristic earthquakes on these structures. We considered the effects of low-vel...

Thermal drawdown induced flow channeling in fractured geothermal reservoirs has been found to be inevitable in previous work. This hypothesis is questioned and found not to be true for all scenarios. We investigate flow in a simple two-fracture network to investigate the basic behavior of thermal drawdown induced flow pattern changes, using a Therm...

Sudden steam-driven eruptions strike without warning and are a leading cause of fatalities at touristic volcanoes. Recent deaths following the 2019 Whakaari eruption in New Zealand expose a need for accurate, short-term forecasting. However, current volcano alert systems are heuristic and too slowly updated with human input. Here, we show that a st...

When injection-induced seismicity poses a risk to communities, it is common to reduce the injection rate or halt operations. This applies both to individual wells and well clusters, such as those within the Area of Interest for Triggered Seismicity in Western Oklahoma, where in 2016 a 40% volume reduction mandate was imposed by the state regulator....

The goal of hydraulic stimulation is to increase formation permeability in the near vicinity of a well. However, there remain technical challenges around measuring the outcome of this operation. During two enhanced geothermal system stimulations in South Australia, Paralana in 2011 and Habanero in 2003, extensive catalogs of microseismicity were re...

Geothermal fluid flow is related to geologic context from the global or district scale down to the reservoir scale. We present a discussion of that relationship that is based on a review of high-temperature geothermal reservoirs worldwide. Initially we focus on large-scale geological controls on productive geothermal reservoirs, such as the role cr...

Earthquakes induced by natural gas extraction from the Groningen reservoir, the Netherlands, put local communities at risk. Responsible operation of a reservoir whose gas reserves are of strategic importance to the country requires understanding of the link between extraction and earthquakes. We synthesize observations and a model for Groningen sei...

The capacity for fault systems to be reactivated by fluid injection is well-known. In the context of CO2 sequestration, however, the consequence of reactivated faults with respect to leakage and monitoring is poorly understood. Using multi-phase fluid flow simulations, this study addresses key questions concerning the likelihood of ruptures, the ti...

The Area of Review is one of the most important aspects of a geologic CO2 storage site permit application and regulatory requirement. The US Environmental Protection Agency (EPA) has implemented an Area of Review (AoR) requirement for Class VI CO2 injection wells for geologic sequestration. Reservoir permeability is one of the most important parame...

The geometry of a geothermal reservoir constrains the exploitable resource volume and the sustainable power capacity. That geometry is typically illustrated by a conceptual model and is primarily defined using a pattern of natural state isotherms. Prior to drilling, surface exploration methods set out to constrain the reservoir geometry even though...

Induced seismicity is of increasing concern for oil and gas, geothermal, and carbon sequestration operations, with several M > 5 events triggered in recent years. Modeling plays an important role in understanding the causes of this seismicity and in constraining seismic hazard. Here we study the collective properties of induced earthquake sequences...

Probabilistic seismic hazard assessment for induced seismicity depends on reliable estimates of the locations, rate, and magnitude frequency properties of earthquake sequences. The purpose of this paper is to investigate how variations in these properties emerge from interactions between an evolving fluid pressure distribution and the mechanics of...

Induced seismicity as a result of the fluid injection to enhance formation is a recognized safety risk in geothermal projects. Development of new models to better understand the underlying triggering mechanics and co-evolution of seismicity with subsurface pressure, stress and permeability is essential both for field management and hazard assessmen...

Creation of an Enhanced Geothermal System relies on stimulation of fracture permeability through self-propping shear failure that creates a complex fracture network with high surface area for efficient heat transfer. In 2010, shear stimulation was carried out in well 27-15 at Desert Peak geothermal field, Nevada, by injecting cold water at pressure...

Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Topographically driven flow, overpressured shale reservoirs, permeable pathways such as faults or leaky wellbores, the increased formation...

Spatial heterogeneity and variability across many orders of magnitude are two properties of formation permeability that are challenging to effectively capture in subsurface flow models. With regard to geological CO2 storage, heterogeneity affects storage capacity, plume migration, pressure build-up around injection wells and, if utilized, the possi...

Antarctic coastal sea ice often grows in water that has been supercooled by interaction with an ice shelf. In these situations, ice crystals can form at depth, rise and deposit under the sea-ice cover to form a porous layer that eventually consolidates near the base of the existing sea ice. The least consolidated portion is called the sub-ice plate...

Shallow aquifer monitoring is likely to be a required aspect to any geologic CO2 sequestration operation. Collecting groundwater samples and analyzing for geochemical parameters such as pH, alkalinity, total dissolved carbon, and trace metals has been suggested by a number of authors as a possible strategy to detect CO2 leakage. The effectiveness o...

Assessment of potential CO2 and brine leakage from wellbores is central to any consideration of the viability of geological CO2 sequestration. Depleted oil and gas reservoirs are some of the potential candidates for consideration as sequestration sites. The sequestration sites are expected to cover laterally extensive areas to be of practical inter...

Many technical, regulatory and public perception challenges remain to be addressed before large-scale deployment of CO2 geologic storage becomes a reality. Two major risks associated with injection of CO2 into the subsurface are the possibility of induced earthquakes compromising long-term seal integrity, and the displacement of saline brines resul...

If carbon dioxide (CO2) sequestration into deep geologic reservoirs is to be accepted by the public and environmental regulators, the possibility of upward leakage into shallow groundwater should be acknowledged and those processes well-understood. Studies of natural CO2 reservoirs and their connection (or lack thereof) with the shallow subsurface...

Recent experiences with large-scale injection of fluids into geological formations within the Oil & Gas, Geothermal and Waste Disposal industries have demonstrated a risk of induced seismicity. In the case of geological sequestration of CO2, reactivation of faults may result in leakage pathways for the buoyant plume and thus compromise the integrit...

A numerical model has been developed to describe injectivity improvements at well 27-15 in the Desert Peak geothermal field, Nevada, under shear stimulation conditions. A Mohr-Coulomb failure criterion, coupled with an empirical fracture permeability law based on laboratory experiments, describes permeability evolution of a fractured rock volume un...

[1] Earthquakes cause a variety of hydrological effects, including changes in well levels, streamflow, hot-spring temperatures, and geyser periodicity. These may be produced by changes in pore-fluid pressure or by changes in permeability. We investigate near-field effects of normal earthquakes on fault-controlled hydrothermal plumes, e.g., at the T...

Hydrothermal convection in the Taupo Volcanic Zone (TVZ), New Zealand, is driven by heat extracted at the brittle-ductile transition, which is in turn supplied by magmatic intrusion of the lower-crust. We present a numerical model that approximates this circulation in a statistical sense, being constrained by TVZ dimensions and mean thermal propert...

Sea ice that forms in McMurdo Sound, Antarctica, which is adjacent to the McMurdo and Ross Ice Shelves, exhibits the uncommon ice fabric, incorporated platelet ice. This ice fabric is the subject of a high resolution thin sectioning study, consisting of ten closely spaced horizontal sections bounded above and below by vertical sections. From these...

Tectonic extension applied to the upper crust commonly results in the development of sub-parallel arrays of faults and fault belts. A computational model of an extending elastic-viscous-plastic crust is presented, which describes the evolution of stress, strain and displacement in the vicinity of an active normal fault. Within the upper crust discr...

Natural geothermal convection abounds within the Taupo Volcanic Zone (TVZ) of New Zealand's Central North Island. In many locations the highly porous eruptive products that blanket the landscape have been altered by the throughput of hydrothermal fluids and the consequent deposition of silica. We detail a numerical model that considers the evolutio...

That earthquakes release vast quantities of energy is widely accepted; however, the most commonly experienced component, radiated seismic energy, is a minor contribution to the total energy budget. The elastic rebound model for earthquakes recognizes that elastic strain energy does work displacing, deforming, and accelerating the crust, as well as...

The geothermal modelling group in Engineering Science (University of Auckland) is involved with several geothermal R&D projects. On the development side we are running models of Ohaaki, Wairakei, Ngawha, Reporoa, Wayang Windu and Lihir. Our experiences in these projects have led on to several parallel research projects, which help to overcome the...

In the Taupo Volcanic Zone (TVZ) of New Zealand geothermal circulation is facilitated high porosity quaternary volcanic deposits, and by fracture permeability associated with widespread normal faulting. High temperature fluids (>250°C) exit the crust at more than 20 locations, many of which are associated with outcropping faults or fault tips, e.g....

A computational model for an elastic-plastic-viscous faulted crust under tectonic extension is used to investigate energy flows around faults over several seismic cycles. The static effects of an earthquake are modeled by considering regular episodes of dip-slip between two surfaces in frictional contact, approximating a fault. An energy budget is...

Here we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m dee...

The Taupo Volcanic Zone of New Zealand's Central North Island is characterized by widespread normal faulting that accommodates a high proportion of the region's 7-15 mm yr-1 tectonic extension. Abundant volcanism and geothermal activity are accompanied by seismicity to depths of ~6-8 km. This suggests a shallow brittle-ductile transition (BDT) cons...

During the annual growth of landfast ice in McMurdo Sound, Antarctica, an episodic flux of platelet ice crystals from the ocean contributes to the build up of a porous subice platelet layer, which is steadily incorporated into the sea ice cover as it thickens over winter. In November 2007, we examined the spatial variability of these processes by c...