Borehole Temperature Research Articles

ESR-thermochronometry of the MIZ1 borehole, Tono, Japan

Previous studies have shown that the low-relief Tono region (Japan) underwent Quaternary exhumation at rates of <1 mm yr-1. Here we explore whether electron spin resonance (ESR) signals of quartz from six samples from the MIZ1 borehole can resolve such low rates of exhumation, or whether the samples are in thermal equilibrium with ambient borehole temperature. ESR thermochronometry requires that both sample-specific signal saturation and thermal decay are constrained in the laboratory, which makes measurements highly time-consuming. To overcome this, the development of a standardised growth curve (SGC) was explored, which allowed more rapid constraint of the trapped-charge concentrations of each of the samples. Thermal kinetic parameters were determined using an isothermal decay experiment for each individual sample and except for sample MIZ1-01, it was possible to fit all the isothermal decay data together to yield a single set of kinetic parameters that successfully described the dataset. Using a single set of kinetic parameters also allows faster measurement. The ESR thermochronometry results show that the MIZ1 samples are not in thermal equilibrium, but rather reflect ongoing exhumation in this region. Exhumation rates determined from the Al-centre are consistent with existing thermochronometric data and indicate total exhumation of 385 ± 220 m over the past 1 Myr. In contrast the different Ti-centre options (A, B, D) yield a total exhumation of ∼1 km over the same period. The cause of this discrepancy is unclear but may relate to sub-linearity of Ti-centre dose response, that lead to underestimation of the trapped-charge concentration and hence an overestimation of exhumation rates. Our results indicate that ESR-thermochronometry can be successfully applied in low-relief zones undergoing exhumation at rates <1 mm yr-1, and that an SGC and common thermal kinetic parameters may be used to expediate sample measurements.

An updated terrestrial heat flow data set for the Junggar basin, northwest China: implications for geothermal resources

SUMMARY Terrestrial heat flow plays a vital role in determining the present thermal regimes of sedimentary basins, offering a robust foundation for understanding hydrocarbon maturation processes and the geothermal resource potential. The Junggar basin is one of the largest and most petroliferous superimposed petroleum basins in China. However, research on heat flow is scarce. In this study, 94 new high-quality heat flow values are derived from through borehole temperature analysis and thermal conductivity measurements of rocks. The results indicate that (1) the geothermal gradient in the basin varies from 11.4 to 28.3 °C km−1, with a mean value of 20.9 ± 3.4 °C km−1, and the heat flow varies from 23.4 to 64.5 mW m−2, with a mean value of 45.1 ± 8.4 mW m−2. The overall low geothermal gradient and heat flow are attributed to the continuous cooling during the Meso-Cenozoic. (2) At basin scale, the high heat flow values are primarily concentrated in areas characterized by basement uplift, whereas the low heat flow values are mainly located in the depressions. This suggests that thermal refraction is the primary factor influencing the heat flow variations. (3) Although large-scale development and utilization of geothermal resources face challenges, certain local areas in the basin show promise for geothermal resource utilization.

20-year permafrost evolution documented through petrophysical joint inversion, thermal and soil moisture data

This study investigates the ground characteristics of the high altitude (3410 m a.s.l.) permafrost site Stockhorn in the Swiss Alps using a combination of surface and subsurface temperature, soil moisture, electrical resistivity and P-wave velocity time series data including a novel approach to explicitly quantify changes in ground ice content. This study was motivated by the clear signal of permafrost degradation visible in the full dataset at this long-term monitoring site within the PERMOS (Permafrost Monitoring Switzerland) network. Firstly, we assess the spatio-temporal evolution of the ground ice and water content by a combined analysis of all available in situ thermal (borehole and ground surface temperature), hydrological (soil moisture) and geophysical (geoelectric and seismic refraction) data over two decades (2002–2022) regarding the driving factors for the spatially different warming. Secondly, we explicitly quantify the volumetric water and ice content and their changes in the subsurface from 2015 to 2022 using a time-consistent petrophysical joint inversion scheme within the open-source library pyGIMLi. The petrophysical joint inversion scheme has been improved by constraining the rock content to be constant in time for six subsequent inversions to obtain consistent changes in ice and water content over the monitoring period based on jointly inverted resistivity and traveltime data. All different data show a warming trend of the permafrost. The ice content modeled from the petrophysical joint inversion has decreased by about 15 vol.% between 2015 and 2022. Changes in ice content are first observed in the lower, south-facing part of the profile. As a result, resistivity and P-wave velocity have been decreasing significantly. Permafrost temperatures measured in the boreholes have increased between 0.5 °C and 1 °C in 20 years. Our study shows the high value of joint and quantitative analysis of datasets comprising complementary subsurface variables for long-term permafrost monitoring.

Robust Reconstruction of Historical Climate Change From Permafrost Boreholes

AbstractReconstructing historical climate change from deep ground temperature measurements in cold regions is often complicated by the presence of permafrost. Existing methods are typically unable to account for latent heat effects due to the freezing and thawing of the active layer. In this work, we propose a novel method for reconstructing historical ground surface temperature (GST) from borehole temperature measurements that accounts for seasonal thawing and refreezing of the active layer. Our method couples a recently developed fast numerical modeling scheme for two‐phase heat transport in permafrost soils with an ensemble‐based method for approximate Bayesian inference. We evaluate our method on two synthetic test cases covering both cold and warm permafrost conditions as well as using real data from a 100 m deep borehole on Sardakh Island in northeastern Siberia. Our analysis of the Sardakh Island borehole data confirms previous findings that GST in the region have likely risen by 5–9°C between the pre‐industrial period of 1750–1855 and 2012. We also show that latent heat effects due to seasonal freeze‐thaw have a substantial impact on the resulting reconstructed surface temperatures. We find that neglecting the thermal dynamics of the active layer can result in biases of roughly −1°C in cold conditions (i.e., mean annual ground temperature below −5°C) and as much as −2.6°C in warmer conditions where substantial active layer thickening (&gt;200 cm) has occurred. Our results highlight the importance of considering seasonal freeze‐thaw in GST reconstructions from permafrost boreholes.

Utilization of abandoned oil well logs and seismic data for modeling and assessing deep geothermal energy resources: A case study

Hammam Faraun (HF) geothermal site in Egypt shows potential for addressing energy demand and fossil fuel shortages. This study utilizes abandoned oil well logs, seismic data, and surface geology to assess HF geothermal energy resources.Seismic interpretation identified a significant clysmic fault parallel to Hammam Faraun fault (HFF), named CLB fault. The two faults together create a renewable geothermal cycle through circulation of mixed formation-sea waters.Petrophysics revealed two main geothermal reservoirs: the Nubian sandstone reservoir and the Eocene Thebes carbonate reservoir with water saturation values approaching 100 %. Corrected borehole temperatures indicated reservoir temperatures around 120 °C and 140 °C for the Thebes and Nubian reservoirs, respectively.Fracture analysis and stress state provided insights into subsurface fractures. A geomechanical model demonstrated the impact of different stresses and pore pressure on geothermal fluid flow. NE-SW oriented fractures showed a higher dilation tendency due to aquathermal expansion.The integrated conceptual geothermal model suggested a magma chamber beneath HF as the heat source, related to Oligo-Miocene volcanic activity. The breached relay ramp and fault-related open fracture system serve as pathways for geothermal fluids.Evaluation of the geothermal potential utilized volumetric calculations and Monte Carlo simulation. The estimated hot water volumes were 1.72 km3, 4.242 km3, and 5.332 km3 for the Nubian reservoir in the onshore part, Thebes reservoir in the offshore part, and Nubian reservoir in the offshore part, respectively.The results indicate a medium enthalpy resource suitable for electricity generation using a Kalina geothermal power plant. The predicted geothermal power output is promising, with an average power output of 9.64 MWe, 21.38 MWe, and 43.76 MWe for the Nubian reservoir in the onshore part, Thebes reservoir in the offshore part, and Nubian reservoir in the offshore part, respectively. These outputs can potentially supply electricity to approximately 12,000, 29,000 and 53,000 households, respectively.

Holocene climate variability in Slovenia: A review

The Slovenian climate has undergone significant fluctuations, and an understanding of the past climate is necessary to improve models and recognise long-term patterns. The cryosphere environment, such as ice core samples, provides valuable palaeoclimate data. Palynology and dendroclimatology are also effective ways to study long-term changes in vegetation and reconstruct past climates using pollen and tree proxies. Sediment cores from various locations in Slovenia have been studied to understand past environmental changes. Borehole temperature profiles as well as historical records were also used to reconstruct past climate conditions. Studies have shown specific periods when climatic changes likely played a major role, but a complete timeline of the Slovenian climate throughout the Holocene has not yet been fully developed.

First comprehensive assessment of industrial-era land heat uptake from multiple sources

Abstract. The anthropogenically intensified greenhouse effect has caused a radiative imbalance at the top of the atmosphere during the industrial period. This, in turn, has led to an energy surplus in various components of the Earth system, with the ocean storing the largest part. The land contribution ranks second with the latest observational estimates based on borehole temperature profiles, which quantify the terrestrial energy surplus to be 6 % in the last 5 decades, whereas studies based on state-of-the-art climate models scale it down to 2 %. This underestimation stems from land surface models (LSMs) having a subsurface that is too shallow, which severely constrains the land heat uptake simulated by Earth system models (ESMs). A forced simulation of the last 2000 years with the Max Planck Institute ESM (MPI-ESM) using a deep LSM captures 4 times more heat than the standard shallow MPI-ESM simulations in the historical period, well above the estimates provided by other ESMs. However, deepening the LSM does not remarkably affect the simulated surface temperature. It is shown that the heat stored during the historical period by an ESM using a deep LSM component can be accurately estimated by considering the surface temperatures simulated by the ESM using a shallow LSM and propagating them with a standalone forward model. This result is used to derive estimates of land heat uptake using all available observational datasets, reanalysis products, and state-of-the-art ESM experiments. This approach yields values of 10.5–16.0 ZJ for 1971–2018, which are 12 %–42 % smaller than the latest borehole-based estimates (18.2 ZJ).

Characteristics of geothermal field and tectono-thermal evolution in Baiyun Sag, Pearl River Mouth Basin, China

Baiyun Sag has become the primary focus of deepwater exploration in the Pearl River Mouth Basin. However, its complex and high-variate geothermal characteristics have severely constrained further oil and gas exploration and resource evaluation. In this study, the present-day geothermal field and tectono-thermal evolution histories of Baiyun Sag were systematically studied based on measured rock thermal conductivity and heat generation data, borehole temperatures, low-temperature thermochronometer analyses, and geodynamic methodologies. The thermal conductivity of 251 core samples ranges from 1.131 to 4.478 W/(m·K), with an average of 2.258 W/(m·K), while the heat generation rate of 106 samples ranges from 0.868 to 1.735 μW/m³, averaging 1.499 μW/m³. The thermal conductivity in Baiyun Sag exhibits a gradual decrease from the Wenchang Formation to the Hanjiang Formation, whereas the heat generation rate decreases with depth. The present-day heat flow in Baiyun Sag ranges from 66.6 to 139.1 mW/m2, with an average of 89.7 ± 14.7 mW/m2, showing a gradual increasing trend from northwest to southeast. Formation temperature at depths of 1–5 km increases proportionally with depth. Thermal inversion, as inferred from low-temperature thermochronological data of six basement samples, reveals distinct temperature paths for each tectonic unit in Baiyun Sag. These paths are primarily linked to regional tectonic uplift-subsidence and basement heat flow variation. Geodynamic simulations further indicate two extensional events in Baiyun Sag during the Eocene and Middle Miocene, leading to a rapid increase in basement heat flow. This study systematically elucidates the present-day geothermal field characteristics and tectono-thermal evolution history of Baiyun Sag, bearing significant implications for regional tectonic evolution and future deepwater oil and gas explorations.

Impact of boundary conditions on the modeled thermal regime of the Antarctic ice sheet

Abstract. A realistic initialization of ice flow models is critical for predicting future changes in ice sheet mass balance and their associated contribution to sea level rise. The initial thermal state of an ice sheet is particularly important, as it controls ice viscosity and basal conditions, thereby influencing the overall ice velocity. Englacial and subglacial conditions, however, remain poorly understood due to insufficient direct measurements, which complicate the initialization and validation of thermal models. Here, we investigate the impact of using different geothermal heat flux (GHF) datasets and vertical velocity profiles on the thermal state of the Antarctic ice sheet and compare our modeled temperatures to in situ measurements from 15 boreholes. We find that the temperature profile is more sensitive to vertical velocity than to GHF. The basal temperature of grounded ice and the amount of basal melting are influenced by both selection of GHF and vertical velocity. More importantly, we find that the standard approach, which consists of combining basal sliding speed and incompressibility to derive vertical velocities, provides reasonably good results in fast-flow regions (ice velocity &gt;50 m yr−1) but performs poorly in slow-flow regions (ice velocity &lt;50 m yr−1). Furthermore, the modeled temperature profiles in ice streams, where bed geometry is generated using a mass conservation approach, show better agreement with observed borehole temperatures compared to kriging-based bed geometry.

Material and mechanical properties of young basalt in drill cores from the oceanic island of Surtsey, Iceland

Characterization of 2017 drill core samples from Surtsey, an oceanic island produced by 1963−1967 eruptions in the offshore extension of Iceland’s east rift zone, reveals highly heterogeneous microstructural, physical, and mechanical properties in subaerial, submarine, and subseafloor basaltic deposits. The connected porosity varies from 42% in weakly consolidated lapilli tuff in a submarine inflow zone to 21% in strongly lithified lapilli tuff in upper subseafloor deposits near the explosively excavated conduit. Permeability, however, varies over six orders of magnitude, from 10−18 m2 to 10−13 m2. Uniaxial compressive strength, P-wave velocity, and thermal conductivity are also highly variable: 10−70 MPa, 1.48−3.74 km·s−1, and 0.472−0.862 W·m−1·K−1, respectively. Synchrotron X-ray microdiffraction analyses integrated with major-element geochemistry and quantitative X-ray powder diffraction analyses describe the initial alteration of fresh glass, incipient consolidation of a fine-ash matrix, and partial closure of pores with mineral cements. Permeability, micromechanical, and thermal property modeling highlight how porosity and pore size in eruptive fabrics—modified through diverse cementing microstructures—influence the physical properties of the pyroclastic deposits. Borehole temperatures, 25−141 °C (measured from 1980 to 2018), do not directly correlate with rock strength properties; rather, the abundance and consolidation of a binding fine-ash matrix appears to be a primary factor. Analytical results integrated with archival data from 1979 drill core samples provide reference parameters for geophysical and heat transfer studies, the physical characteristics of pyroclastic deposits that lithify on a decadal scale, and the stability and survival of oceanic islands over time.

Coupled thermo–geophysical inversion for permafrost monitoring

Abstract. This study explores an alternative way of deriving soil thermal properties from surface geophysical measurements. We combined ground surface temperature time series with time lapse geoelectrical acquisitions measured from the ground surface in a fully coupled inversion scheme to calibrate a heat conduction model. The quantitative link between the thermal and geoelectrical parts of the modeling framework is the temperature-dependent unfrozen water content, which is also the main factor influencing electrical response of the ground. The apparent resistivity data were incorporated into the coupled framework without being inverted separately, thus reducing the uncertainty inevitably associated with inverted resistivity models. We show that geoelectrical time lapse data are useful as alternative calibration data and can provide as good results as borehole temperature measurements. The fully coupled modeling framework using field data achieved performance comparable to calibration on borehole temperature records in terms of model fit within 0.6 ∘C, inversion convergence metrics, as well as the predictive performance of the calibrated model.

Distributed fiber-optic temperature monitoring in boreholes of a seasonal geothermal energy storage

Monitoring the in-situ temperature is key for the characterization of a seasonal geothermal energy storage. Distributed fiber-optic temperature sensing (DTS) systems provide temporally and spatially continuous measurement data in near real-time that captures borehole temperature dynamics. In the presented project, three boreholes of a seasonal geothermal energy storage with a vertical depth of down to 500 meters were instrumented with distributed fiber-optic sensors. For this purpose, a standard armored sensor cable was modified to allow for combined DTS, distributed acoustic sensing (DAS) and distributed strain sensing (DSS) at temperatures up to 120 °C. The cable was installed in a loop configuration to allow for temperature calibration using a temperature matching approach. A “Mini-Bend” solution was selected to cope with the limited space between formation and casing and the single cable feedthrough at the wellhead of the pressurized system. Using specially designed centralizers, the fiber-optic cable was installed and hold in place successfully in the 2” annulus outside the casing. Within the present paper, the focus lies on the DTS system and the data acquired during cementation. This continuous temperature data showcases the ability of DTS systems to detect small temperature changes in high detail. During the whole cementation process, the rising level of cement as well as setting temperatures were captured by rising temperature values in the range of a few degrees Celsius. As next project steps, not part of this paper, spatiotemporally temperature data will be recorded during stimulation treatments of the seasonal geothermal energy storage. The authors believe that the experience made with this fiber-optic monitoring system can be a good demonstration of the capabilities of fiber-optic sensing in deep borehole environments.

Simultaneous Inversion of Subsurface Thermal Conductivity Using Surface Heat Flow and Borehole Temperature Data

Simultaneous Inversion of Subsurface Thermal Conductivity Using Surface Heat Flow and Borehole Temperature Data

Deep subsurface microbial life in impact-altered Late Paleozoic granitoid rocks from the Chicxulub impact crater.

In 2016, IODP-ICDP Expedition 364 recovered an 829-meter-long core within the peak ring of the Chicxulub impact crater (Yucatán, Mexico), allowing us to investigate the post-impact recovery of the heat-sterilized deep continental microbial biosphere at the impact site. We recently reported increased cell biomass in the impact suevite, which was deposited within the first few hours of the Cenozoic, and that the overall microbial communities differed significantly between the suevite and the other main core lithologies (i.e., the granitic basement and the overlying Early Eocene marine sediments; Cockell etal., 2021). However, only seven rock intervals were previously analyzed from the geologically heterogenic and impact-deformed 587-m-long granitic core section below the suevite interval. Here, we used 16S rRNA gene profiling to study the microbial community composition in 45 intervals including (a) 31 impact-shocked granites, (b) 7 non-granitic rocks (i.e., consisting of suevite and impact melt rocks intercalated into the granites during crater formation and strongly serpentinized pre-impact sub-volcanic, ultramafic basanite/dolerite), and (c) 7 cross-cut mineral veins of anhydride and silica. Most recovered microbial taxa resemble those found in hydrothermal systems. Spearman correlation analysis confirmed that the borehole temperature, which gradually increased from 47 to 69°C with core depth, significantly shaped a subset of the vertically stratified modern microbial community composition in the granitic basement rocks. However, bacterial communities differed significantly between the impoverished shattered granites and nutrient-enriched non-granite rocks, even though both lithologies were at similar depths and temperatures. Furthermore, Spearman analysis revealed a strong correlation between the microbial communities and bioavailable chemical compounds and suggests the presence of chemolithoautotrophs, which most likely still play an active role in metal and sulfur cycling. These results indicate that post-impact microbial niche separation has also occurred in the granitic basement lithologies, as previously shown for the newly formed lithologies. Moreover, our data suggest that the impact-induced geochemical boundaries continue to shape the modern-day deep biosphere in the granitic basement underlying the Chicxulub crater.

The Thermal Structure of Central Te Riu a Māui/Zealandia Continent as Determined From the Depth to Base of Magnetic Sources

AbstractTo investigate the thermal structure of central Te Riu a Māui/Zealandia continent we estimate the depth to base of magnetic sources (DBMS), using an updated compilation of magnetic data. We calculate thousands of power spectral density estimates and solve an analytic solution for the fractal distribution of magnetization using a Bayesian method constrained by a sediment thickness model. Our DBMS results are broadly similar to a global model yet show complex relationships with independent models of crust and elastic thicknesses, and depth to base of seismicity (D90). Across central Zealandia, DBMS occurs above, coincident with, and below the Moho reflecting that factors additional to temperature influence DBMS. Away from subduction zones crustal thickness likely modulates DBMS while plate margins show varying influence dependent on margin characteristics. In the Taupō Volcanic Zone we interpret DBMS to reflect Curie point temperature at an average depth of 9–11 km. Error analysis shows that the choice of prior influences the posterior standard deviation and can result in counter intuitive error versus DBMS relationships. We use the DBMS as a basal temperature isotherm in heat flow models to compare with heat flow derived from borehole temperature measurements. Using the standard Curie point temperature of 580°C we find that DBMS derived heat flow models over estimate heat flow in areas underlain by plutonic rocks. We propose this reflects a higher titanomagnetite content in these rocks, although detailed studies of the magnetic mineralogy of Zealandia's plutonic rocks are required.

Ground-air temperature tracking from a geothermal climate-change observatory in South India

A geothermal climate-change observatory at Choutuppal (17.29 oN, 78.92°E) in south India was put into operation in July 2009 to study the inter-relationships between subsurface, ground-surface and surface-air temperature. The observatory is the first of its kind in a low latitude region with a significant monsoon season. Subsurface temperature measurements were obtained at different times of the year in two boreholes at the observatory drilled into granite to 21 m and 210 m depths respectively; a continuous record of shallow ground temperatures was acquired at six different depths to 1.2 m next to the deeper boreholes. Meteorological parameters recorded at the same site include surface air temperature (SAT), relative humidity, precipitation, solar radiation, wind speed and direction. Analysis of ground-air temperature data for the first five years (2009–2014) reveals the following. (i) Air temperature changes diffuse into the granitic subsurface with associated attenuation of high frequency components and phase lag characteristic of conductive heat transfer. (ii) Ground temperatures measured in the 1.2 m and 21 m deep holes track the diurnal and seasonal changes in SAT respectively, (iii) The ground is warmer than air on average; the difference is not constant but varies from −2.1 to +7.6 °C with a mean of 3.4 ± 1.8 (SD) oC, (iv) Ground-air temperature difference is influenced by incoming solar radiation and monsoon precipitation. On average, the ground warms by 2.5 °C for 100 Wm−2 increase in average incident solar radiation. The study highlights the importance of long-term monitoring to test ground-air temperature tracking critical to using borehole temperatures to infer climate change.

Characteristic period of response spectrum for surface and borehole ground motions

Determining the characteristic period of ground motions (Tg) is an essential step in the seismic design of structures, but a critical problem that needs to be solved is figuring out how to do so when bedrock ground motion is utilized as an input. In this paper, the relationship between surface and borehole temperature data is investigated, and the main factors affecting the Tg are analyzed. Then, the algorithm for predicting Tg is presented. It is found that two factors, i.e., peak ground acceleration (PGA) and fault distance have significant effects on the value of Tg. By comparing the values of Tg for borehole and surface motions, we find that, contrary to expectation, Tg for borehole motions is usually larger than that for surface motions in most cases. Finally, the conversion equation between the Tg for borehole and surface motions is given using regression analysis.

Topographical effect of high embankments on resistivity investigation of the underlying permafrost table

AbstractInvestigation of resistivity has been effectively used in assessing the risks of embankmentation and failure. A two‐dimensional (2D) approximation of the surveyed object is commonly assumed for a survey line on the road surface. However, this approximation may not be met when resistivity investigations are conducted over a raised high embankment; under these conditions, regular inversions might yield erroneous results. This study explored the topographical effect of a high embankment on resistivity measurements by forward and inverse modeling of a 3D high embankment model. The results show that a 2D approximation of the survey lines on the road surface significantly increases the apparent resistivity within the depth of the raised embankment. Maximum relative errors reached 21% and 11% for the road shoulder and midline survey lines, respectively. The biased apparent resistivity resulted in an inverted resistivity that was higher than the true values, although resistivity contrasts can still identify interfaces between layers. A geometric factor was used to correct the biased apparent resistivity to eliminate the high embankment topographical effect. Inversion results of the corrected apparent resistivity agreed well with the forward model. The method was then verified by field application. The apparent resistivity of the field data collected on a high embankment in permafrost regions on the Qinghai–Tibet Plateau was corrected before inversion. The permafrost table derived from the inverted resistivity was verified based on borehole temperatures. These findings indicate that the topographical influence of high embankments on resistivity measurements needs to be considered. Correction of the apparent resistivity is indispensable for quantitative interpretation of the inverted resistivity.

Optimization of Heat Interaction Between Borehole Heat Exchanger and Ground Using Taguchi Method During Space Cooling and Heating Operation of GSHP System

Abstract In this research work, optimization of heat exchange between borehole heat exchanger (BHE) and the ground soil for space cooling and heating applications, incorporating the optimum thermal effectiveness of BHE has been reported. Initially, Taguchi technique is employed to optimize the effectiveness of borehole heat exchanger. Later, the experimental data of 24 h are coupled with the theoretically optimized parameters to compute the optimum heat exchange during peak summer and peak winter seasons. In the Taguchi optimization approach, six control variables at three levels are employed and a standard, L27 (36) orthogonal array is selected for the analysis. Among the six control variables, thermal conductivity of the grouting material is observed to be the most influential parameter and tube radius of BHE as the least parameter in the optimized thermal effectiveness of the BHE. Both the experiments for space heating and cooling were conducted on a 17.5 kW cooling capacity ground source heat pump system (GSHP), connected with five parallelly connected double U-tube BHE and one single U-tube BHE. To compute the optimum heat transfer to/ from the BHE, time-dependent borehole temperature was incorporated to include the dynamic thermal load of the GSHP system. After incorporating the Taguchi-optimized thermal effectiveness in the experimental data, there is an enhancement of 30% to 48% of heat rejection into the ground during the summer season, whereas in the winter season, there is an enhancement of 35– 52% of heat extraction from the ground.

State of the art of hydraulic packer testing in clay rocks in the Swiss national radioactive waste disposal program

Abstract. To provide input for site selection and the safety case for deep geological repositories for radioactive waste, Nagra has drilled a series of deep boreholes in northern Switzerland. As part of the hydrogeological characterization of this exploratory drilling campaign, hydraulic packer tests in the sedimentary rocks were carried out under the supervision of Nagra, in depths from 300 to 1130 m b.g.l. (below ground level). Single or straddle-packer hydraulic tests were conducted in different low-permeability rocks. The hydraulic testing equipment used was specially designed to reduce uncertainty associated with the low-permeability testing conditions and included a downhole hydraulic shut-in valve (DHSIV), a hydraulic piston, a probe carrier, and inflatable packer(s). Pressure and temperature are monitored with sensors connected to the bottom of the borehole, the test interval, the annulus above the upper packer, and the test tubing above the DHSIV. Hydraulic test sequences consisted of multiple withdrawal and shut-in phases. Test phases started with a compliance phase to allow some of the pressure and temperature transients caused by tool installation and packer inflation to dissipate, followed by a pressure static recovery phase to allow the test interval pressure to establish a recovery trend after being isolated. In general, in the low-permeability formations the active test sequence consisted of a pulse withdrawal performed using a hydraulically operated piston displacing a precisely defined volume, a slug withdrawal, and lastly a slug-withdrawal shut-in phase. The pressure data collected during the hydraulic tests were analyzed using the well-test simulator nSIGHTS®. The use of a numerical model is necessary for the analyses of the tests in low-permeability rocks as borehole history and temperature equilibration during the test can have a significant impact on test responses. Analyses started with the definition of a borehole-formation conceptual flow model based on geological information and interpretation of diagnostic plots. Input model parameters composed of non-fitting parameters and fitting parameters are defined. Preliminary automated calibrations are performed to identify the appropriate conceptual model and obtain baseline estimates of the fitting parameters for each tested interval. Perturbation analyses entailing hundreds to thousands of optimizations are performed to obtain the final best-fit parameter values and the corresponding uncertainty ranges. Our presentation will give an overview on methodologies and workflows with emphasis on the numerical design and analysis techniques applied to the hydraulic packer tests performed. The applied methodology provided high-quality results and reliable estimates of the hydraulic conductivities in the low-permeability rocks. The results from the hydraulic packer tests will be presented, and implications for further research by Nagra will be discussed.