Multiple Boreholes Research Articles

Efficient simulation of multiple borehole heat exchanger storage sites

In this paper, an adapted model is developed for borehole heat exchangers (BHEs) to simulate geothermal applications such as heat storage on a large scale efficiently and with high accuracy. The adapted numerical model represents all BHE components, allowing for a detailed representation of the governing processes. The approach is calibrated and validated for a single U-tube BHE using a high-resolution experimental data set from a laboratory thermal response test. It is found that the computational effort can be reduced by factors of ~50, ~50 and ~25 for single U-tube, double U-tube and coaxial BHEs, respectively, if an absolute deviation of less than 1 % compared to a conventional fully discretised model is allowed. Computation times can be reduced further by accepting higher deviations. The adapted modelling approach allows for a detailed and correct representation of the temporal and spatial temperature distribution under highly transient conditions by applying it to a high-temperature heat storage scenario using multiple BHEs. The model is especially suited to represent coupled flow and heat transport processes, to account for groundwater flow in the BHE region as well as geological heterogeneities and especially interaction between a large number of BHEs.

Variations of hydraulic conductivity of fracture sets and fractured rock mass with test scale: Case study at Heshe well site in Central Taiwan

Variations of the equivalent hydraulic conductivity of a fractured rock mass with investigating scale typically cause difficulties in the hydraulic characterization of a site. A mechanistic model regarding water flow through fractured rock masses and associated methods of multiple regression analysis have been established to determine the transmissivities of dominant fracture sets with statistical significance using in situ hydrogeological test results. Accordingly, this manuscript decomposes the transmissivity of a fracture set into the product of its hydraulic conductivity and its aperture. The ultimate purpose is to investigate the effects of scale on the hydraulic characteristics of fractured rock masses. The results of in situ hydrogeological tests at various scales, performed at the Heshe well site, are utilized to determine the equivalent hydraulic conductivity thereof. A series of laboratory tests are planned and carried out to obtain the apertures of fracture sets, which are applied in determining the respective hydraulic conductivities of those sets. Analyzed results reveal that the hydraulic conductivities of fracture sets can be determined with statistical significance. The comparison of equivalent hydraulic conductivities that are obtained in in situ multiple borehole pumping (MBP) tests with those determined using the combination of the hydraulic conductivities and geometric parameters of fracture sets verifies the proposed method. The hydraulic conductivities of fracture sets that are determined from their apertures, which are obtained from laboratory tests, differ from those obtained by in situ MBP tests by eight orders of magnitude. The magnitudes of hydraulic conductivities of fracture sets initially increase with the test scale, subsequently approaching an asymptote, exhibiting a negative exponential distribution. The proposed method yields the hydraulic conductivities of fracture sets and the equivalent hydraulic conductivity of a fractured rock site from the spatial distribution, hydraulic conductivities, and aperture of each dominant fracture set. Such an approach can minimize the deviation of the hydraulic conductivity of a fracture set with the measured aperture, and can be used to determine hydraulic conductivity, which is affected by both test scale and in situ stress.

Fracture size estimation using data from multiple boreholes

The size of a fracture within a rock mass can be obtained using data sampled from boreholes or exposed surfaces. In this study, fracture sizes are estimated using data from multiple boreholes. The fractures are assumed to be elliptical, all having the same aspect ratio, but not necessarily occurring parallel to each other. Formulae for estimating the mean and standard deviation of the characteristic fracture size are developed by considering the number of intersections between the fractures and the boreholes. A method is proposed to infer the probability density function of the fracture size from the number of fractures, which is obtained by random sampling of fractures in adjacent boreholes; the fracture distribution is assumed to follow the form of a fractal, exponential, or lognormal distribution. The performance of the method is validated using Monte Carlo simulations with different inputs of size distribution, fracture density, orientation, and mean fracture size.

Study on the Representative Borehole in Ground Heat Exchanger Design

In view of the fact that the large-scale projects of the ground-coupled heat pump systems become more popular, a new concept is proposed that a representative borehole takes the place of the least-favorable borehole in thermal analysis of the ground heat exchangers so that more precise designs and simulations may be achieved. Borehole temperatures and their average are calculated in fields with multiple boreholes, and the mean square error coefficient is devised in order to analyze the imbalance between the boreholes. Conclusions are obtained to locate the representative borehole in different configurations of the borehole fields.

Feasibility Study of a Novel Soil Thermal Recovery Method for HGSHP System

In order to solve the heat accumulation problem of GSHP system in cooling load dominated area with high humidity in summer, this paper presents a novel HGSHP system based on CT cool-storage in transition season. Then the performance evaluation model for HGSHP system is built on the base of the heat and moisture transfer principle of CT, the GHE multiple borehole finite line heat source model and TRNSYS software. At last, using the evaluation model, an actual project in Nanjing is used as an example to predict the thermal performance of this novel HGSHP system. The results show that using novel HGSHP system, the heat accumulation problem can be well solved.

Zoning operation of multiple borehole ground heat exchangers to alleviate the ground thermal accumulation caused by unbalanced seasonal loads

When the heat extraction and injection of a borehole ground heat exchanger (GHE) are not seasonally balanced, ground thermal accumulation will occur and will cause a decline in the ground source heat pump's operational efficiency, especially for large multiple borehole ground heat exchangers (LMBGHE). A zoning operation strategy, in which only the relatively central part of the GHE runs during the low load season, is adopted in this paper to alleviate the thermal accumulation. By analyzing the case in which the heat injected into the ground in the summer is greater than that extracted from the ground in the winter, it was found that, in comparison with the full operation mode, the highest and average field temperatures significantly decrease when only the central part of the GHE runs during the winter. Analysis shows that the thermal accumulation can be effectively alleviated by this GHE zoning operation. This study also indicates that the zoning operation method is more effective when the ground has a smaller thermal conductivity.

Did borers make corals more susceptible to a catastrophic disease outbreak in Hong Kong?

A massive outbreak of coral disease was recently recorded in Tolo Harbour and Channel (22.46°N, 114.29°E), a semienclosed embayment in northeastern Hong Kong. Preliminary photo-quadrat surveys registered a 20.6-51.4 % loss in total coral cover in five communities within March 2015. Platygyra spp., the most dominant corals, were most susceptible and often suffered near-complete colony mortality (Fig. 1a). Such large scale and rapid disease outbreak in Hong Kong is now documented for the first time since periodic monitoring started here over 20 years ago. The disease was first noticed by abnormal extensive protrusion of mesenterial filaments or tentacles from the coral colony surfaces of Cyphastrea, Dipsastraea, Echinophyllia, and Platygyra (Fig. 1b). Colony surfaces immediately around holes of boring bivalves, mainly Lithophaga spp. (ValentichScott 2003; Bouchet and Gofas 2015), appeared to suffer most from the disease attack. Lesion areas initiated around the boreholes (Fig. 1c) and from there radiated to other parts of the colony (Fig. 1d-f). As borer infestations are most common among massive, submassive, and encrusting corals, disease origins from multiple boreholes may have contributed to the rapid spread over the colony surface. The reasons behind this apparent association between borers and the disease are unclear. The disease pattern of rapid tissue loss closely resembles that exhibited by the white syndrome that is often associated with ciliates (Sweet and Bythell 2012). In collected samples, a complex microbial community involving ciliates, dinoflagellates, and diatoms was observed under the microscope in tissues of diseased lesions but not in the seemingly healthy areas nearby. Ciliates, the apparent causative agent, may always be present in the coral tissue (Qiu et al. 2010) and could spread out when conditions become favourable. Coral tissues around boreholes may be inherently stressed through physical movement or chemicals secreted by the borers (Glynn 1997) and thus, may be more vulnerable to disease attack.

Analyses on soil temperature responses to intermittent heat rejection from BHEs in soils with groundwater advection

Multiple boreholes heat exchangers (BHEs) are widely used in ground source heat pump systems (GSHPS) and are operated in intermittent mode in general. The soil type and groundwater advection that possibly exist may affect their heat transfer performances. Thus, based on the moving finite line heat source model (MFLS) and the superposition principle, an analytical solution to soil temperature response to heat rejection from multiple BHEs was proposed. It is programmed by using MATLAB and the calculation results were validated by 3D transient numerical simulation results. Then the influence of groundwater advection velocity and soil type on the soil temperature responses to heat rejection from a single BHE in intermittent and continuous operational modes was compared and discussed. The dynamic temperature responses of the positions monitored for the multiple BHEs can be calculated by the new analytical model. Results indicate that thermal accumulation phenomenon in the soil can be mitigated by intermittence and the amplitude of soil temperature recovery tends to be large. Not only the advection velocity of groundwater, but also the soil properties play an important part in the soil temperature recovery characteristics when BHEs reject heat intermittently. In addition, soil temperature variation distributions around two kinds of multiple BHEs layouts were also illustrated. It is suggested that the consideration of both groundwater advection and boreholes layout be important in designing the BHEs configurations.

Discrete 3D fracture network extraction and characterization from 3D seismic data — A case study at Teapot Dome

Three-dimensional discrete fracture networks (DFNs) extracted from the seismic data of the Tensleep Formation at Teapot Dome successfully matched 1D fracture data from multiple boreholes within the area. The extraction process used four seismic attributes, i.e., variance, chaos, curvature, and spectral edge, and their multiple realizations to define seismic discontinuities that could potentially represent fractures within the Tensleep Formation. All of the potential fracture attributes were further enhanced using a fracture-tracking attribute for better extraction and analysis of seismic discontinuity surfaces and their network properties. A state-of-the-art discontinuity surface extraction and characterization workflow uniformly extracted and interactively characterized the seismic discontinuity surfaces and networks that correlate with borehole fracture data. Among the attributes, a fracture-tracking attribute cube created out of the high-resolution spectral-edge attribute provided the best match with the borehole fracture data from the Tensleep Formation. Therefore, the extracted discontinuity planes were classified as fractures and then characterized. The extracted fracture population also matched earlier published records of faults and fractures at Teapot Dome. Unlike the conventional method, which uses 1D borehole fracture data as primary input and 3D seismic data as a guide volume during DFN modeling, I used 3D seismic attributes as the primary data and the 1D borehole fracture data only for quality control. I also evaluated the power of converting seismic fracture attribute volumes into discrete surfaces and networks for effective correlation with 1D fracture logs from boreholes.

Investigating uncertainties in empirical Green's function analysis of earthquake source parameters

AbstractI use a well‐recorded earthquake sequence to investigate the uncertainties of earthquake stress drops calculated using an empirical Green's function (EGF) approach. The earthquakes in the largest (M ~ 2.1) repeating sequence targeted by the San Andreas Observatory at Depth (SAFOD), Parkfield, California, are recorded by multiple borehole stations and have simple sources, well‐constrained stress drops, and abundant smaller earthquakes to use as EGFs. I perform three tests to estimate quantitatively the likely uncertainties to arise in less optimal settings. I use EGF earthquakes with a range of cross‐correlation values and separation distances from the main earthquakes. The stress drop measurements decrease by a factor of 3 as the quality of the EGF assumption decreases; a good EGF must be located within approximately one source dimension of the large earthquake, with high cross correlation. I subsample the large number of measurements for the main earthquakes to investigate the expected stress drop uncertainties in studies where fewer stations or EGFs are available. If only one measurement is available, the uncertainties are likely to be at least 50%. The uncertainties decrease to <20% with five or more measurements; using multiple EGFs is a good alternative to multiple stations. To investigate the effects of limited frequency bandwidth, I recalculate the corner frequencies after progressively decimating the sample rate. Decreasing the high‐frequency limit of the bandwidth decreases the estimate of the corner frequency (and stress drop). The corner frequency may be underestimated if it is within a factor of 3 of the maximum frequency of the signal.

Land subsidence due to groundwater withdrawal in the northern Beijing plain, China

Beijing is an international metropolis, where over-exploration of water resource makes land subsidence becoming more and more serious. The related problems cannot be avoided in the coming years because of the giant increase of population. The aims of this study are to quantify land subsidence over the period 2003 to 2010, grasp the evolution of the process, and investigate the relation with the triggering factors in the northern area of the Beijing plain. Various data, including deep compaction from vertical multiple borehole extensometers, land subsidence from Persistent Scatterer Interferometry and leveling surveys, groundwater levels, hydrogeological setting from wellbores, and Landsat TM image were collected and effectively used to detect the spatial and temporal features of land subsidence and its possible relation with groundwater level changes, compressible layer thickness, and urban development. Results show that land subsidence is unevenly distributed and continuously increased from 2003 to 2010. The average loss of elevation over the monitoring period amounted to 92.5mm, with rates up to 52mm/y. The distribution of the subsidence bowl is only partially consistent with that of the groundwater depression cone because of the variable thickness of the most compressible fine deposits. In fact, extensometers reveal that silty-clay layers account for the larger contribution to land subsidence, with the 15m thick silty-clay layer between 102 and 117m depth accounting for about 25% of the total subsidence. Finally, no clear correlation has been observed between the subsidence rates and the increase of the load on the land surface connected to the impressive urban development. This study represents a first step toward the development of a physically-based model of the subsidence occurrence to be used for planning remediation strategies in the northern Beijing plain.

Simulation Modelling and Design Optimization of Vertical Ground Heat Exchanger-GEOSTAR Program

This paper presents the analytical heat transfer models for the vertical ground heat exchangers (GHEs) and the attendant program named “GEOSTAR” developed for use in design and simulation of vertical GHEs. The program used spatial superposition for multiple boreholes and sequential temporal superposition principles to account for time-varying loads. Coupled with the models of the heat pump and building loads, the GEOSTAR can predict time-varying heat pump energy consumption, heat transfer rates of GHEs, and other variables of interest. Finally, the program is applied to design an existing GCHP project and to simulate the system performance. The results demonstrate the usefulness of the simulation model and attendant program as a tool for designing GCHP systems.

Phase correction of electromagnetic coupling effects in cross-borehole EIT measurements

Borehole EIT measurements in a broad frequency range (mHz to kHz) are used to study subsurface geophysical properties. However, accurate measurements have long been difficult because the required long electric cables introduce undesired inductive and capacitive coupling effects. Recently, it has been shown that such effects can successfully be corrected in the case of single-borehole measurements. The aim of this paper is to extend the previously developed correction procedure for inductive coupling during EIT measurements in a single borehole to cross-borehole EIT measurements with multiple borehole electrode chains. In order to accelerate and simplify the previously developed correction procedure for inductive coupling, a pole–pole matrix of mutual inductances is defined. This consists of the inductances of each individual chain obtained from calibration measurements and the inductances between two chains calculated from the known cable positions using numerical modelling. The new correction procedure is successfully verified with measurements in a water-filled pool under controlled conditions where the errors introduced by capacitive coupling were well-defined and could be estimated by FEM forward modelling. In addition, EIT field measurements demonstrate that the correction methods increase the phase accuracy considerably. Overall, the phase accuracy of cross-hole EIT measurements after correction of inductive and capacitive coupling is improved to better than 1 mrad up to a frequency of 1 kHz, which substantially improves our ability to characterize the frequency-dependent complex electrical resistivity of weakly polarizable soils and sediments in situ.

Tunnel reinforcement in columnar jointed basalts: The role of rock mass anisotropy

The extent of the loosening zone in tunnels excavated through columnar jointed basalts is studied with the numerical discontinuous deformation analysis (DDA) method. The structure of the rock mass and tunnel geometry are modeled on the basis of a real field case study of deep tunneling performed in such a rock mass in south west China. Slender prismatic keyblocks formed by the intersection of broadly-spaced, gently-dipping breccia layers and closely-spaced, orthogonally oriented, steeply dipping columnar joints result in a highly anisotropic rock mass structure and give rise to sliding and toppling failure modes in the sidewalls and to an excessive height of loosening zone in the roof, the geometry of which is shown to be controlled by the orientation of the steeply inclined columnar joints. Results of displacement monitoring performed during tunnel excavation in a similar rock mass with multiple point borehole extensometers confirm the numerically obtained depth of the loosening zone both in the sidewalls and the roof. We find that the height of the loosening zone in the roof as obtained with DDA is greater than would have been predicted by Terzaghi’s empirical rock load classification for “blocky” rock masses, and show that its shape and orientation are controlled by the anisotropy of the rock mass structure. Moreover, we demonstrate that dimensioning rock bolt reinforcement using well-established empirical criteria without consideration of the anisotropic nature of the rock mass may lead to un-conservative design.

Impact of Groundwater Flow and Energy Load on Multiple Borehole Heat Exchangers

The effect of array configuration, that is, number, layout, and spacing, on the performance of multiple borehole heat exchangers (BHEs) is generally known under the assumption of fully conductive transport. The effect of groundwater flow on BHE performance is also well established, but most commonly for single BHEs. In multiple-BHE systems the effect of groundwater advection can be more complicated due to the induced thermal interference between the boreholes. To ascertain the influence of groundwater flow and borehole arrangement, this study investigates single- and multi-BHE systems of various configurations. Moreover, the influence of energy load balance is also examined. The results from corresponding cases with and without groundwater flow as well as balanced and unbalanced energy loads are cross-compared. The groundwater flux value, 10(-7) m/s, is chosen based on the findings of previous studies on groundwater flow interaction with BHEs and thermal response tests. It is observed that multi-BHE systems with balanced loads are less sensitive to array configuration attributes and groundwater flow, in the long-term. Conversely, multi-BHE systems with unbalanced loads are influenced by borehole array configuration as well as groundwater flow; these effects become more pronounced with time, unlike when the load is balanced. Groundwater flow has more influence on stabilizing loop temperatures, compared to array characteristics. Although borehole thermal energy storage (BTES) systems have a balanced energy load function, preliminary investigation on their efficiency shows a negative impact by groundwater which is due to their dependency on high temperature gradients between the boreholes and surroundings.

Oscillatory subglacial drainage in the absence of surface melt

Abstract. The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations.

A Review of the Modelling of Thermally Interacting Multiple Boreholes

Much attention is now focused on utilizing ground heat pumps for heating and cooling buildings, as well as water heating, refrigeration and other thermal tasks. Modeling such systems is important for understanding, designing and optimizing their performance and characteristics. Several heat transfer models exist for ground heat exchangers. In this review article, challenges of modelling heat transfer in vertical heat exchangers are described, some analytical and numerical models are reviewed and compared, recent related developments are described and the importance of modelling these systems is discussed from a variety of aspects, such as sustainability of geothermal systems or their potential impacts on the ecosystems nearby.

Minimum-structure borehole gravity inversion for mineral exploration: A synthetic modeling study

A borehole gravimeter for the diameters of holes typically used in mineral exploration has recently been developed. Investigating how the data from such instruments can contribute to the gravity interpretation procedures used in mineral exploration is therefore appropriate. Here, results are presented from a study in which synthetic data for 3D exploration-relevant earth models were inverted and the impact of borehole data assessed. The inversions were carried out using a minimum-structure procedure that is typical of those commonly used to invert surface gravity data. Examples involving data from a single borehole, from multiple boreholes, and combinations of borehole and surface data, are considered. Also, a range of options for the particulars of the inversion algorithm are investigated, including using a reference model and cell weights to incorporate along-borehole density information, and an [Formula: see text]-type measure of model structure. The selection of examples presented demonstrates what one can and cannot expect to determine about the density variation around and between boreholes when borehole gravity data are inverted using a minimum-structure approach. Specifically, the density variation along a borehole can be accurately determined, even without constraints in the inversion, but this capability decreases dramatically a few tens of meters from a borehole.

Geometric arrangement and operation mode adjustment in low-enthalpy geothermal borehole fields for heating

The efficient operation of ground source heat pump (GSHP) systems with multiple borehole heat exchangers (BHEs) over a lifetime of decades implies an optimized performance of the BHEs and a mitigation of the environmental impact of the system. This paper introduces a new combined optimization approach, which adjusts the BHE positions as well as the individually regulated energy extraction for each single BHE within a given borehole field in conduction dominated media for a given seasonal changing load profile. The optimization of only the BHE positions without optimizing the individual BHE loads nearly produces the same improvement of the underground temperature change of approximately 12% as an optimization of the BHE loads without optimized positioning. The combination of both optimization approaches results in only slightly better results compared to a result achieved by only one of the optimization approaches. Thus for homogeneous fields without groundwater flow, an optimal load assignment can be substituted by an optimal BHE placement, which leads to a considerably reduced complexity of the borehole field.

Estimation of dynamic petrophysical properties of water-bearing sands invaded with oil-base mud from the interpretation of multiple borehole geophysical measurements

Nonmiscible fluid displacement without salt exchange takes place when oil-base mud (OBM) invades connate water-saturated rocks. This is a favorable condition for the estimation of dynamic petrophysical properties, including saturation-dependent capillary pressure. We developed and successfully tested a new method to estimate porosity, fluid saturation, permeability, capillary pressure, and relative permeability of water-bearing sands invaded with OBM from multiple borehole geophysical measurements. The estimation method simulates the process of mud-filtrate invasion to calculate the corresponding radial distribution of water saturation. Porosity, permeability, capillary pressure, and relative permeability are iteratively adjusted in the simulation of invasion until density, photoelectric factor, neutron porosity, and apparent resistivity logs are accurately reproduced with numerical simulations that honor the postinvasion radial distribution of water saturation. Examples of application include oil- and gas-bearing reservoirs that exhibit a complete capillary fluid transition between water at the bottom and hydrocarbon at irreducible water saturation at the top. We show that the estimated dynamic petrophysical properties in the water-bearing portion of the reservoir are in agreement with vertical variations of water saturation above the free water-hydrocarbon contact, thereby validating our estimation method. Additionally, it is shown that the radial distribution of water saturation inferred from apparent resistivity and nuclear logs can be used for fluid-substitution analysis of acoustic compressional and shear logs.