Borehole

Summary

A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water (drilled water well and tube well), other liquids (such as petroleum), or gases (such as natural gas). It may also be part of a geotechnical investigation, environmental site assessment, mineral exploration, temperature measurement, as a pilot hole for installing piers or underground utilities, for geothermal installations, or for underground storage of unwanted substances, e.g. in carbon capture and storage. Importance Engineers and environmental consultants use the term borehole to collectively describe all of the various types of holes drilled as part of a geotechnical investigation or environmental site assessment (a so-called Phase II ESA). This includes holes advanced to collect soil samples, water samples or rock cores, to advance in situ sampling equipment, or to install monitoring wells o

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https://en.wikipedia.org/wiki/Borehole

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Experimental studies suggest that the fracture toughness of rocks increases with the confining pressure. Among many methods to quantify this dependency, a so-called burst experiment (Abou-Sayed, 1978) may be the most widely applied in practice. Its thick wall cylinder geometry leads to a stress state resembling the subsurface condition of a pressurized wellbore with bi-wing fractures. The fracture toughness of a sample, under a given confinement pressure, can be recovered from the critical pressure upon which the bi-wing cracks propagate. Traditionally, this critical pressure is thought to correspond to a sudden drop in injection pressure. However, as the standard configuration was deliberately designed to obtain stable fracture growth at the onset, propagation can take place well before this drop in pressure, and one may overestimate the fracture toughness from measured pressures. Here, we study crack stability in the burst experiment and propose modifications to the experimental design which promotes unstable fracture growth and makes the critical pressure less ambiguous to interpret. We found that experiments with the original, stable design can lead to inconsistent measurement of fracture toughness under confining pressure, while results from unstable configurations are more consistent. Our claim on the stability was also supported by the recorded acoustic emissions from both stable and unstable experiments.

SPRINGER WIEN2022

Estimating soil thermal diffusivity with interference analyses

Lyesse Laloui, Thomas Mimouni

The development of ground source heat pumps has facilitated the use of geothermal power at shallow depths. Initially, ground heat exchangers were buried in trenches or boreholes, but recent investigations and increasing congestion of landscapes have suggested the use of foundation structures as heat exchangers with the ground. Foundations are shorter than conventional borehole heat exchangers and are closer to each other to ensure structural support. The thermal inertia of grounds wherein seasonal heat storage is achieved, therefore, becomes an important parameter for the design of such structures. Although thermal response tests have been developed to estimate bulk thermal conductivity on the scale of geo-thermal boreholes, only laboratory or shallow in situ test methods have been developed to estimate the thermal diffusivity of the ground. This paper investigates the potential for using a periodic pumping test procedure to measure the thermal diffusivity of soils in a scaled model of a geothermal borehole. The thermal diffusivity estimates obtained are in agreement with values reported in the literature and illustrate the potential of the proposed method.

Springer Verlag2015

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Shallow geothermal power represents an important energy resource for the heating and cooling of the buildings. Due to relatively low temperature levels encountered at shallow depths in the soil, between 10°C and 20°C, heat pumps are required to process the extracted heat, forming the so called ground source heat pump system. Different types of heat exchangers with the ground were developed in order to optimize the heat exchanges, from simple geothermal loops grouted in boreholes reaching depths up to a couple of hundreds of meters to complex energy geostructures. Indeed, embedding geothermal loops within concrete foundation structures increases the heat exchange efficiency as well as it saves the cost of additional drillings. Recent developments suggested that applying the concept of energy geostructures to tunnel structures that are in contact with the ground should also be efficient. The present study investigates the potential of using tunnel anchors and nails as heat exchangers with the surrounding soil. Two main structures of urban tunnels were investigated. A cut and cover tunnel, whose diaphragm walls are maintained with long anchors, was modelled first. Thermal influence of the soil surface and unsaturated conditions were taken into account because of the shallow depth of the tunnel body. Nevertheless, mechanical implications of the heat extraction on the cut and cover tunnel were neglected because of the low mechanical confinement observed on the structure. Then, an urban bored tunnel was investigated. Soil conditions encountered at this depth were assumed always saturated and the thermal influence of the surface was neglected. Mechanical implications of the heat exploitation were assessed because of the high confinement of the bored tunnel body induced by the soil weight. Different types of heat exploitation cycles were tested for the different configurations. The heat extraction is based on the external air temperature in order to meet a simplified building heat demand. Cycles with and without heat injection were also investigated. All the exploitation cycles were optimized in order to reach a temperature threshold in the ground to prevent freezing it. Next, comparisons between extracted and injected heat of the different cycles allow drawing an optimum exploitation method. It is found that injecting heat during the hot period is necessary for the cut and cover tunnel as the natural heat reload isn’t high enough to ensure the sustainability of the heat storage. Conversely, the bored tunnel beneficiates from an increased natural heat reload, turning the heat injection into a more expensive solution. Furthermore, mechanical implications of the heat exploitation on the bored tunnel are found to be more significant when injecting heat. This shows the importance of a thermo-mechanical design of such a system. Finally, considering heat injection or not, it is estimated that heat extraction ranges from 0.6 to 1.2 MWh per year and per meter of cut and cover tunnel, and from 2.8 to 4.0 MWh per year and per meter of bored tunnel.

Swiss Association of Road and Transportation Experts2013

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Swiss Association of Road and Transportation Experts2013