Hot Rock Masses Research Articles

Research on Hydrodynamic Parameters and Heat Transfer with Thermal-Hydraulic-Mechanical Coupling in Hot Rock Masses

Research on Hydrodynamic Parameters and Heat Transfer with Thermal-Hydraulic-Mechanical Coupling in Hot Rock Masses

Fracture Network Volume Fracturing Technology in High‐temperature Hard Formation of Hot Dry Rock

AbstractIt is more difficult for a hot dry rock to form a fracture network system than shale due to its special lithology, physical and mechanical properties under high temperature. The essential characteristics, rock mechanics and in‐situ stress characteristics of a hot rock mass have been systematically studied by means of laboratory tests and true tri‐axial physical simulation. The fracture initiation and propagation characteristics under different geological and engineering conditions are physically simulated, and the main controlling factors for the formation of a complex fracture network are revealed. The technology of low displacement for enhancing thermal cracking, gel fluid for expanding fracture and variable displacement cyclic injection for increasing a fracture network has been applied in the field, and good results have been achieved. Micro‐seismic monitoring results demonstrate that complex fractures were formed in the field test, and the stimulation volume for heat exchanging reaches more than 3 million cubic meters. The research results play an important role in the stimulation technology of an enhanced geothermal system (EGS) and realize a breakthrough for power generation.

Acoustic emission monitoring of hydraulic fracturing using carbon dioxide in a small-scale field experiment

Carbon capture and storage (CCS) is a promising method for mitigating the greenhouse effect. If we could utilize carbon dioxide (CO2) for recovery of geothermal energy and shale gas, CCS will be much more eagerly developed and adopted by compensating its cost. We clarified that hydraulic fracturing (HF) using CO2 tended to induce three-dimensionally sinuous cracks with many secondary branches, which appear to be desirable pathways for energy recovery. However, in a laboratory experiment, it was difficult to evaluate crack extension size and effects of macroscopic pre-existing cracks. Thus, we conducted a HF experiment using CO2 in a hole 10 m long that was drilled in a hot rock mass, thus satisfying the temperature necessary to form supercritical (SC) CO2. A bi-wing crack 3.33 m in length was induced along the hole; this crack was significantly larger than the approximately 0.5 m long crack we made with water at another site. The fact that acoustic emission (AE) hypocenters distributed in almost perpendicular to an initial HF crack direction without pressure suggested that CO2 could easily intrude and enhance AE occurrence along a pre-existing crack. Focal mechanisms using P wave first-motion polarities elucidated that many compression-dominant AE events were recorded; these were never observed in similar HF experiment we conducted using water. The compression-dominant events were probably induced by crack closure due to the degassing of injected CO2. Even if CO2 injection induces many AE events, the compressive-dominant events out of them unlikely trigger natural earthquakes because they never create new cracks.

Thermal jet drilling of granite rock: a numerical 3D finite-element study.

This paper presents a numerical study on thermal jet drilling of granite rock that is based on a thermal spallation phenomenon. For this end, a numerical method based on finite elements and a damage-viscoplasticity model are developed for solving the underlying coupled thermo-mechanical problem. An explicit time-stepping scheme is applied in solving the global problem, which in the present case is amenable to extreme mass scaling. Rock heterogeneity is accounted for as random clusters of finite elements representing rock constituent minerals. The numerical approach is validated based on experiments on thermal shock weakening effect of granite in a dynamic Brazilian disc test. The validated model is applied in three-dimensional simulations of thermal jet drilling with a short duration (0.2 s) and high intensity (approx. 3 MW m-2) thermal flux. The present numerical approach predicts the spalling as highly (tensile) damaged rock. Finally, it was shown that thermal drilling exploiting heating-forced cooling cycles is a viable method when drilling in hot rock mass. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'.

Effects of fracture density, roughness, and percolation of fracture network on heat-flow coupling in hot rock masses with embedded three-dimensional fracture network

This paper presents a model to reveal the heat transfer mechanism and simulate the heat-flow coupling process in fractured rock masses. Specifically, effects of fracture density, roughness, and percolation on heat-flow coupling are investigated systematically. The fractured rock masses are composed of a discrete fracture network and a rock matrix. Regarding the mesh discretisation for finite element analysis, the rock matrix is discretised into three-dimensional (3D) solid elements, whereas the discrete fractures are modelled by zero-thickness elements. Fracture and matrix elements share the same nodes. Considering the effect of temperature on fluid density and dynamic viscosity, a heat-flow coupled model of fractured rock masses is established with an embedded 3D fracture network. The reliability of the model is verified by comparing it with the analytical solution of a two-dimensional single-fracture heat-flow coupling problem. The effects of fracture density and diameter on percolation probability are studied based on Monte-Carlo tests with each group for 10,000 times. Finally, numerical samples of the 3D discrete fracture network with different geometric parameters are generated to characterise fractured rock masses, and heat-flow coupling numerical simulation is conducted simultaneously. Results show that the percolation of the fracture network is the decisive factor affecting heat-flow coupling. The average outlet flow rate of the percolation network under the same fracture density is much larger than that of the nonpercolation fracture network, which results in a more rapid decrease in the outlet temperature. Other factors such as fracture roughness are also investigated. It is discovered that the effect of fracture roughness on heat-flow coupling is almost negligible for the nonpercolation fracture network model.

Excavation based enhanced geothermal system (EGS-E): introduction to a new concept

The current technology for enhanced geothermal system (EGS) typically involves drilling of deep injection and production wells to inject cold water into the injection well to the EGS reservoir depth, to extract heat by permeating through the fractured hot rock masses, collected by the production well and return to the surface as heated water. In this article, a new approach is proposed to develop EGS based excavation technology (EGS-E). This method consists of (1) excavation to deep rocks, including a deep shaft from surface to the EGS depth, a room-and-pillar mined cavern complex to be filled with water to become a large underground heated water reservoir in the hot rocks; (2) enhanced heat extraction from rock, aided by additional drillholes spreading out and down from the caverns, and induced fracturing of the surrounding rocks for enhanced rock–water heat transfer; (3) enclosed heat transmission from heated water reservoir to the power plant by using independent heat conducting pipes running between them, continuously supplying the heat for power generation, without water movement in the reservoir. EGS-E offers the following advantages: (1) utilizing mechanized rock excavation technology for large volume energy production; (2) greatly increasing heat extraction volume by easily extending vertically and horizontally; (3) maximize the energy transfer from hot rock to the water reservoir; and (4) separated heat transmission from heated water reservoir in EGS to power plant to minimize environmental pollution.

Analytical and numerical modeling for the effects of thermal insulation in underground tunnels

The heat flow generated from the infinite rock mass surrounding the underground tunnels is a major cause for the increasing cooling demands in deep mine tunnels. Insulation layers with lower thermal conductivities on tunnel walls and roof ceilings are believed to supply a thermo-barrier for heat abatement. However, it is found that no systematic theoretical investigations were made to predict and confirm the effectiveness of underground thermal insulation. Specifically, investigations on the underground insulation problems involving heat flows through the semi-infinite hot rock mass and insulation layer were not sufficient. Thus, in this paper, the thermal characteristics, accompanied with heat flow through the semi-infinite rock mass and the insulation layer, were modeled by both analytical and numerical methods with focus on underground mine tunnels. The close agreements between models have indicated that the thermal insulation applied on tunnel surfaces is able to provide promising heat abatement effects.

Remarks on Heat Emission during Operation in Hot Rock Mass and on Possibilities of Its Use

Heat flow in a hot mining department, where the original temperature of rock is greater than 50 0 C has been studied. The total power of electrical equipment installed in the department amounts to about 6 MW. In accordance with the current climate regulations of Polish mining, the air temperature in the workplace, measured with a dry-bulb thermometer may not be higher than 33 0 C. In order to be able to ensure a temperature consistent with the regulations in such department, fresh air supply workings must be equipped with chillers of a total refrigeration capacity of 2 MW. The carried out analysis of the results of climate forecasts and of the results of the rock mass cooling modelling around these workings has shown that approximately 30% of the cooling power is needed to receive the heat from the hot rock massif. Assuming the required air temperature at 28 0 C (enabling an unshortened working time), then the cooling power needed to extract heat from the rock mass would be much greater. The studies carried out show that the amount of heat emitted in the department also depends on the phase of the longwall course (the length of main gates and changes in rock temperature in them). It has been proposed that hot water from condensers supplies the crystallizers of surface (absorption) cooling devices. In this way, reduction of the air-conditioning expenditure will be obtained.

Using the migration of the induced seismicity as a constraint for fractured Hot Dry Rock reservoir modelling

The challenge for Hot Dry Rock technology is to develop a heat exchanger in deep hot rock masses and to circulate a fluid in order to extract its energy to be used at the ground surface. The present day strategy is to take advantage of natural fractures that pre-exist at these depths and to improve their hydraulic properties. The extension of the area with enhanced properties must then be evaluated so that the best locations for further boreholes can be proposed. To date, this development procedure is based on fluid injection at high rates, forcing hydro-mechanical interactions to take place along pressurised fractures. These pore-pressure-driven mechanisms are accompanied by seismic activity. Assuming the validity of poro-elastic theory in the fractured host rock, some authors have derived the virgin hydraulic diffusivity of the fractured reservoir from the analysis of the spatio-temporal growth of the induced seismicity. The present work is aimed at verifying this approach using a numerical code to solve directly for hydro-mechanical interactions in random fracture networks. Our approach assumes that the seismic activity is controlled by a Coulomb shear criterion and we show how the interpretation of spreading rate of the modelled shear activity in a given network coincides with the upscaled virgin hydraulic diffusivity of the same fracture network, calculated from an independent numerical procedure at the reservoir scale. Therefore, it is shown that the direct analysis of the seismicity migration is appropriate to give reliable estimates of virgin hydraulic and mechanic parameters. These parameters can then be used for performing any further quantitative analysis of a reservoir open to the far field. This is of importance, as fluid mass-balance in multi-well exploitation systems will include the contribution from areas surrounding the stimulated zone.

ROUGH SIZE ESTIMATION OF A THERMALLY FRACTURED ZONE IN AN INFINITE HOT ROCK MASS

Concerning the extraction of geothermal energy from a deep thermal reservoir by the downhole coaxial heat exchanger with a thermally insulated inner pipe proposed by Morita et al, we obtained rough estimates of a size of the fractured zone induced by thermal stresses due to injecting cold water into the hot rock mass through the pipe. We assumed complete spherical symmetry of the temperature and stress fields. At the rough estimation, we considered three typical or extreme cases. (1) The fracturing affects neither the loading capacity of a fractured rock mass nor the temperature distribution within the formation. (2) The fractured zone completely loses its loading capacity and is fully invaded by the borehole water. No disturbance of the fracturing makes any difference in the temperature. (3) The rock formation is assumed to have an appropriately increased fictitious conduction substituted for the heat transfer enhanced by the expected convection within the fractured zone in order to discuss the effects of an occurrence of heat convection within the fractured zone on the temperature and stress distributions and the fractured zone size. As a result, the size of the zone has been estimated to be about ten or more times the borehole radius.

Elastic deformation models of Krafla Volcano, Iceland, for the decade 1975 through 1985

The dynamics of the shallow magma reservoir at Krafla Volcano in NE Iceland have been analyzed with three types of elastic models based on over 70 surveys of tilt and displacement made from 1975 to 1985, a period of continuous volcano-tectonic activity. Modeling results are integrated with geophysical and geological information to estimate the position, geometry, and volume change of magma reservoir domains subjected to periodic inflation and deflation. Dominating influences on magma reservoir dynamics are examined in the context of activity in Krafla's associated rift system and the deformation of its caldera from 1975–1985. Rather than the fluid-filled cavity concept of Mogi (1958) and most recent workers, our models are idealized as strained regions with spherical, double-spherical, or general ellipsoidal symmetry. The models are mathematically generalized from that of Mogi (1958) and are derived by inversion of displacements. Model results from different displacement components are remarkably consistent, although models based on vertical displacements typically have errors-of-fit much closer to expected measurement errors than those based on tilt or horizontal displacements. About one half of the double-sphere and ellipsoid models have significantly better fits than single-sphere models. Double-sphere model results are consistent with a 2 to 3 km center-to-center separation of magma storage zones from at least 2 km to 4 km depth by portions of the fissure system, as implied by S-wave attenuation patterns for 1976–1977. However, all models suggest pressurization zones of more limited extent than possible domains of storage inferred from S-wave attenuation. Ellipsoid models typically implied unrealistically shallow depths of magma storage. Caldera inflation rates decreased after January 1978 when the caldera periodically reinflated to its level prior to the initial December 1975 deflation event. From 1975–1980 the volume and duration of inflation between deflation events was strongly correlated with the volume of the previous deflation. After 1980 there was a significant increase in the duration of inflation periods and a decrease in rates of caldera inflation and fissure system widening. Consistent with these results, the magma reservoir is conceptualized as a hot rock mass containing numerous magma chambers and pressure-sensitive conduits connecting the chambers and deep magma sources. Magma is injected into the fissure system at critical pressures determined by the confining stress and rock mass strength. The duration and volume of inflation required to reach a critical pressure threshold is largely dependent on the volume of magma released in the previous deflation. Reduction of the extension rate across the Krafla fissure system after 1980 suggests that extensional forces were also reduced. A resultant rise of confining pressure on the magma reservoir and a reduced capacity of the fissure system to accommodate dike injection in combination would have increased critical stress levels for reservoir deflation and reduced the pressure gradient driving magma supply from deep sources.

Geothermal heat extraction by water circulation through a large crack in dry hot rock mass

AbstractOne proposed geothermal heat extraction scheme relies on water circulation in a large vertical crack created by hydraulic fracturing in a hot dry impermeable rock mass. Water flow, heat convection and crack opening widths are analysed by finite elements. Governing field equations of the problem are first set up rigorously and then various small terms are identified and neglected, retaining the effects of pressure gradient, buoyancy, velocity head (kinetic energy) and head loss due to viscous friction in the water flow equation, and the effects of heat convection in water and heat conduction in rock in the heat transfer equation. The finite element scheme for water flow is based on a variational principle that is typical for diffusion problems, and for heat transfer it is based on the method of least‐square residuals. The system of differential equations is highly non‐linear. The non‐linear terms and coefficients are treated in the fiaite element analysis as constant; the finite element analysisof, the steady‐state pressures, fluxes and temperatures is then iterated, evaluating all non‐linear terms and coefficients on the basis of the solution obtained in the previous iteration. Numerically calculated fields at various times after the start ofcooling are presented. They indicate some features favourable for the geothermal scheme, such as formation of eddy currents, and downward flux of water toward hotter rock. However, other important questions would have to be solved to gain full understanding, of this proposed geothermal scheme.

The analysis and study of hot-spring water in Foochow

The hot spring waters are of two types, natural springs and artificially dug wells. Chemical analyses show that they are of volcanic origin, descending surface waters being heated by coming in contact with hot rock masses.