This paper presents a study of the micromechanics of the coupled hydro-mechanical (H-M) behavior of brittle rock during hydraulic fracturing. The study is conducted using the discrete element method (DEM), which spatially discretizes a rock mass into discrete disc particles, coupled with a solver for modeling fluid flow through a network of connected pores. In the coupled H-M DEM modeling, fluid flows through newly formed hydraulic fractures due to pore pressure increase from fluid injection in a wellbore and is coupled with rock mechanical response across a wide range of flow rates. The micromechanical insights from the DEM modeling provide better understanding of coupled H-M processes which precede rock breakdown during hydraulic fracturing, and the transition in deformation in brittle rock from a single hydraulic fracture to branched hydraulic fractures and a diffused damage zone. The effects of the properties of the fracturing fluid and the rock matrix as well as the effects of loading flow rate on the development of pressure-induced deformation and fracturing in crystalline brittle rocks fracturing are investigated. The DEM models used properties that were obtained by calibrating flow-rate-dependent stress–strain response against previously published experimental data on brittle rocks (granite in particular). DEM results are compared with experimental results from a true-triaxial scale model testing on hydraulic fracturing in an analogue rock. The presented results are expected to enable better understanding of conditions which lead to successful fracture propagation versus damage and fracture arrest in geo-reservoirs.

This paper presents the results of a comprehensive micromechanical study to improve the understanding of the coupled hydro-thermo-mechanical (HTM) processes during injection of pressurized cold fluid in permeable hot rock. At the microscopic level, it is expected that fluid flow–induced convective temperature changes in the voids will dominate the conductive heat flow from grain to grain. However, the coupled interactions between fluid flow, thermal convection, and conduction, and the resulting changes in local permeability due to thermo-poroelastic stresses at the grain scale remain poorly understood. Moreover, almost all previous studies at a large scale have focused only on thermal conduction. This study used the discrete-element method (DEM) modified to account for convective heat transport to model the particulate interactions between rock grains and between rock deformation, fluid pressure, and temperature. The results indicate that wellbore pressure, provided it is less than the fracturing threshold, has a less significant role in rock cooling around the wellbore, which is governed predominately by rock permeability, as previously thought. Different applied flow rates into the wellbore resulted in low and high wellbore pressures at short times in the absence of rock fracturing. As expected, convective heat transport dominated over conduction, resulting in a cooled ring around the wellbore without a significant thermal gradient zone due to conductive heat flow. As the fluid infiltrates and cools the rock, preferential fluid flow paths occur as fingering instabilities that are oriented toward the minimum principal horizontal stress even in the absence of initial local anisotropic porosity variations.

This paper shows results and analysis of experimental investigation for coarse, medium and fine sand flow and transport into narrow slots. The presented research is conducted within a context of proppant flow and transport in geo-reservoirs during placement into fractures. Proppant is granular material which is placed into hydraulic fracture for propping the fracture open and enhancing production in geo-reservoirs. The experimental investigation presented in this paper contributes to better understanding of proppant-fluid slurry flow and transport in narrow fractures and complement current empirical relationships derived from wider slot experiments. At higher proppant concentrations in narrow and constrained fractures inter-particle and particle-wall contact interactions affect the general slurry flow and transport, phase motions and their relationships. In this work, various volumetric concentrations of coarse, medium and fine silica sands are injected into a relatively narrow fracture under different flow rates and using Newtonian carrying fluids. Results indicate and quantify the significance of particle size in a narrow slot on various flow regimes observed. Sediment transport theories are used to quantify flow regimes and investigate whether the Rouse number can be used to correctly predict the flow regime in a narrow fracture. Presence of turbulence in slurry motion is evaluated towards optimal flow and transport outcomes. The presented results will aid geothermal and petroleum engineers to better design proppant transport, increase utilization of sustainable sand materials and better understand fundamental flow and transport of dense-phase slurries in narrow fracture zones.

This paper presents the results of an experimental study of dense sand particles’ settling micromechanics in narrow smooth and rough fractures. Particle Image Velocimetry (PIV) is used for the analysis of velocities of individual particles and groups of particles and their relative paths, collisions, and agglomerating in viscous Newtonian fluid. The displacement vectors obtained through PIV analysis show the movements and velocities of individual and groups of particles and the global velocity trends of the observed area. Experiments were conducted in a relatively narrow 2-mm slot compared to the main particle size of 0.66 mm across. Smooth and rough fracture walls are considered, in which acrylic plates and 3-D–printed hydraulic fracture replica were used. The measured results from this experimental study give new insights into the effects of particle and agglomerate size and shape as well as fluid dynamic viscosity on slurry settling velocity. It was found that an increase in fluid dynamic viscosity promotes the formation of larger agglomerated particles of sand, which, in return, affect overall slurry settling velocity. Observations from rough fracture wall experiments yielded significantly different slurry behavior, with increased erratic particle motions and fewer agglomerations compared to the smooth, idealized fracture walls. Experimental results give new insights into slurry settling in narrow fractures, which has applications to proppant settling in fractures for the oil, gas, and geothermal industries.

This review paper summarizes recent advances and challenges in the assessment of rock behavior and performance in deep low-permeability and high-temperature geothermal reservoirs. Geothermal energy systems for electricity production target deep rock between ca. 2 km and 5 km depth to obtain sufficiently elevated temperatures. Rock permeability enhancement faces many challenges, and therefore the development of Enhanced Geothermal Systems (EGS) still represents a pioneering effort. The potential and advantage of EGS above conventional geothermal reservoirs is its independence of the location that supplies sufficient heat and fluid. Several issues prevent the successful application of EGS technology. First, the effects of non-uniform in-situ stresses and loading history on rock fracturing are not well understood. Second, the role of rock anisotropy, heterogeneity and thermal effects on rock properties in the design of hydraulic fracturing operations is not clear. Third, the reduction of induced seismicity effects raises safety and public acceptance issues. This manuscript formulates outlines for future research directions. Specifically, the recommendations focus on the development of tools for better understanding and mitigating problems, which occur during stimulation of deep geothermal reservoirs.

This paper presents fundamental analysis and micromechanical understanding of dense slurry behavior during settling in narrow smooth and rough slots. Particularly, this study seeks to contribute toward better understanding of dynamics of particle–particle and particle–wall interactions in viscous fluids using simple experiments. The findings of this study are applicable in a wide variety of problems, for example sediment transport, flow and transport of slurry in pipes, and industrial applications. However, the results interpretation focuses on better understanding of proppant flow and transport in narrow fractures. A sequence of experiments image frames captured by video camera is analyzed with particle image velocimetry (GeoPIV). The measurements include vertical velocities and displacement vectors of singular and agglomerated particles and larger area of formed slurry. Results present novel insights into the formation and effects of agglomerates on general slurry settling, and are supplemented with a comparison with previously published theoretical and empirical relationships. This work also emphasizes a role of particle–particle interactions in promoting agglomeration in viscous fluid. Particularly, a thin layer of viscous fluid between approaching particles dissipates particle kinetic energy due to lubrication effect. Lubrication effect is more pronounced when particles are constrained between two narrow walls and interact frequently with each other. Fluid tends to flow around agglomerated particles, and agglomerates remain stable for prolonged time periods gravitationally moving downward. The relative amount and size of agglomerated affects general settling of the slurry. It was found that fluid viscosity due to lubrication effect promotes agglomeration, and therefore, the overall slurry settling relatively increases at higher fluid viscosities. The results of the presented work have impact on various industrial and engineering processes, such as proppant flow and transport in hydraulic fractures, sand production in oil reservoirs, piping failure of dams and scour of foundation bridges.

This paper presents an investigation into the suitability of abandoned wells in California for Enhanced Geothermal Systems (EGS) and low temperature deep Borehole Heat Exchanger (BHE) applications. The study identifies three counties characterized by high numbers of abandoned wells, medium to high crustal heat flows (75–100 mW/m²), and suitable sedimentary geology: Santa Clara, Monterey, and Santa Barbara. Thermal gradients range between 4 and 7.3 °C/100 m and enable access to the bottom hole temperatures between 40 and 73 °C for an average 1000 m deep well. These rock temperatures are sufficient for low-temperature direct use EGS such as district heating, greenhouse heating, and aquaculture. Economically, the mitigation of drilling costs and the documented lithology both reduce the risk associated with EGS. However, hydraulic fracturing of loosely to moderately consolidated sedimentary rock in transitional stress regimes remains one limitation to the EGS conversion of these abandoned wells. Alternatively, the feasibility of deep BHE applications within abandoned oil and gas wells is demonstrated here with a mathematical model. Predictions show that outlet fluid temperatures >40 ^°C can be achieved for 1000 m deep wells in regions with temperature gradients >7 °C/100 m.

This paper presents an improved understanding of coupled hydro-thermo-mechanical (HTM) hydraulic fracturing of quasi-brittle rock using the bonded particle model (BPM) within the discrete element method (DEM). BPM has been recently extended by the authors to account for coupled convective–conductive heat flow and transport, and to enable full hydro-thermal fluid–solid coupled modeling. The application of the work is on enhanced geothermal systems (EGSs), and hydraulic fracturing of hot dry rock (HDR) is studied in terms of the impact of temperature difference between rock and a flowing fracturing fluid. Micro-mechanical investigation of temperature and fracturing fluid effects on hydraulic fracturing damage in rocks is presented. It was found that fracture is shorter with pronounced secondary microcracking along the main fracture for the case when the convective–conductive thermal heat exchange is considered. First, the convection heat exchange during low-viscosity fluid infiltration in permeable rock around the wellbore causes significant rock cooling, where a finger-like fluid infiltration was observed. Second, fluid infiltration inhibits pressure rise during pumping and delays fracture initiation and propagation. Additionally, thermal damage occurs in the whole area around the wellbore due to rock cooling and cold fluid infiltration. The size of a damaged area around the wellbore increases with decreasing fluid dynamic viscosity. Fluid and rock compressibility ratio was found to have significant effect on the fracture propagation velocity.