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Documentation of Yade-DEM. Please refer to a more recent version of this document at https://zenodo.org/record/5705394

A pore scale numerical method dedicated to the simulation of heat transfer and associated thermo–hydro-mechanical couplings in granular media is described. The proposed thermo–hydro-mechanical approach builds on an existing hydro-mechanical model that employs the discrete element method for simulating the mechanical behavior of dense sphere packings and combines it with the finite volume method for simulating pore space fluid flow and the resulting hydro-mechanical coupling. Within the hydro-mechanical framework, the pore space is discretized as a tetrahedral network whose geometry is defined by the triangulation of discrete element method (DEM) particle centers. It is this discretization of DEM particle contacts and tetrahedral pore spaces that enables the efficient conductive and advective heat transfer models proposed herein. In particular, conductive heat transfer is modeled explicitly between and within solid and fluid phases: across DEM particle contacts, between adjacent tetrahedral pores, and between pores and incident particles. Meanwhile, advective heat transfer is added to the existing implicit fluid flow scheme by estimating mass–energy–flux from pressure induced fluid fluxes. In addition to the heat transfer model, a thermo-mechanical coupling is implemented by considering volume changes based on the thermal expansion of particles and fluid. The conduction and advection models are verified by presenting comparisons to an analytical solution for conduction and a fully resolved numerical solution for conduction and advection. Finally, the relevance of the fully coupled thermo–hydro-mechanical model is illustrated by simulating an experiment where a saturated porous rock sample is subjected to a cyclic temperature loading.

Spam, the Software for the Practical Analysis of Materials is a Python library that has evolved to cover needs of data analysis from 3D x-ray tomography work and correlated random fields with mechanical applications. Spam is first and foremost a measurement package that extends the extremely convenient framework of NumPy and SciPy by providing or accelerating tools for the material- science/mechanics oriented analysis of 2D images or 3D volumes representing field measurements. Typical uses are either the measurement of displacements fields between images of a deforming sample from which strains can be computed, or the characterisation of a particular microstructure (correlation length or particle orientation). The package is organised into a library of Python tools which are expected to be used in user-written scripts and a number of more sophisticated standalone scripts. Please see the online documentation. Spam is developed on a git repository hosted at Université Grenoble Alpes. This deposit is the version 0.5.2 that was reviewed and accepted on the Journal of Open Source Software

This study details the acceleration techniques and associated performance gains in the time integration of coupled poromechanical problems using the Discrete Element Method (DEM) and a Pore scale Finite Volume (PFV) scheme in Yade open DEM software. Specifically, the model is tailored for accuracy by reducing the frequency of costly matrix factorizations (matrix factor reuse), moving the matrix factorizations to background POSIX threads (multithreaded factorization), factorizing the matrix on a GPU (accelerated factorization), and running PFV pressure and force calculations in parallel to the DEM interaction loop using OpenMP threads (parallel task management). Findings show that these four acceleration techniques combine to accelerate the numerical poroelastic oedometer solution by 170x, which enables more frequent triangulation of large scale time-dependent DEM+PFV simulations (356 thousand+ particles, 2.1 million DOFs).

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.

A method was developed for experimentally observing the evolution of fracture aperture/permeability within a granite-based Enhanced Geothermal System (EGS) in an attempt to better understand the reduction of fracture permeability due to chemical and mechanical processes. Specimens of granite were subjected to flow-through column-like experiments to characterize the evolution of fracture aperture/permeability in a near-field setting for 20 and 40 days. Near-field experimental conditions approached in-situ shallow EGS conditions: granite rock, tensile fracture, 120 °C temperature, and 25–35 MPa effective stress. Following each 20–40 day experiment, permeability, fracture aperture, and mass of minerals dissolved were computed using pore-pressure observations, effluent chemistry, and non-concurrent X-ray computed tomography (CT) scan imaging. Results showed an increased upstream pore pressure, which could indicate a decline in permeability due to a combination of the dissolution of fracture propping asperities and mechanical creep. Effluent solution composition was in agreement with the dissolution of silicates (most notably feldspars) which points to a geochemically driven decrease in aperture due to mineral dissolution rather than mechanical effects. In comparison, the pore pressure measurements corresponded to a greater aperture change than the effluent chemistry (0.127 vs 0.101 μm, respectively). This discrepancy may be attributed to the contribution of other physical processes to fracture aperture change, including mechanical creep. Similarly, the CT scan showed a decreased fracture aperture, but the low resolution and non-concurrent scans influenced higher aperture closure estimates in comparison to the effluent chemistry and pore pressure measurements. However, the CT-scan confirmed the scale-dependency of the chemical and mechanical processes. Fundamentally, the study demonstrated a method for ensuring near-field conditions and observing the chemical and physical processes associated with fracture permeability decline.

The combination of the Ground Source Heat Pump (GSHP) with energy piles has become a viable alternative to traditional indoor climate control. As geothermal energy pile applications increase in size and demand, maintaining the highest possible efficiency is paramount. Therefore, a numerical analysis was performed to investigate the relationship between construction specifications and energy pile efficiency. In particular, inlet flow rate and concrete cover were parameterized within two heat exchanger tube configurations including “U-shape” and “W-shape” configurations. Additionally, cyclic and constant thermal loadings were compared using COMSOL multi-physics software. Simulation results indicate that heat rejected is limited by surface area of the heat exchangers and residence time of working fluid. For this model, an apparent maximum heat rejected was achieved at 240 cm 3 /s/pile (3.8 gpm) for both U-shape and W-shape configurations. Beyond 240 cm 3 /s, the increase of rejected heat was negligible. The study also demonstrated that concrete cover contributes significantly to energy pile efficiency. A reduction of concrete cover decreased potential cyclic heat rejected by up to 20%. The comparison of tube configurations bolstered conclusions in the literature that the W-shape configuration is most effective at rejecting heat, improving efficiency by approximately 20% compared to the U-shape configuration.