This paper presents an investigation on 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/m 2 ), 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 40-73 °C for an average 1,000 m deep well. These rock temperatures are sufficient for low-temperature direct use EGS such as district heating, greenhouse heating, and aquaculture. Abandoned wells reuse 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. The feasibility of deep BHE applications within abandoned oil and gas wells is demonstrated here by a mathematical model. Predictions show that outlet fluid temperatures >40°C can be achieved for 1,000 m deep wells in regions with temperature gradients >7 °C /100 m.

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The micromechanics of proppant settling in quiescent fluid in a rough and relatively narrow rock fracture is investigated. The study focuses on particleparticle and particle-wall interactions in a dense-phase particle settling. The study used a coupled discrete element method and computational fluid dynamics (DEM-CFD) code because DEM-CFD is the most suitable computational method for modeling the frequent interactions of dense assembly of rigid particles and enables modeling of two-way solid-fluid interactions. Due to frequent particleparticle interactions of grains submerged in fluids, the particle interaction model in DEM is improved by the incorporation of the effects of lubrication due to a layer of fluid surrounding particles. Results of the numerical study are compared to previous experimental and theoretical relationships. The findings of the study highlight conditions that lead to proppant aggregation due to the fluid viscosity and fracture width in relation to particle diameter ratio. In the light of the DEM-CFD results, it was found that published relationships are inadequate in describing the settling rates for proppant in a rough and narrow hydraulic fracture and high fluid viscosity. Micromechanical particle interactions during settling and erratic upward and fluid counter-flow may cause proppant trajectories that are not always in the direction of gravity in a rough fracture resulting in clogging of the fracture or forming faster settling particle agglomerates. The maximum packing density 0.3-0.4 (3.9-5.9 lbs/gal) of proppant in a narrow and rough hydraulic fracture was obtained in this study, which is lower than the usually assumed one of 0.5 (9.8 lbs/gal) for any given fracture roughness.Keywords DEM-CFD Á Lubrication Á Fluid-solid coupling Á Dense-phase flow Á Proppant settling Á Hydraulic fracture List of symbols