Thermally Conductive Proppants to Improve Heat Extraction in Geothermal Systems

Abstract

Abstract Due to the low permeability of geothermal reservoirs, hydraulic fracturing could provide conductive pathways to improve fluid flow and heat exchange. Fractures in geothermal systems not only provide channels for fluid flow, but also provide a larger contact area for heat transfer to achieve an efficient and economic heat extraction. Since commercially available proppants have low thermal conductivity, we present a method to improve heat exchange inside propped fractures by increasing thermal conductivity of the proppant packs using conductive coating. Accurate numerical modeling requires a representative proppant pack sample with particle size distribution, sorting, and shape similar to a physical sample. Thus, using a dynamic method of generating a granular packing, we construct a proppant pack under the influence of gravity through discrete element method to obtain a sample under representative in-situ conditions. We apply uniform conductive coatings of different thickness to the proppants by taking advantage of image processing techniques. Finally, we simulate heat transfer through the proppant pack using finite volume methods to compute the effective thermal conductivity of the coated proppants and determine the improvement in heat exchange inside the propped fractures achieved by applying such a conductive coating. To calibrate our model, we conducted a validation simulation and compared the results to the available analytical results. Particle-scale analysis revealed the effectiveness of using conductive coating in improving the thermal conductivity of the proppant used. Adding a thin layer of copper coating to sand proppant is observed to increase its thermal conductivity by more than a 100%. Results also showed a positive correlation between the thickness of the conductive coating and the resulting effective thermal conductivity of the proppant. The observed increase in the proppant thermal conductivity can increase the rate of heat exchange by providing additional surface area for heating up the circulating fluid and thus allowing more heat extraction from geothermal reservoirs. Through modeling proppants at the particle-scale, we find that applying a conductive coating can remarkably increase the effective thermal conductivity of the proppant pack and promote heat exchange between the proppants and the circulating fluid in addition to fracture walls. Particle-scale modeling allows to understand how individual proppants interacts with each other under elevated temperatures and high closure stress and how such particles interactions control the effective thermal conductivity of the proppant pack.