MULTIPHYSICAL COUPLED MODELING FOR DISTRIBUTED THERMAL RESPONSE TESTING OF HETEROGENEOUS LITHOLOGIES

A thorough assessment of the thermal properties of geologic materials in the subsurface is critical for designing shallow borehole heat exchange (BHE) and optimize ground source heat pumps systems that reduces their installation costs. A distributed thermal response test (DTRT) is a hybrid method coupling a conventional thermal response test (TRT) and fiber-optic distributed temperature sensing (FO-DTS). Performing a DRTR allows for an analysis of thermal properties in the heterogenous subsurface layer by layer. During a study of a deep borehole at University of Illinois at Urbana-Champaign DTRT data was collected to a depth of 97.5 m. We obtained the thermal gradient with 1-meter spatial resolution both inside and outside of a geothermal loop installed in the borehole.

To simulate the in-situ thermal regime during the DTRT, a 3-D multiphysical model with temperature was developed using COMSOL^® software that allowed estimating the transient heat transfer within the geologic materials and as non-isothermal pipe flow. The simulations combining a heterogeneous thermogeology with complex heat conduction and groundwater flow were modeled in tandem with the BHE system using typical design parameters (i.e., drilling depth and borehole diameter, grout type, fluid flow, and heating power). Thermal properties of materials along the length of borehole (i.e., thermal conductivity, thermal diffusivity and borehole resistance), were measured in the laboratory. In addition, the thermal conductivity values were derived from the analytical models based on the infinite line source method. This procedure utilized the FO-DTS data recorded inside the geothermal loop, and at the interface between the grout and the outside wall of the tubing during the heat injection phase of the DTRT.

During the modeling, simulations were run for four parameters and three scenarios to determine heat transfer and non-isothermal pipe flow. An orthogonal experimental design was used in order to optimize the modeling procedure; reducing the number of runs from 64 to 9. The variance in ground temperature between model and FO-DTS was evaluated and used to calibrate the model. Due to various impacts on the subsurface thermal regime, the changes in temperature returned from simulations using the different parameters and scenarios was evaluated and predicted during both the heating and cooling phases.

Keywords: distributed thermal response test, borehole thermal resistance, thermal conductivity, borehole heat exchanger, distribute temperature sensing