Interpretation of Thermal Response Tests in Borehole Heat Exchangers Affected by Advection

ABSTRACT We focus on the treatment of thermal response test data when both advection and short-period changes of surface temperature occur. We used a moving line source model to simulate temperature-time signals under an advective thermal regime. The subsurface thermal conductivity, the Darcy velocity and the borehole thermal resistance were inferred by means of an optimisation procedure. In case of Darcy velocity lower than 10 -7 m s -1 , the underground thermal conductivity is comparable to that obtained by means of the infinite line source model, which assumes a purely conductive thermal regime. The optimisation analysis was finally applied to real thermal response test data. The temperature-time curves were filtered to remove the disturbing spectral components associated with a non-optimal thermostatic behaviour of the apparatus. This produced reliable estimates of thermal and hydraulic parameters. An independent method based on the analysis of temperature-depth logs was also used to validate the inferred groundwater flow. INTRODUCTION The thermal power that can be extracted with borehole heat exchangers (BHE) depends mainly on the thermal properties of the underground, and in particular, on thermal conductivity. Laboratory measurements of thermo-physical properties can be unfeasible, as core samples are often unavailable in boreholes. Thus, in-situ tests are routinely used to determine subsurface and borehole thermal properties. Tests record the underground temperature variation with time due to a constant heat that is injected (or extracted) by means of a carrier fluid into a borehole acting as a heat exchanger. The most commonly used model to analyse temperature-time curves obtained from these tests is the infinite line source (ILS). If some conditions are fulfilled, this model can give rapid and appropriate estimations of thermal parameters. On the other hand, several flaws can often affect the data interpretation. Some of them are related to the model assumptions, which imply a purely conductive heat transfer regime, a homogeneous medium, no vertical heat-flow and infinite length of the borehole. Others have to do with the difficulty in the proper thermal insulation of the test equipment, and consequently with the oscillations of the carrier fluid temperature due to surface air temperature changes, which generally produce a periodic offset in the recorded temperature-time curves. In this paper, we discuss the treatment of the thermal response test (TRT) data when both advection caused by groundwater flow and periodic changes of air surface temperature occur. An approach based on the moving line source (MLS) model is tested with simulations of temperaturetime signals obtained under different hypothesis of thermal and hydraulic conditions. Then, the same procedure is applied to real TRT data to estimate thermal conductivity, borehole thermal resistance and Darcy velocity. The magnitude of the inferred groundwater flow is finally checked by means of an independent method based on the analysis of the undisturbed temperaturedepth records

A computational approach for 3D modeling and integration of heterogeneous geo-data

This paper tackles the volumetric representation of geophysical and geotechnical data, gathered during exploration surveys of the subsoil; in particular, we focus on the modeling and analysis of underwater deposits. The creation of a 3D model as support to geological interpretation has to take into account the heterogeneity of the input data, coming from offshore acquisition campaigns. Some data are massive, but cover the domain unevenly, e.g., along dense differently spaced lines, while others are very sparse, e.g., borehole locations with soil sampling and CPTU (Piezocone Penetration Test) locations. A automatic process is presented to generate the subsurfaces and volume defining a sub-seabed deposit, starting from the identification of relevant morphological features in seismic data. In particular, simplification and refinement based on geostatistics have been applied to generate regular 2D meshes from strongly anisotropic data, in order to improve the quality of the final 3D tetrahedral mesh. Furthermore, we also use geostatistics to predict geotechnical parameters from local surveys and estimate their distribution on the whole domain: in this way the 3D model will include relevant geological features of the deposit and allow extrapolating different geotechnical information with associated uncertainty. The volume characterization and its 3D inspection will support geological analysis and planning of future engineering activities. The developed methodology has been tested on two real case studies