Multivariate Statistical Analysis of Borehole Heat Exchanger Drilling Efficiency Using Field Measurements

Drilling operations for shallow geothermal borehole heat exchangers (BHEs) represent a crucial and energy-intensive stage of system installation, where operational parameters directly affect drilling duration and fuel consumption. This study evaluates the operational efficiency of down-the-hole (DTH) rotary-percussive drilling using a Beretta T-57 rig during the construction of 18 boreholes at an experimental geothermal field in Młoszowa, Poland. A multivariate statistical framework was applied to investigate the combined effects of rotational speed (40 rpm, 50 rpm, and 60 rpm) and compressed air pressure (20 bar, 22 bar, and 24 bar) on drilling time and fuel consumption (compressor and drilling rig) within a controlled 65–71 m depth interval in massive limestone. While univariate analysis of variance (ANOVA) did not reveal statistically significant individual parameter effects, multivariate analysis of variance (MANOVA) identified rotational speed as a dominant factor governing the coupled system response, as confirmed across multiple multivariate test statistics (Wilks’ Lambda = 0.1702, p = 0.0301; Pillai’s Trace, p = 0.0203; Roy’s Largest Root, p = 0.0192). Also, a significant parameter interaction (rotation × pressure) was indicated by Roy’s Largest Root (p = 0.0292), highlighting potential localized coupled operational effects. Descriptive patterns indicate that operating at intermediate pressure (22 bar) and rotational speed (50–60 rpm) enhances process stability while reducing operational energy demand. The findings demonstrate that BHE drilling performance should be evaluated as an integrated mechanical–energy system, providing a quantitative basis for drilling contractors to optimize fuel consumption and execution time in shallow geothermal projects.