The mathematical description of the variation of the rate of penetration, and, consequently, the cumulative rotating time, with depth is potentially a powerful analytical tool in the study of well drilling, especially in relation to well cost estimating. This report describes the theory and practical aspects of one suggested method of mathematical treatment and presents results from the subsequent analysis of twenty-six wells drilled recently in two land areas of the U.K. with potential low-enthalpy geothermal energy resources