Estimation of borehole temperature disturbed by drilling

Abstract

Four new thermal gradients have been determined for the Choogaryung Rift Valley. The thermal regime in the interior part of the Rift Valley is characterized by low temperature gradients in the range 16.2 to 17.3°C km⁻¹. Disturbances of borehole temperature by drilling depend on the physical properties and rock type, drilling fluid, drilling rate, casing and depth of penetration. We calculated the effects of drilling fluid in the borehole through an integrated analysis of temperature and geological data for the formation temperature in the early stage after drilling. Compared to four boreholes drilled in connection with the intensive search for thermal regime between November 16 and 30 of 2000, holes in which temperature logs were obtained are plentiful. From an intense data set of temperature logs, formation temperature provides the opportunity for comparisons and calculations of geotherms. The temperature divergence due to drilling fluid at time t≥90 hours is negligible, while the result is shown as a valuable indicator for different time, t=40 hours. The temperature disturbance has been satisfactorily tested with the analytic calculations using heat conduction equation. With evidence for recent tectonothermal events the variations of the heat flow values in the study area can be mostly explained on the basis of heat conductivity of the crustal rock.