Distribution of the temperature field in a wellbore and surrounding rocks after completion of logging

Abstract

Background. The possibilities of using well geophysical logging data to assess the zone of wellbore influence on the temperature field of formations in the near-wellbore space are considered. The study is based on the results of interpretation of geophysical logging data from a well drilled within the Solotvyno Depression of the Transcarpathian Trough.

Methods. The thermophysical properties — thermal conductivity and heat capacity — of rocks of different composition and genesis within the well section were calculated. Maxwell's equation was used to calculate the thermal conductivity. The value of the thermal conductivity of the fluid filler was calculated based on the additive effect of individual components: formation water and gas. Using classical equations, namely the Fourier equation describing radial heat-flow propagation from the wellbore and the heat-capacity equation, the radial heat-flux density and the extent (depth) of the zone of significant wellbore influence on the temperature distribution in the near-wellbore space were determined.

Results. The effect of the shut-in time after cessation of drilling-fluid circulation on temperature changes in the wellbore, the radial heat-flux density, and the extent of the wellbore influence zone was investigated. With increasing time after well shutdown (downtime, or relaxation time), the relative error in estimating the true formation temperature relative to the measured temperature in the well decreases sharply. The new approach in calculations is that the temperature distribution around the well is represented by analogy with the formation of a drilling fluid filtrate invasion zone. The largest gradient in error reduction occurs during the first two days after the circulation of the flushing fluid in the well is stopped. It was established that, in addition to the thermophysical parameters of the formations, the substantial variability of the wellbore diameter should be taken into account when calculating the current formation temperature in the well.

Conclusions. An express method is proposed for calculating the radial heat-flux density and the extent (depth) of the zone of significant wellbore influence on the temperature distribution in the near-wellbore space. The proposed calculations make it possible to assess the parameters of wellbore impact on rock temperature in the radial direction at different times after termination of drilling-fluid circulation.