Analysis of Lost Circulation During Drilling in Cooled Formations

Abstract

Abstract A few of the lost circulation incidents that have occurred at the Statfjord field and the Veslefrikk field appeared to have been caused by drilling through cooled formations. These lost circulation incidents have been analyzed and compared with offset wells drilled with full returns. We have made plausible that the presented cases of lost circulation are caused by the fact that these wells have been drilled too close to a water injector. Injection data have been analyzed to determine the thermo-elastic stress changes around these injectors. To quantify the thermal cooling effect, downhole pressure data measured during the mud loss incident are analyzed. We found a significant reduction both in the breakdown pressure and minimum horizontal stress. When comparing these data with a thermo-elastic model, the theory predicts a much stronger effect than the interpreted downhole pressure measurements. Numerical simulations of near-wellbore thermal stress effects showed that differences between the mud temperature and the formation temperature could significantly increase the wellbore stresses toward compression. However, the combined near-wellbore thermal effects of formation cooling and a warm mud could not explain the discrepancy between the model and the interpreted stress measurements.