Time of refreezing of surrounding the wellbore thawed formations

Because deep wells in permafrost areas are usually drilled with a warm mud, there is some unknown degree of formation thawing around the wells. The warm mud alters the temperature field and as a result thaws the permafrost. To determine the static temperature of the formation and permafrost thickness, one must wait for some period after completion of drilling before conducting temperature logs. The duration of the refreezing of the layer thawed during drilling strongly depends on the natural temperature of formation; therefore, the rocks at the bottom of the permafrost refreeze very slowly. A lengthy restoration period of up to ten years or more is required to determine the temperature and thickness of permafrost with a sufficient accuracy. In this study, we present a new method of estimation of the time of refreezing of thawed formations (surrounding the borehole) by utilizing temperature logs. It also shown that the same basic equation (modified Horner plot) can be used for predicting the formation temperature. To our best knowledge, this study is the first effort to determine the duration of the refreezing period. The physical basis of this approach consists of non-conventional utilizing of three temperature observations in borehole after the freezeback completion. To demonstrate the applicability of the new method we present six examples: three field cases, one laboratory experiment, and two cases of numerical modelling.