ABSTRACT
Temperature logs are commonly used to determine the permafrost temperature and thickness. When wells are drilled through permafrost, the natural temperature fi eld of the formations (in the vicinity of the borehole) is disturbed and the frozen rocks thaw for some distances from the borehole axis. To determine the static temperature of the formation and permafrost thickness, one must wait for some period after completion of drilling before making geothermal measurements. This is so-called restoration time. Usually a number of temperature logs (3-10) are taken after the well’s shut-in. Signifi cant expenses (manpower, transportation) are required to monitor the temperature regime of deep wells. In this chapter we show that in most of the cases when the time of refreezing formations is relatively short (in comparison with the shut-in time) two temperature logs are suffi cient to predict formations temperatures during shut-in, to determine the geothermal gradients, and to evaluate the thickness of the permafrost zone. Thus the cost of monitoring the temperature regime of deep wells after shut-in can be drastically reduced. The presence of permafrost has a marked effect on the time required for the near-well-bore formations to recover their static temperatures. The duration of the refreezing of the layer thawed during drilling is greatly dependent 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 suffi cient accuracy (Melnikov et al. 1973; Taylor and Judge 1977; Judge et al. 1979, 1981; Taylor et al. 1982; Lachenbruch et al. 1988). Earlier we suggested a “two point method ” (Kutasov 1988) which permits one to determine the permafrost thickness from short term (in comparison with the time required for temperature restoration) downhole temperature logs. The “two point method” of predicting the permafrost thickness is based on determining the geothermal gradient in a uniform layer below the permafrost zone. Only temperature measurements for two depths are
needed to determine the geothermal gradient. The position of the permafrost base is predicted by the extrapolation of the static formation temperature -depth curve to 0ºC. It should be noted that here the permafrost base is defi ned as the 0ºC isotherm. Precise temperature measurements (Taylor and Judge 1977; Judge et al. 1979) taken in 15 deep wells located in Northern Canada (Arctic Islands and Mackenzie Delta ) were used to verify the proposed method. Let us assume that at the moment of time t = tep the phase transitions (water-ice) in formations at a selected depth are completed, i.e., the thermally disturbed formation has frozen. In this case at t > tep the cooling process is similar to that of temperature recovery in sections of the well below the permafrost base. It is well known (Tsytovich 1975) that the freezing of the water occurs in some temperature interval below 0ºC (Figure 14-1, Chapter 14). In practice, however, the moment of time t = tep (Figure 14-1) cannot be determined. Only shut-in temperatures Ts1, Ts2, and Ts3 are measured at a given depth (Figure 14-1). For this case we proposed a method of predicting the formation temperatures (see Chapter 14).
