Geothermal Investigations in Permafrost Regions—The Duration of Temperature Monitoring after Wellbores Shut-In

Izzy M Kutasov,Lev V Eppelbaum

doi:10.4236/gm.2012.24013

Abstract

The most important data on the thermal regime of the Earth’s interior come from temperature measurements in deep boreholes. The drilling process greatly alters the temperature field of formations surrounding the wellbore. In permafrost regions, due to thawing of the formation surrounding the wellbore during drilling, representative data can be obtained only by repeated observations over a long period of time (up to 10 years). Usually a number of temperature logs (3 - 10) are taken after the well’s shut-in. Significant expenses (manpower, transportation) are required to monitor the temperature regime of deep wells. In this paper we show that in most of the cases (when the time of refreezing formations is less than the shut-in time) two temperature logs are sufficient 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. A simple method to process field data (for the well sections below and above the permafrost base) is presented. Temperature logs conducted in two wells were used to demonstrate utilization of this method.

Highlights

Temperature logs are commonly used to determine the permafrost temperature and thickness
In this paper we show that in most of the cases two temperature logs are sufficient to predict formations temperatures during shut-in, to determine the geothermal gradients, and to evaluate the thickness of the permafrost zone
Earlier we suggested a “two point method” [7] which permits one to determine the permafrost thickness from short term downhole temperature logs

Summary

Introduction

Temperature logs are commonly used to determine the permafrost temperature and thickness. 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 socalled restoration time, after which the difference between the temperature of the formation and that of the fluid is less than the needed measurement accuracy. 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. The objective of this paper is to show that in many cases (when the time of refreezing formations is less than the shut-in time) two temperature logs are sufficient to predict formations temperatures during shut-in, to determine the geothermal gradients, and to evaluate the thickness of the permafrost zone

Shut-In Temperatures—Permafrost Zone

Temperature Gradient and Estimation of the Permafrost Thickness

Onset of Formations Freezeback

Conclusion