Computerized Temperature Decay An Asset to Temperature Logging
Authors: Cocanower R.D. , Morris Billy P. , Dillingham Mat 
Published: December 2007

By determining a temperature Progression or regression rate after shut-in, and by further extrapolating through a computer program, temperature surveys can be interpreted quantitatively to yield a precise definition of zones of injection. precise definition of zones of injection.

Introduction

There has been renewed interest in the use of temperature surveys for fluid movement analysis in both producing and injection wells. Many techniques have producing and injection wells. Many techniques have been tried and the results publicized. The major problem of defining accurately the vertical dimensions of problem of defining accurately the vertical dimensions of the injection zone was inherent in all the analog techniques. Additional problems of the effect of borehole mechanical conditions and the statistical errors common to analog recordings made interpretation more difficult. To obtain better definition of injection zones, calculation methods have been developed for comparing shut-in temperature surveys with injection temperature surveys or with normal gradients. The calculations for each depth interval were time-consuming and still contained the errors common to analog recordings. The digitizing of the signals from the temperature element at the well site and the use of computers for converting these signals to temperatures and to computer projections to a selected time interval provide more accurate definition of injection zones. provide more accurate definition of injection zones.

Conditions

Movement of fluid into a formation cools the zone invaded as well as the strata above and below (Fig. 1). The extent of the cooling, both horizontal and vertical, increases with continued injection until steady-state heat flow is approached. When the well is shut in for survey purposes, the normal formation temperature continues to transfer heat to the wellbore.

The rate at which the wellbore returns to nominal temperature depends upon the mechanical conditions of the borehole and the degree to which the strata have cooled.

Temperature surveys performed after shut-in show the effect of packers, hole size changes, and cooling of areas (Fig. 2) vertically adjacent to the injection zone. Repeated surveys show a decreasing effect of the mechanical conditions and provide more information on the injection zone. A single-run temperature survey after shut-in can be very misleading. Multiple runs are necessary to distinguish the mechanical conditions from the injection zones.

Repeated runs to define the upper and lower limits of an injection zone are at first affected by the cooling of strata adjacent to the zone, and later by the vertical heat transfer to the injection zone near the wellbore. In the first case, the zone may appear too broad, and in the second, the zone could be eliminated from view.

To gather valid data, it is not necessary that the down-hole sensors be extremely sensitive, but they must show linear reaction over the temperature range being investigated. With the development of the surface differential systems, there has been considerable confusion over the term "sensitivity". These systems are capable of expanding a down-hole signal to a high degree, but the sensitivity of the system is only that of the sensor in the borehole.

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