Instructor : Hamad ur RahimClass: DDE

Semester: III

Lecture :Temperature Logs and its Applications

SECTION 1: THEORY OF TEMPERATURE LOGS Introduction TheoryLog principleTools

SECTION 2: APPLICATIONS Uses of Temperature LogREFERENCES

Contents

INTRODUCTION

Section 1

Well logging

The continuous recording of a geophysical parameter along a borehole produces a geophysical well log.

Main purpose of well-logging is formation evaluation (lithology, porosity, permeability, bed thickness and water and hydrocarbon saturation). Well-logging is done in most oil wells, mining exploration wells, and in many water wells. (Robinson & Coruh, 1988)

Section 1

INTRODUCTION

Temperature sensors are attached to every tool combination that is run in a well for the measurement of the maximum temperature.

Few modern tools exist that can continuously measure temperature as the tool travels down the well.

Readings from a number of the maximum thermometers attached to different tool combinations and run at different times are analyzed to give the corrected temperature at the bottom of the borehole (bottom hole temperature, BHT).

Section 1

THEORY

Subsurface temperature increase with depth known as geothermal gradient or geotherm. G = T (formation)- T(surface)/ Depth

T (formation)= Formation Temperature

T (surface) = Average mean surface temperature; (-5 oC Permafrost; +5 oC Cold Zones; 15 oC Temperature zones; 25 oC tropical zones).

Section 1

THEORY

Typical geotherms for reservoirs are about 20 to 35

oC /km.Higher values (up to 85 oC /km) can be found in

tectonically active areasLower ones (0.05 oC /km) in stable continental

platforms. • Hence, the bottom hole temperature (BHT) for a 3000

m well with a geotherm of 25 oC and a surface temperature of 15 oC is 90 oC.

Section 1

THEORY

Temperature in the sub-surface increases with depth. The rate at which it does so is called the geothermal gradient or geotherm.

Graph of geothermal gradients. The zone of typical oilfield gradients is indicated.

Section 1

THEORY

Geothermal gradient depend upon a formation thermal conductivity (the efficiency with which that formation transmits heat or, in the case of the earth permits heat loss).

o Low thermal conductivity rocks, such as shale.

o high thermal conductivity rocks, such as salt.

Section 1

THEORY

Section 1

THEORY

Table: gives some range of thermal conductivity for typical lithologies.

When a rock with high thermal conductivity is encountered, it will a show a low thermal gradient.

In shale, where the passage of heat is slow, the gradients will be higher. In other words the blanket of shale would keep us warm at night while a blanket of salt would not!

Thus, the real temperature gradient in a well is not a straight line but a series of gradients related to the thermal conductivities of the various strata.

the gradient varying inversely to the thermal conductivity.

Section 1

THEORY

In oil fields temperature gradients vary from the extremes of 0.05 oC /km to 85 oC /km although typical figures are 20-35 oC /km.

Section 1

THEORY

• Notions of temperature variations with depth and with position in a basin may be expressed in map form, using contours of equal geothermal gradient.

• Present day north sea geothermal gradients. (after Cornelius, 1975: Carstens and Finstad, 1981: Harper, 1971). Taken from Cornford (1984).

Each tool combination is equipped with a temperature sensor.Temperature measurements are always made at the bottom of the

well.A special thermometer tool ( Thermistor or metal whose resistance

is sensitive to temperature changes ) may be used to give continuous temperature readings.

The absolute accuracy of temperature measurements is low (±2.5

oC).

Section 1

LOG PRINCIPLE

The actual temperature measured is that of the drilling fluid not the formation temperature.

It is the temperature of the mud in the bore hole.The drilling mud is cold compared to the formation.The cold drilling fluid invades the formation and cools it

down very efficiently via heat convection.

Section 1

LOG PRINCIPLE

True formation temperature True formation temperature

During circulation of drilling fluid the temperature of the borehole reaches an equilibrium defined by the cooling effect of the drilling fluid and the heating effect of the formation.

Equilibrium may only be attained after several months after stopping the circulation of the drilling fluid as in figure.

Section 1

LOG PRINCIPLE

• To correct bore hole temperature values, several methods have been devised.

The most frequently used method is Horner plot method (Ferlt and Timko, 1972)

Section 1

LOG PRINCIPLE

It is most common method.It plots the measured temperature on the concept of straight

line relationship between BHT and Log . log(T/(t+T)).T = Time in hours since circulation of the drilling fluid was

stopped.t = length of time of circulation of drilling fluid before logging.

Section 1

LOG PRINCIPLE

Horner method

The Horner plot generated with this data, and the resulting formation temperature.

This plot a straight line that intersects T(t+T)=1 at formation temperature .

The correction is negative on the surface but Temperature 30 oC increase at 3000m.

Section 1

TOOLS

Section 2

Uses of Temperature Logs

The sensors of other logging tools are sensitive to temperature. The temperature measurement can be used to correct for this .

Section 2

Uses of Temperature Logs

Correction of other tools

Some parameters measured by other tools are sensitive to temperature. The best example is resistivity logs. The temperature data is used to correct all resistivity data to a standard 24 oC so they are not depth dependent and can be compared.

Section 2

Uses of Temperature Logs

Correction of measurements

The maturity of hydrocarbons depends upon the maximum temperature that the organic remains have been subjected to, as well as time and pressure.

Section 2

Uses of Temperature Logs

Hydrocarbon maturation

Continuous temperature logs record differences in thermal gradient that result from differences in the thermal conductivity of the formations. These difference can be used for correlation.

Section 2

Uses of Temperature Logs

Correlation

Continuous logs also note the presence of over pressured zones, where the hot over pressured fluids escape into the borehole and are noted by a rise in the measured temperature.

Section 2

Uses of Temperature Logs

over pressured zones

Continuous logs can observe intervals of raised (or lowered) temperature caused by the influx of hotter (or colder) fluids into the borehole through the rock matrix, or more usually, through patent fractures. This effect may also be due to cold drilling fluid escaping into the rock.

Section 2

Uses of Temperature Logs

Fluid movement

Schlumberger, 1998, log interpretation charts; Schlumberger wire-line and testing, SMP-7006, sugar land, Texas.

Rider, M.H., 1986. The geological interpretation of well logs, Blacky and Son Limited, Bishopbrigg, Glasgow.175P

Western Atlas logging services, 1985, log Interpretation charts, Rev. 12/95; Baker atlas, Houston, Texas.•