Geophysical prospecting and interpretation

Geothermal well logging

Edited by G. Peth & P. Vass

Geothermal well logging

The expected bottom hole temperature (which depends on the geothermal gradient and the depth) fundamentally determines the suit of applicable logging methods and tools in geothermal exploration.

Below 150 C, standard well logging methods and tools coming from the petroleum industry can be effectively used.

But the standard logging tools cannot be applied above 150 Cowing to the limited heat tolerance of their electronics and sensors.

In such an extreme situations, so-called memory tools are used, which do not send real-time data to the surface, but store the collected data in their built-in memory.The data can be read out from the tool memory after it has been pulled up to the surface.

Geothermal well logging

These tools are primarily measure the temperature and pressure as a function of time.For a memory tool, the electronics are placed inside a Dewar flask (vacuum flask) to isolate the electronics from the high well temperatures and keep the internal tool temperaturebelow 175C for hours even at 350C well temperature.

In order to understand the characteristics of a geothermal system, principally the properties of the geothermal fluid and reservoir rock are needed to determine.

There are two main types of geothermal systems:

geothermal systems without local magmatic heat source, geothermal systems with local magmatic heat source.

The capability of well logging for helping the exploration strongly depends on the type of the geothermal system.

Geothermal well logging

Geothermal systems without local magmatic heat source The geothermal gradient higher than the continental average (25-30 C/km), but lower than that of the other type of geothermal reservoirs.

The least problematic situation when the geological environment of the geothermal reservoir is a clastic sedimentary basin, or buried valley.

In this case, the porosity is mostly of intergranular type.The well logging methods and interpretation techniques (based on rather empirical relations than on theoretical considerations) were actually developed for such circumstances in the petroleum industry.

Geothermal well logging

Open hole logging methods can be used for measuring the temperature as a function of depth, identification of the lithology (SP, GR, resistivity methods, CAL), determination of the bed boundaries (SP, shallow and micro

resistivity logs), porosity determination (density, neutron and acoustic logs).Permeability can also be estimated from the processed and evaluated log curves.

Several empirical relations have been found between the measured quantities and reservoir parameters which are valid in sedimentary rocks.

But, the number of geothermal reservoirs found in sedimentary rocks is not significant (e.g. California, Pannonian basin).

Geothermal well logging

The average geothermal gradient is 50-60 C/km in Hungary (in Budapest 60-80 C/km), because the Earth's crust is thinner (2426 km) than the average continental crust (~40 km).

It means that the formation temperature is about 110 C at a depth of 2 km.

The utilization of geothermal energy is connected to hot water production which is based on hot water bearing beds. But, the temperature of the produced water rarely exceeds 100 C on the surface, because it is getting cool during the flow up. So, the geothermal energy is utilized for communal and glasshouse heating as well as thermal baths rather than geothermal power plants.

Geothermal well logging

When the reservoir was formed in fractured hard rocks (e.g. limestone, dolomite) within a fault zone, the porosity and permeability estimation is problematic.There is no reliable, empirical relationship among the characteristics of fractures (size, size distribution, numbers, density), the porosity and permeability.The precise detection and quantification of fractures requires special, and expensive logging services (acoustic and electrical borehole wall imaging logging, e.g. CBIL, FMI, FMI-HD, XRMI).But the permeable zones can usually be recognized by means of standard well logging methods.

Resistivity imaging devices provide

micro-resistivity formation images with

borehole dip and azimuth data.

Water-base mud is required.

On the wide pads, arrays of button

electrodes are placed to cover about

the 80% of the borehole wall.

Caliper measurement is also provided

with different directions, so more exact

shape of the borehole can be

determined.

Electrical borehole wall imaging

Schlumberger:FMI (Fullbore

Formation MultiImager) brochure

Micro-resistivity image logs

Electrical borehole wall imaging

Schlumberger:FMI-HD High-definition formation

microimager brochure

Acoustic pulse-echo imaging tools

provide the complete 360 degree

circumferential coverage of the

borehole size and shape.

A centralized ultrasonic transmitter and

receiver rotates rapidly while the tool is

being pulled up slowly.

As a result of this spiral movement of

the transmitter/receiver pair, a finely

detailed image of the reflected signal

from the borehole wall is obtained.

Images can be made from both the

amplitude and the transit time of the

reflected signal.

Acoustic borehole wall imagingThe CBIL imager tool, Baker

Hughes Wireline Sevice Catalog

High-resolution acoustic image showing

formation fractures, Baker Hughes Wireline

Sevice Catalog

A spiral plot of the acoustic radius

information gives the 3D image of

a section of the hole.

Acoustic caliper measurement

Schlumberger: UBI (Ultrasonic

Borehole Imager) brochure

Geothermal well logging

Geothermal systems with local magmatic heat sourceThe geothermal gradient and heat flow is significantly higher than the average continental values.The geological environment of these geothermal systems is various, and significantly differs from the clastic sedimentary basins.The reservoirs are mostly connected to volcanic rocks for which much less experience is available in geophysical logging.There are no proper tool calibration methods for volcanic rocks.If the bottom hole temperature is not so high, the following logging methods can aid the lithological identification and the determination of mineral composition: spectral gamma ray logging (SL or SGR), combined density and photoelectric absorption logging (Litho-

density, Z-density),

Geothermal well logging

spectral neutron logging, mainly the neutron-induced gammalogging which is based on the inelastic scattering of epithermal neutrons (SNL). From the measured gamma ray (energy) spectrum carbon, nitrogen, oxygen, sodium, aluminium, silicon, chlorine, calcium, chromium, iron, nickel, copper, zinc content or occurrence can be determined.

Of course, the above-mentioned methods are expensive and sensitive to the temperature.These geothermal reservoirs are mostly of fractured type. In most cases, faults and fractures control the permeability of the geothermal reservoirs.The detection and quantification of fractures requires special, and expensive logging services (acoustic and electrical borehole wall imaging logging, e.g. CBIL, FMI, XRMI).

Geothermal well logging

The determination of porosity, permeability is problematical, therefore the resource assessment of the geothermal field is difficult.In addition, the temperature can be reach 350C in dry stream fields.In such cases, the standard logging tools cannot be used, so the range of data acquisition is limited to temperature, pressure, flow rate measurements and fluid sampling.But, geothermal power plants can generally be located in these geothermal fields.

Dry steam fields can be found inItaly (Larderello, Mt Amiata, surroundings of Vesuv, 140 C /km) Japan, New Zealand, Iceland, Kamchatka in Siberia etc.

Temperature logging

The subsurface temperature measurement is very important in geothermal surveys.The two main purposes of temperature data acquisition: determination of the average geothermal gradient and the

variations of the geothermal gradient along the borehole, identification of porous permeable beds or fractured zones

where the drilling mud enter the formation or the formation fluid flows into the borehole.

The neutral zone is a depth interval whose temperature is constant because either the temperature variations of the atmosphere or the heat flow of the Earths interior is not able to modify its temperature.The interval of this zone depends on several factors but mostly the geographic location and the geological structure.In most cases, this zone can be found in the range of 10-30 m.

Temperature logging

Below the neutral zone, the temperature normally increases with

depth.

In a homogeneous medium this change can be described by a

linear relationship, whose slope gives the geothermal gradient:

T(z) = a(z-zn)+Tnwhere

T(z) is the subsurface temperature at the depth of z,

zn is the depth of the bottom of the neutral zone,

Tn is the temperature of the neutral zone

a = T/z (below zn) is the geothermal gradient.

Temperature logging

Hubert Guyod: Temperature well logging

The value of the geothermal gradient

depends on the thermal (or heat)

conductivity of the medium (K, [W/m2/C]) and the heat flow rate intensity (or heat

flux density) which is a flow of energy per

unit of area per unit of time (J/m2/s =

W/m2).

The latter can be considered as constant

but its value depends on the location

more exactly the local characteristics of

the Earths interior (e.g. magmatic

intrusions, volcanic structures, the

thickness of the crust etc.)

Generally, there is an inverse relationship

between the heat conductivity of the

medium (that is the rock formation) and

the geothermal gradient.

Temperature logging

When the sequence of strata is horizontally layered and each bed

is homogeneous but different from its neighbours, the heat

conductivity is constant for each bed but its value depends on the

type of rock.

So, the heat conductivity can be described as a step function of

depth.

The geothermal gradient is also a step function and the constant

values of each bed is proportional to the reciprocal of the heat

conductivity.

T/z ~ 1/K

In consequence, the temperature increases with depth as a

piecewise linear function.

Temperature logging

Hubert Guyod: Temperature well logging

The main factors influence the heat

conductivity of rocks:

heat conductivity of the solid parts,

geometry of the solid parts,

heat conductivity of the fluid filling

the pore space,

porosity of the rock.

Igneous and metamorphic rocks are

generally better heat conductors than

sedimentary rocks.

In clastic sediments, the heat

conductivity increases with the rate of

compaction and consolidation.

The heat conductivity of sands is

usually higher than that of shales or

clays, because of the high conductivity

of quartz.

Temperature logging

When the situation is more complex than the horizontally layered

structure the change of the temperature is irregular, so it is very

hard to draw any conclusion on the geological structure from the

shape of a temperature curve.

The above-mentioned relations are valid only in the case of

thermal equilibrium.

It means that the temperature field of the subsurface is not

disturbed by any external influence, so it is static.

But the drilling process modifies the geothermal gradient and the

temperature of the subsurface compared to the thermal equilibrium

because of the circulation of drilling mud between the surface and

the actual bottom of the borehole.

Temperature logging

The mud circulation cools

down the bottom part of the

borehole and warms up the

upper part of the borehole.

So, the temperature

difference between the

surface and the bottom of

the hole reduces.

The intersection of the two

curves determines a point

called neutral point.

At the depth of the neutral

point the temperature does

not change.

O. & L. Serra: Well Logging Data Acquisition and Application (2004)

Temperature logging

The temperature field of the subsurface disturbed by the drilling

process is transitional.

After the circulation has stopped, the temperature begins to return

to the static condition.

The process is not linear. The rate of change is higher in the

beginning and it is gradually slowing down with time.

The whole process takes a long time. A few days are required to

reach the thermal equilibrium (the time depends on the depth of the

borehole).

There are two types of temperature measurement in boreholes

bottom hole temperature measurement with maximum

thermometers (mercury thermometers)

(continuous) temperature logging for measuring the temperature

profile along the borehole.

Temperature logging

Bottom hole temperature measurement

(BHT)

For open hole logging operations, two

mercury thermometers are generally used for

measuring the maximum temperature (that is

the bottom hole temperature) in the borehole.

The thermometers are placed into metal

capsules which prevent them from

mechanical effects and the mud pressure.

These capsules can be fixed to the cable

head which provides electrical and

mechanical contact between the cable and

the logging tool string.http://www.weatherford.com/en/st

andard-cable-head

Temperature logging

Because the borehole is not in thermal equilibrium during the logging

operation, the measured temperature is not the true formation

temperature but the actual mud temperature at the bottom of the hole.

Usually, more than one logging run is performed in the same portion of

the borehole, and the maximum temperature is repeatedly measured.

If at least three values of the bottom hole temperature are measured

which belongs to the same depth, an increase in temperature is observed

with time.

These temperature values can be used for extrapolating the bottom hole

temperature of thermal equilibrium (true formation temperature of the

bottom of the hole).

By means of the true or initial formation temperature, an average

geothermal gradient can be calculated for the logged depth interval.

The applied extrapolation method is called Horner method

Temperature logging

The data used for the Horner method:

tk the cooling time at the bottom of the hole (time taken to drill the

last metre + circulation time in minutes)

t1 the time of warming up, it corresponds to the time elapsed

between the end of the mud circulation and the arrival of the

first logging tool at the bottom of the hole (the time of the first

temperature measurement)

T1 the bottom hole temperature measured at first (it belongs to the

time of t1)

ti the time of warming up for the ith temperature measurement

Ti the bottom hole temperature measured at time of ti

From these data, pairs of coordinates can be derived, and the

points can be displayed in a suitable coordinate system (Horner

plot).

Temperature logging

The coordinates of the ith point:

xi = log[(ti + tk)/ ti], yi = TiThe Horner plot has a semi-

logarithmic grid.

The axis of abscissas is

logarithmically scaled for

expression of (ti + tk)/ ti.

The axis of ordinates represents

the temperature, which is plotted

on a linear scale.

A straight line can be fitted to the

plotted points which intersects the

value of initial formation

temperature (T) on the axis of

ordinates. O. & L. Serra: Well Logging Data Acquisition and Application (2004)

Temperature logging

Continuous temperature logging for measuring the temperature

profile along the borehole

A special logging sonde with built-in thermometer is used for continuous

temperature measurement.

The logging operation is usually made while the tool is being lowered

slowly in the borehole.

In such a way, the disturbance of the thermal equilibrium can be

minimalized.

If the temperature field of the borehole is almost static (there was enough

time for the borehole to approximate the thermal equilibrium), the

formation temperature and the temperature gradient of the different depth

intervals can be directly determined from the temperature log curve.

If the borehole is not close to the thermal equilibrium, repeated

measurements are required to extrapolate the initial temperature profile

of the given depth interval.

Temperature logging

The temperature log can be used

for the identification of permeable

zones, because the infiltration of

mud causes negative temperature

anomaly opposite the permeable

beds.

TEL: measured temperature log

Temperature logging

The continuous temperature logging is always applied in

geothermal exploration, but not in petroleum exploration because

it requires a long time (and the rig time is very expensive in

petroleum industry).

The main applications of temperature logs are connected to the

fluid production phase:

detection of producing zones,

determination of the depth of the bubble point,

detection of zones of fluid injection entry.

Temperature logging

Suggested literature (on the Web)

GEOTHERMAL WELL LOGGING: TEMPERATURE AND PRESSURE LOGS

www.os.is/gogn/unu-gtp-sc/UNU-GTP-SC-16-21.pdf

GEOTHERMAL WELL LOGGING: GEOLOGICAL WIRELINE LOGS AND

FRACTURE IMAGING

www.os.is/gogn/unu-gtp-sc/UNU-GTP-SC-12-24.pdf

GEOTHERMAL LOGGING l - Orkustofnun

www.os.is/gogn/Skyrslur/1980/OS-80017-JHD09.pdf

ON GEOPHYSICAL LOGGING OF GEOTHERMAL WELLS WITH EXAMPLES

FROH WELL KJ-13 IN THE KRAFLA GEOTHERMAL FIELD, N. ICELAND

Zosimo F. Sarmiento, UNU Geothermal Training Programme, National Energy

Authority, Grensasvegur 9, 108 Reykjavik , Iceland

www.os.is/gogn/unu-gtp-report/UNU-GTP-1980-05.pdf