Geothermal Power Plantstheargeo.org/shortcourses/drilling/Benedikt...Temperature, Pressure and Flow logging ARGeo C-6 Oct 31-Nov 1, 2016 – Short Course 9 . 10 K10G Operation and

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Benedikt Steingrímsson Iceland GeoSurvey (ÍSOR), Reykjavík, Iceland

GEOTHERMAL WELLS Logging and Testing during Drilling

ARGeo C-6 Oct 31-Nov 1, 2016 – Short Course

Outline

• Geothermal well logging • Use of temperature logs during drilling

– To locate feed zones in well – To determine cross-low in wells – to estimate formation temperature (Horner plot)

• Injection well tests during drilling – One step injection tests to determine injectivity index and

get idea about the

• Multi step injection tests to determine and model the hydrological connection and properties of the well and the reservoir system

ARGeo C-6 Oct 31-Nov 1, 2016 – Short Course 2

What are Well Logs ?

• Well logs are all measurements carried in a borehole (well)

• They cover a wide range of very different measurements

• The measuring device, called logging tool or logging probe (sensor), is moved either up or down the well while the measurements are carrying out on the run or at certain location


• Information on the measurements is

either fed from the probe to the surface through the cable and registered there or recorded in internal memory and extracted at surface

• Log is defined as: • Record of sequential data

• A well log is a series of measuring data stating the depth and the measuring

What are Well Logs? (cont)


• Measurements or special operations regarding the well design, geometry and completion. Often carried out during the drilling or during the operational life of the well.

Examples: Cement Bond Log, caliper, perforations, free point logging, casing inspection tools and ….

• Measurements done in order to obtain information on the structure and the physical properties of the geothermal system. Both regarding the rock formations (porosity, permeability and resistivity) and the geothermal fluid (temperature, pressure and salinity)

Purposes of Logging Geothermal Wells


Temperature

Pressure

Caliper

Resistivity

Neutron logs (n-n)

Natural gamma ray

TeleViewer

Water sampling in wells

Cement bound logs (CBL)

Casing Collar Locator(CCL)

Downhole Video Camera

Gyroscopic logging

Geothermal Well Logging in Iceland

Several Logging trucks • Specially equipped trucks for real-time

logging in wells (conductor cable) • Equipped for logging with memory tools

(slick line)


TEMPERATURE and PRESSURE logs are the most important logs in geothermal exploration and

utilization


Portable temperature logging unit for shallow gradient wells

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Slick-line Truck for logging deep Geothermal Wells


•Heat shielded (Dewar flask) electronic T+P logging tool

•Battery driven electronic HT Strain sensor tool (9), which measures temperature and pressure (T corrected) simultaneously

•Today ~100% of T+P measurements in hot HT-wells in Iceland are done with the K10G tools and we have been using PTS for the last two years

The K10 Geothermal PT and PTS

The main Logging Tool used today for Temperature, Pressure and Flow logging


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K10G Operation and T+P ratings

The K10G can withstand pressures up to 345bar (at 350°C) before it collapses

The internal electronics are only protected for a certain time by the Dewar flask;

•300°C for max. 9 hours

•315°C for max. 6 hours

•370°C for max. 4 hours (pers. comm. from Kusterco CEO, 2006)

•400°C for max. 2 hours (pers. comm. from Kusterco CEO, 2008)

•The K10G is run with one or two centralizers to protect the Dewar flask from wear, especially in deviated wells.

Temperature Logging in cooled Wells during Drilling !!

How can that be useful?



Analysis of Temperature Logs during Drilling

Temperature logs run in a “cold” well during drilling give valuable information on well conditions:

1. Location of aquifers. Aquifer temperature can however very seldom be determined during drilling.

2. Cross-flow between aquifers.

3. Estimation of cooling efficiency cold water injection (Blow-out risk evaluation).

4. Determination of “bottom-hole temperature” (BHT)


Analysis of Temperature Logs during Drilling

Temperature logs run in a “cold” well during drilling give valuable information on well conditions:

1. Location of aquifers. Aquifer temperature can however very seldom be determined during drilling.

2. Cross-flow between aquifers.

3. Estimation of cooling efficiency cold water injection (Blow-out risk evaluation).

4. Determination of “bottom-hole temperature” (BHT)


2. Temperature Logs of Wells under Injection during Drilling

Where does water enter and exit this well ??

During injection the log shows:

1. Slope change where feed zones are

accepting the injection

2. Step change where feed zones

contribute to the total downflow

Cross-flow


3. Temperature Logs of Wells under Injection during Drilling

Where does water enter and exit this well ??

Blow-out conditions :

1. Surface casing to 110 m in a 193 m deep

well.

2. Drill string at 178 m

3. Injection on kill line ~13 L/s

4. Pressure on kill line 10 bar

5. Temperature logged inside drill string with

float valve at 178 m

6. Water level in string at 60 m

Underground blow-out. Hot up-flow from 184 m that flows out at 153 m together with the injection.

How do you know when you come to the planned casing depth in a new field that T> 230 ??

1. Ask the wellsite geologist about the alteration about

the alteration and if he sees “fresh” epidote”

2. You could also take a cuttings and study fluid

inclusions to estimate the temperature.

3. If the geologist is sure the temperature is higher than

230°C, run the casing

4. If the geologist is sure the temperature is lower,

continue drilling.

5. What if he is in doubt?

Is sometimes done during drilling for the production casing.

Short heat-up period 12-24 hours to check if T at bottom

>220°C.


4. Determination of Temperature at “Casing Depth” It is important in high temperature drilling to sink the production casing into temperatures higher than ~230°C to seal of all colder aquifers.

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If you are in serious doubts on the temperature at the casing depths do heat recovery i.e. 1. Stop the drilling and the cooling of the well.

2. Lower a temperature tool to the bottom of the

well

3. Monitor the temperature recovery for some

period of time (12-48 hours).

4. Plot the temperature recovery with the heat up

time.

5. Use available software to extrapolate the recovery

curve to “equilibrium” (i.e. Horner plot).



Thermal recovery after drilling is stopped

Methods to estimate formation

temperature from recovery data

•The Horner plot

•The Albright method

The computer program “Berghiti”

Is sometimes done during drilling at

production casing for short heat-up period

12-24 hours to check if T at bottom

>220°C.



• Plot T(t1), T(t2),…. as function of ln((t - t0/)/t)

• For very long times (t - t0)/t = 1 and ln((t - t0)/t) = 0.

• Equivalent to the Horner method in reservoir engineering.

• The method does not require that q and k are known, but it does require that q is constant during drilling.

The Horner plot

Assume that the drilling process lasts for time t0. During this time, heat is removed (cooling) from the rock at constant rate q. Set the time passed since the cooling stopped t, the formation temperature as Tf and the temperature at any time in the well after the cooling was stopped as T(t). A solution to the heat diffusion equation, this time in radial coordinates, is found by integrating the instantaneous response of a linear heat source over the cooling time duration t0:


Heat Recovery at 1500 m Depth in a Well after Cooling for 54 hours


Heat Recovery at 1500 m Depth in a Well Horner Plot Estimation


PRESSURE TRANSIENT TESTING OF GEOTHERMAL WELLS


Well Pressure Transient Tests

Q

Water level

Pressure gauge

To logging truck Tests where fluid is injected into a well (or produced from) at a known (possibly variable) rate, while the down hole pressure is monitored. Sometimes is the pressure measured in an observation well close by. Such tests are called interference tests

Production/injection


Drawdown test Buildup test

Injection test Falloff test


Different types of pressure transient tests

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Step test


Different types of pressure transient tests

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Q

Water level

Pressure gauge

To logging truck

Main objectives of pressure transient tests

• To determine Injectivity (or productivity) indices: II (or PI) = ΔQ/ΔP

• To estimate the deliverability and flow characteristics of the well.

• In situ measurement of hydrological parameters for the reservoir close to the well (transmissivity and formation storage).

• To determine the hydrological connection between the well and the reservoir (skin effect).

• To enhance the transmissivity close to the well (cleaning of feed zones and fracturing, stimulation)

Production/injection


Injection Testing of Geothermal Wells during Drilling


1. Circulation losses during drilling are indicator on future productivity of the well

2. One step injection tests are commonly done in Iceland during the drilling of the production part to determine the injectivity index to get more accurate estimate on the potential production capacity and use this information when deciding if drilling should be continued or stopped and the well eventually stimulated.

Injectivity Index >4 l/s per is considered to give acceptable productivity but reservoir temperature will influence the

outcome.

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The one step Injection Test Procedure


1. Stable injection before the test starts (typically ~15 l/s)

2. T- and P-logs down to the bottom of the well

3. Position the PT-tool at the “best feed zone and monitor the pressure while the injection is increase in one step, (typically ~10 l/s increase), and kept for ~1 hour to determine the injectivity index.

4. T- and P- logs from the bottom up the well.

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Three recent injection tests in well See the temperature logs on previous slide


The II increased from 1.6 to 2.1 and ended as 3.7 l/s per bar Not very good injectivity but the reservoir temperature is >300°C

so the guess is a 5-7 MW well

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Well Completion Injection Step Rate Test Procedure


Carried out after the liner has been set (if set)

1. Stable injection while liner is lowered into to well and continued until the test starts (typically ~15 l/s)

2. T-, P- and Spinner-logs down to the bottom of the well 3. Position the PTS-tool at the “best feed zone and monitor the

pressure while the injection is increase in two 10-15 l/s steps and the pressure monitored in each step for ~3 hours.

4. Fall-off step to ~20 l/s and pressure monitored for 3-5 hours. 5. T-, P- and Spinner logs from the bottom up to the top of the

well. 1. .

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Injection Step Rate Test


Injection Step Rate Test Average Injectivity Index determination


Further analyses of step rate test data is through model simulation

reservoir

parameters

k, s, C

p

t

reservoir response

Match

field input

t

q

model

parameters

k, s, C

p

t

model response

model input


The Pressure Diffusion Equation

Is the basic equation in well testing theory where S is

storativity, and T transmissivity.

Used to calculate the pressure (P(r,t)) in the reservoir

At a certain distance (r) from the producing well that produces at rate

(Q) after a given time (t).

The most used solution through the years of the pressure diffusion equation

is the so called Theis solution, which is based on a simplified model of the

geothermal reservoir and is also approximate solution.

Numerical models and simulation codes are now available for analyzing well

test. The code used in Iceland is WellTester ARGeo C-6 Oct 31-Nov 1, 2016 – Short Course 34

The Theis Model of a Confined Reservoir

•Horizontal, constant thickness

•Confined between impermeable

formations

•Infinite in horizontal extent

•Homogeneous and isotropic

•Flow according to Darcy’s law

V = -k/µ *dp/dr

Observation well Production well

Impermeable basement

Impermeable caprock

Confined horisontal reservoir

Q(kg/s)


Approximate time dependent

solution for the draw down cone

close to the well and some time

after start of production/injection:

This is called the

Theis solution

It tells us that the draw down close

to the well will change as

logarithm of time

The Theis model of a confined reservoir

P(0) - P(r,t) = 2.3Q/4T *log(2.25Tt/r2S)

Production well Observation well

Impermeable basement Confined horisontal reservoir


Injection test of well KJ-17 in Krafla Showing injection rate, measured and simulated pressure


Well tester analyses of the 3 Step Test above

Reservoir/Well properties Step 1

Transmissivity [10-8 m³/(Pa s)] 9.3

Storativity [10-8 m/Pa] 1.8

Response distance [m] 14

Skin factor [-] -0.7

Wellbore storage [10-5 m³/Pa] 1.2

Transmissivity ratio [10-4] 0.9

Storativity ratio [10-4] 9.3

Reservoir Thickness 50-100

Injectivity Index [(l/s)/bar] 14.2

Effective Permeability [10-15 m²] 20-40


WELL COMPLETION


The multi step injection test is usually the last work with the rig on site. The cold water pumps are stopped and the well is then shut in.

Then starts the heating-up period, that usually lasts few months until thermal equilibrium is obtained. The well is then flow tested.

This will be discussed in a later lecture at the short course.

Thank you

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Geothermal Power Plantstheargeo.org/shortcourses/drilling/Benedikt...Temperature, Pressure and Flow logging ARGeo C-6 Oct 31-Nov 1, 2016 – Short Course 9 . 10 K10G Operation and