UNU-GTP, KenGen and MEMD-DGSM Short Course on Geothermal Project Management and Development. j g pEntebbe, Nov. 20-22, 2008

Subsurface ExplorationE l i d illi d b h l i i iExploration drilling and borehole investigations

Benedikt SteingrímssonSverrir ÞórhallssonSverrir ÞórhallssonIceland GeoSurvey

ISOR

GeothermalGeothermal DrillingDrilling givesgives usus accessaccess toto thethe reservoirreservoir for for ExplorationExploration andand ProductionProduction

Ground surface Well

ExplorationExploration andand ProductionProduction

Geothermal reservoir

2

Geothermal drilling is similar to oil and groundwater drillingdrilling.

•Differences for HT-drilling:

•Temperatures up to 350°C•Temperatures up to 350 C•Pressure gradient hydrostatic or less.•Danger of steam blow-out•Igneous rocks•Drilling fluids are mud, water, air•Large or total circulation losses•Large or total circulation losses•Big well diameter-large production.•No fire or explosion hazard.•Low pollution

What is an oil exploration well?What is an oil exploration well?

• Definition of “e ploration” from the oil ind str• Definition of “exploration” from the oil industry:

• The initial phase in petroleum operations that includes generation of a prospect or play or both, and drilling of an exploration well. Appraisal, development and production phases follow successful exploration. (Ref. Sclumberger glossary)

What is a geothermal exploration well?What is a geothermal exploration well?

f h h l d ll• Use of terms in the geothermal industry is generally:

• An exploration well is any well, shallow or deep, drilled to investigate the geothermal resource until the decision is made to develop the prospect. Deep exploration‐ and production wells are often identical in design.

ICELAND GEOSURVEY

Main types of wells drilled in Geothermal 

1. Exploration wells.1. Thermal gradient wells (< 200 m deep)

2. Core wells (uncommon, < 500 m deep).

3. Slim holes (5‐8” in diameter and up to 2000 m deep).

4. Deep wells of production design (typical HT‐ expl. wells)

2. Production wells.

3. Reinjection wells.

Main Types of Low Temp. Wells

• Temperature gradient wellsD h 40 200 ( d i I l d f l i )– Depth 40‐200 m (used in Iceland for exploration)

– Well diameter 3‐4” (<10 cm) , plastic liner HDPE

/Cost 30‐50 USD/m

• Exploration wells.– Depth <800 m depth and diameter  < 8” (20 cm)

Cost 200‐400 USD/m

• Production wells– Depth 400‐3100 mp

– Mainly three casing diameters for production 14", 11" and 8".

– Casing depth í 100‐500 m.  Barefoot ‐ no liner.Cas g dept 00 500 a e oot o e

– Cost 400‐700 USD/m

Main Types of High Temp WellsMain Types of High Temp. Wells

• Typical HT well profile:• Typical HT‐well profile:– Depth of well 1200‐2200 mCasing depth 600‐1100 m– Casing depth 600‐1100 m

– Production casing dia. 9 5/8", liner 7"– Cost USD/m 1,200‐1,500Cost USD/m 1,200 1,500

• Large diameter well profile:– Depth of well 1200‐3000 mDepth of well 1200 3000 m– Casing depth 600‐1100 m– Production casing dia. 13 3/8", liner 9 5/8"g / , /– Cost USD/m 1,500‐1,800

Types of wellsTypes of wells

Main Reasons for Casing a Well g

• To seal out unwanted aquifers and to prevent fluid o sea out u a ted aqu e s a d to p e e t u dmigration between formations. 

• To support the hole.

• To allow control of blow‐outs and to anchor the wellhead.

• To provide a conduit for the well production.

A Family Photo of Icelandic Geothermal Wells

BBBigBig -- TypicalTypicalBBiidd

Big Big TypicalTypicaldiameterdiameter

Gradient wellGradient well

High temp L ll

Production wellsProduction wells

High temp. Low temperature wells.

GeothermalGeothermal DrillingDrilling inin IcelandIceland: : NationalNational ContractorsContractors

GEYSIR: GEYSIR: TheThe rigrig of of IcelandIceland DrillingDrilling ContractorsContractors..ggSoilmecSoilmec. . ItalianItalian RigRigDepthDepth capcap: 4000 km.: 4000 km.Anno 2004.Anno 2004.

Photo: Jarðboranir hf

The Drill StringThe Drill StringConventional rig Modern rig

K ll

•Swivel •Top drive

•Kelly +•Rotary table

•Drill Pipe

•Drill Pipe

•Drill Collars•(Mud motor)•Drill bit

•Drill Collars•(Mud motor)•Drill bit•Drill bit •Drill bit

The mud circulation systemThe mud circulation system

T iTri-conedrill bit

Checkvalve above bit

Typical:Flow 40 l/sP 100 bPresure 100 bar

Drilling progress days vs depthDrilling progress – days vs. depthÁsG/SSJo22-08-2003Hágöngur

Drilling progress of well HG-01 Days

Shallow (<200 m) exploration wells are drilled

00 4 8 12 16 20 24 28 32 36 40 44

Wire-line logs, casing run. Cementing and Cement Bond Logging.

Drilling with 17 1/2" bit

Installation of rig, cementing of 18 5/8" surface casing.

g p g

exploration wells are drilled in a matter of 1 to 7 days.

500 Drilling with 12 1/4" bit

Wire-line logs, casing run, cementing and Cement Bond Logging

dept

h (m

)

2500 m deep exploration wells are completed in 40‐

1000

Drilling with 8 1/2" bitCasing depth:Surface casing: 107 mAnchor casing: 308 m

Wel

l

wells are completed  in 40‐60 days. 1500

Anchor casing: 308 mProduction casing: 803 mProduction section: 2360 m

2000

POOH, bit change

Wire-line logging, stimulation, step-testingand completion of well with liner

Drilling progress

2500

and completion of well with liner

Well cost

2500 mwell3 million US$in 2004 nowin 2004, now~4 million US$

Geothermalwells are morewells are moreexpensive thanoil and gas wellsof the same depth

Tester 2007

“Problems” of Geothermal DrillingProblems  of Geothermal Drilling

1 High cost of geothermal wells1. High cost of geothermal wells.

2. Well sites in protected areas.

3 l f l3. Large loss of circulation zones.

4. Drilling “blind” after total loss of circulation.

5. Slow drilling in hard lava formations.

6 Well stability6. Well stability.

7. Methods to seal #3 and stabilise #6.

8 ll i h l bili8. Wells with low permeability.

9. High pressure while drilling through steam cap.

10. Temperature limitations of logging tools.

Various types of Various types of formation temperature profilestemperature profiles

Exploration of Geothermal WellsExploration of Geothermal WellsGround surface Well

Wh d kWhat do we want to know about the reservoir??Si V l

Geothermal

•Size –Volume•Geology (Stratigraphy, alteration, fractures and feed

reservoiralteration, fractures and feed zones) •Chemical, physical properties

f k ( it bilit

What information can we get from:

of rock (porosity, permeability and temperature)•Chemical and physical g

Boreholes?? p y

properties of fluid (T+P+TDS)•Productivity of wells•Reservoir production capacity

Data collected from GeothermalData collected from Geothermal Drillholes

• Drilling Parameters, (i.e.penetration rates, circulation losses/gain).losses/gain).

• Gyrosurveys for slope and direction.• Geological samples.  (Drill cuttings, drill cores).g p ( g , )• Temperature logs.• Other measurements in the well (Caliper Pressure, 

Geophysical logs.)• Well tests for productivity/injectivity.

l d l f l f fl ll ( l• Fluid samples: Surface sample of flowing wells ( Ocasional downhole samples).

Exploration Programme for shallow LT‐exploration wells i I l din Iceland

☯ Investigations during drilling:

• Drill cuttings sampled every 2 meters

Occasional temperature logs • Occasional temperature logs.

☯ Investigations at the end of drilling:

• Temperature logs.

• Air lift if well productive.

☯ Investigations after drilling

• Temperature log after T-recovery.

• Flow testing if well productive.

• Fluid samples and analyses

Often the only information we seek from a shallow well is the thermal gradient.

Problem: Find the geothermal gradient from the graph.S l tiSolution:

ΔT/ΔZ = (T2 - T1)/(D2 - D1) = D1 = 0 mT1 = 4 °C

( 25 - 4 )/(76 - 0 ) =0.32 °C/m

or 320°C/km

D2 = 76 mT2 = 25 °CT2 25 C

Thermal gradient map of the Reykjavík area

Temperature gradient map indicating hot fluid up‐flowin a fracture zone

Exploration Programme carried out for deep high t m t lls i I l dtemperature wells in Iceland

☯ Investigations during drilling:☯ Investigations during drilling:

• Analyses of the drilling performance,

• Analyses of drill cuttings.

• Well logging

Sti l ti th d t h • Stimulation methods to enhance well productivity/injectivity. Well injection tests (pressure transient tests).

☯ Investigations after drilling

Monitoring of T+P recovery after • Monitoring of T+P recovery after drilling.

• Flow testing of well.

• Fluid sampling and analyses

Example:Example:Lithology based in drill Lithology based in drill cuttings and logging resultscuttings and logging results

Injectivivity index for HE-15 Hellisheiðij yII = ∆Q/∆P=25/1.8 =14 l/s per bar

Pressure at 1650 m in HE-15 when injection is decrease from 50 to 25 l/s

HeatHeat recoveryrecovery afterafter drillingdrilling

DeterminationDetermination of of thethe formationformation temperaturetemperature

KRAFLA WELL KJ-21Temperature recovery after drillingdrilling

1 During injection2 After 2 hours2 After 2 hours3 After 30 hours4 After 52 hours5 Aft 96 h5 After 96 hours6 After 242 hours7 After 556 hours 78 Estimated rock

temperature 1 32 654 8

Output curves for wells in IcelandOutput curves for wells in Iceland13 3/8”

9 5/8”5/8

That’s all folksThat s all folks.Thank you very muchy y