SLIMHOLES FOR GEOTHERMAL EXPLORATION

Sverrir ThorhallssonBjarni RichterIceland GeoSurvey (ÍSOR)Reykjavik, Iceland

Exploration drilling

To investigate a new geothermal field where surface mapping by a host of geo scientific methods has indicated a resource, exploration drilling is the next phase. The objectives are:

• A) to provide additional information to some key questions for proper understanding of the earlier surveys, e.g. resistivity and temperature gradient. These wells can be slim holes and not so very deep.

• B) to confirm the existence of a viable resource and provide information on the reservoir and its potential. A “conceptual model” combines results of surveys and drilling results and aids in the site selection for new wells. Reservoir simulation allows a developer to evaluate the commercial potential and decide whether to proceed to production drilling. Such exploration wells need to be deep enough to penetrate the reservoir and of large enough diameter to allow fluid to be produced. These wells can also be of slim design

• C) To bring the knowledge and confidence of the geothermal resource to a level that meets the requirements of “Measured”. (See next slide):

Reporting code – clarification of terms for investors

Australia: The Geothermal Reporting Code (2010)Canada: The Canadian Geothermal Code for Public Reporting.Reporting of Exploration Results, Geothermal Resources and Geothermal Reserves (2010)

Most common casing programs for geothermal wells

Why a slimhole?

• High cost of drilling is a barrier to exploration for subsurface resources.• Remote locations – difficult access. Easier transportation. Less infrastructure.• The reduction in bulk allows helicopter transport or small trailer mounting.• The smaller size reduces location area (about 1000 m2 with mud pits).• The power required is low, typically 300-400 hp, saves fuel.• Sensitive areas – minimum footprint needed.• Slimhole drilling = reduced costs – 25-75% (Finger, 1994)• Cheaper drilling tools, casing, cement jobs.• Shorter rig mobilization and de-mob time.• Smaller crews, fewer materials.• Provides information on the resource temperature, pressure and composition and also

indications of permeability and output.• Continuous cores provides high definition stratigraphic information and samples for

determination of reservoir properties.

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Slimhole drilling in Iceland – high temperature exploration

• High temperature exploration wells (5) were drilled in Krísuvík 1970-1972 to depths of ~800 m with a rig purchased especially for slimholes (Wabco).

• Casing program (1970):

• Slimholes have been drilled at Krafla (5) and Þeistareykir (1), Gjástykki (1) with a coring rig for determination of gradient and to correlate geology and resistivity. Max depth ~600 m.

Depth Drill bit Reaming Casing Thickenss(m) (") (") (") (mm)20 12-1/4 10-3/4 6,3

100 6-1/4 9 7/8 7 5250 6-1/4 4-1/2 3,7

200-800 3-7/8 2-1/2

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Slimhole drilling in Iceland – low temperature exploration

• Slimhole drilling of thermal gradient wells, percussion wells to 40-100 m, is common for exploration of low temperature resources. This has been found to be very economical and effective and aids in precise siting of production wells.

352°C/km 97°C/km

Slimhole rig

Mobile rig for low temperature drilling: -Thermal gradient wells 40-100 m-Slim production wells to 1100 m

Photo on right shows percussion drilling with simultaneous casing system (ODEX).

Slimholes (Sandia handbook)

• There is renewed interest in drilling slimholes, wells under 6” in diameter, as exploration wells. Cost USD/m ~850-1500 vs. 2000 USD for a full size well suitable for production.

1. Wells drilled with wireline coring rigs from the mineral exploration industry. Diameter of 3-4”, HQ and NQ coring to 1500 m.

2. Wells drilled with rotary rigs of 60-100 t hook load capacity, having a final diameter of ~6”. Casing program: 10 ¾”, 7” and a 4½” perforated liner to max 2000 m.

Key document on well design – New Zealand standard (2015)

The figure shows possible combinations of API casing and bit sizes that can make up a casing program.- Large casing program red line- Regular casing program blue- Slimhole green

Note for slimhole:The open hole section is with a is 4 ½" slotted liner in a 6 ⅛" hole.

API Casing and bit programs - combinations

Slimhole

Regular

Large

New New Zealand std. (2015) – minimum casing depth criteria

Figure from draft (not approved):The minimum design depths of each cemented casing string or liner shall be chosen so that formations at each casing shoe have sufficient “Effective Containment Pressure” to equal the “Maximum Design Pressure” expected to be encountered in the next open-hole section.

Compact drill site for a coring rig

Tricone bit 6 1/8"Cemented casing HWTDrilling fluid: Bentonite mud150-170 m

Wireline coring NQ (75.8 mm)Liner: Holed NQ rods (not cemented)

Drilling fluid: Bentonite mud or water

1000 - 1500 m

Wireline coring HQ (96 mm)Cemented casing NWDrilling fluid: Bentonite mud

500-700 m

Casing head flange 6" x 600 ANSIwelded on HWT (4 ½") casing

Sverrir 04.10.2014

6 ¼” Annularblow out preventerAPI 7 1/16" x 2M flangesinstalled at 150-170 m

Master ValveGate valve6" x 600 ANSISS trim, rising stem

Stripper rubber to seal aroundcore rods and divert the flow

Flow line withbutterfly valve

Two flanged side outletswith 2" x 600 ANSI valves

Drainto pit

Cellar depth2-2.5 m

depending onwellhead and rigselection

Conductor 7", 4-8 m deep

~Top of cellar

½” Press

GEOTHERMALEXPLORATION WELLSLIMHOLE(wireline coring)

Cored well design for HTModifications required for geothermal drilling:1. Conductor and anchor casing are stronger (API)

and with an added clearance (annulus) for cementing.Drilling with tricone bit or reaming.

2. Production casing is cemented in spite of small annulus – reverse circulation.

3. Casing head has two side outlets for kill and choke with a dedicated extra pump for use after circulation loss.

4. Wellhead consist of a master valve, annular blowout preventer and a stripper to seal around the drill pipes. There is a valve on the flow line. Installation requires a cellar.

5. A lubricator is required for tripping-in to retrieve core barrel in case of a kick (next slide).

6. A NQ technical casing can be installed from the wellhead Tee to ~250 m depth to stimulate the well to self-flow by air lifting.

When doing HT coring, you need…

Slimhole casing design: Cored exploration well to 1200 m

Coring rig

• Compact size of a coring rig.• No substructure but requires a deep cellar

for wellhead and BOP.

Slimhole 6 ⅛"

• This slimhole can be drilled with rigs of 60 to 100 t hook load capacity.

• Such rigs are truck mounted or on a trailer and require a substructure or alternatively a deep cellar.

• Temperature, pressure and spinner logs and samples are possible.

• The wells may be flow tested.• The wells are drilled with conventional tri-

cone bits.• Such wells can reach 2000-2500 m,

depending on the drill string and hook load rating of the rig and pump size.

Slimhole drilling (6 ⅛") with a 100 t hook load rig

Note:- Top drive rig 100 t- Rotary drilling – tri cone- Drilling 6 ⅛" to 1700 m- Compact drill site- High substructure- Concrete mud pit- All major equipment as for

production drilling- Mud cleaning system- Cooling tower for mud- Cementing equipment

Slimhole 2¾" (NQ)

• This well is drilled with a coring rig, the last two sections HQ and NQ.

• The coring rigs are truck mounted and typically have a hook load of 20 t.

• The rig requires a substructure and a cellar for the well control equipment (BOP).

Coring rig on location

Note:-Truck mounted coring rig-Subbase and ramp-Deep cellar (not shown)-Water truck-Water storage in balloons-Rig data logger in container-High altitude drilling 5200 m

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Rig hook load rating requirements

• The diagram shows how deep a particular rig can go, based on its hook load capacity (t) and diameter of drill pipes.

• The weight (in water) of casings is shown as dotted lines.

• NQ and HQ are light weight coring drilling rods.

• Examples for small rigs of 100 t and 50 t hook load are shown for reference, with a safety factor (SF) of 1.33.

Slimhole drilling

• ROP• Rotary drilling ~70-100 m/day• Coring ~25-40 m/day

• Circulation• Rotary ~15-30 L/s• Coring ~5 L/s

• Drilling fluids• Benotnite and viscosifiers, maintain high pH.• In total loss: Water and hi-vis polymer pills every joint. Also pump water down annulus to

lubricate the string.• Problem of bit cooling for such low circulation rates.

• Drill bits• Tri-cone bits with tungsten carbide inserts and PDC bits.

• Cementing• No loss: Cementing through a cement head with plugs or reverse cementing.• In total loss: Cement up to the loss, then reverse.

Analysis of drilling and logging data for reservoir information

• Estimation of formation temperature from:• Alteration mineralogy. By microscopic analysis and X-ray.• Static formation test. Temperature build-up on bottom for ~6 hr.• Various temperature logs.

• Location of productive fractures and permeable zones:• Measurement of fluid losses: continuous in flow line, intermittent with tanks.• Temperature logs.• Spinner logs.• From cutting analysis.• Caliper logs show fracture zones.• Borehole televiewer.

• Structure:• From cutting analysis: lithological logs.• Hardness from drilling data.

Drilling data

Geological description:• Stratigraphy• Alteration type• Intrusion• Feed point• Remarks

Drilling parameters:• Rate of penetration (ROP)• Weight on bit (WOB)• Circulation loss (L/s)• Total weight (t)• Pumping (L/s)• Pump pressure (bar)• Temp. difference (˚C)• Torque (dNm)

Flow testing of cored well – some outcomes

Production test setup – flow line

Slimhole output curve - example

Flow testing with compact equipment designed for a cored well