PROCEEDINGS, 45th Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 10-12, 2020

SGP-TR-216

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Slim Hole Drilling Overview for Geothermal Exploration in Indonesia: Potential and

Challenges

Daniel W. Aditatama, Dorman Purba, Sunarso, Farhan Muhammad

Equity Tower, 49th Floor Jl Jend Sudirman, SCBD, Jakarta

daniel.adityatama@rigsis.com

Keywords: geothermal, drilling, exploration, Indonesia

ABSTRACT

Unlike other energy sources for power generation such as solar and wind, geothermal energy is stored deep under the surface.

Exploration activity and drilling are required to find and confirm the presence and magnitude of the resource before the feasibility of the

project can be assessed. This high upfront cost in the early stage of the project while the uncertainty is still very high makes investor

reluctant to invest in the geothermal exploration activity. One alternative to reduce the resource risk during exploration is by using less

expensive drilling method such as slim hole drilling instead of using standard or big hole drilling. Slim hole drilling was used for early

geothermal exploration in Indonesia since 1993, but the recent trend in Indonesia shows that most geothermal developers prefer to use

standard or big hole instead for exploration activity. This may cause a huge loss for the developer if they are unable to confirm the

geothermal resource during exploration drilling, and currently Government of Indonesia considers using slim hole drilling for reducing

the resource risk in the exploration.

The objectives of this paper are to give an overview of slim hole drilling and how it differs from standard or big hole, both from a

technical and economic aspect. The advantages, limitation, and challenges of slim hole drilling application for geothermal exploration in

Indonesia are also discussed. A case study from recent slim hole drilling in East Java is used for an example of current slim hole

utilization for geothermal exploration. The lessons learned from the case study then can be used to improve exploration and drilling

planning in the future.

1. INTRODUCTION

1.1. Indonesia Geothermal Target

Government of Indonesia (GoI) has set a target for 7,200 MW of geothermal installed capacity in 2025 (Presidential Decree), a steep

increase from the current installed capacity of around 2,000 MW (EBTKE, 2019). This target is ambitious to say the least, and requires

acceleration on exploration phase on various concession areas in Indonesia. Several studies have identified the challenges in developing

geothermal in Indonesia (Poernomo et al., 2015; DiPippo, 2016; Purba et al, 2019):

1. Insufficient 3G (geology, geochemistry, and geophysics) data in the area renders the early assessment inaccurate.

2. Very high resource risk in the early stage of the project.

3. High upfront investment cost, especially due to the commencement of the drilling to confirm the resource.

4. Several uncertainties in legal aspects and lack of cross-sector coordination.

5. Limited number of experts with specific competences in geothermal sector.

6. Social issues such as community rejection.

7. Lack of infrastructure especially in eastern Indonesia.

1.2. Resource Risk in Exploration Phase

Resource risk is defined as the uncertainties on the presence, extent, and the quality of the geothermal resource (Purba et al., 2019;

Matek, 2014). In general, the following aspects need to be found during exploration to be able to economically produce electricity from

geothermal energy (in a conventional hydrothermal system, excluding Enhanced Geothermal System / EGS):

Adequate temperature for preferred power generation option (single / double / triple flash, binary, etc).

Sufficient permeability to accommodate fluid flow in the reservoir.

Benign reservoir fluid.

The lack of subsurface data prior to drilling implies that until there is an enough well data, the resource uncertainty will still high. The

3G survey conducted may not be enough to proof the reservoir quality and characteristic, thus requiring a deep well drilling activity to

proof the result of 3G exploration. The exploration activity itself may contribute to 20% of the total project cost. This high upfront cost

for exploration and exploration drilling in particular in the early phase of the project when there is no means to generate revenue for a

while (up to 5-7 years) is what deters investor to invest in the geothermal exploration.

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1.3. Objective

The purposes of this paper are as follow:

1. Give overview on slim hole drilling and how it may be used for geothermal exploration activity in Indonesia;

2. How slim hole drilling differs from standard or big hole;

3. Overview on the advantages and limitation of slim hole drilling compared to standard or big hole;

4. Overview on the potential and challenges of utilizing slim hole drilling in Indonesia;

5. Extract lessons learned from the case study of the recent slim hole drilling activity in East Java.

This study is admittedly in the early phase. Authors welcome any feedback or question that might be useful to improve the quality of the

information presented on this paper.

2. SLIM HOLE DRILLING

2.1. Definition

There are several definitions of what size can be classified as slim hole drilling. Generically speaking, any well with a smaller diameter

than normal can be considered as slim hole drilling. SPE defines slim hole well as a well with a casing size less than 7” for 90% of its

depth, while ISOR defines slim hole well as a well with a final diameter less than 6” (Thorhallson, 2016). For this study, authors

proposed to define a well with production casing smaller than 7” as a slim hole well (Figure 1).

Figure 1. Common well configuration for big, standard, and slim hole (Purba et al., 2019).

2.2. Drilling Equipment

In contrast with standard or big hole well, a slim hole can be drilled with different equipment: diamond coring and rotary (Table 1).

Diamond coring uses high rpm and low weight on bit (WOB) to cut through rock and produce core, while on rotary drilling the

recovered samples are just rock fragments or cutting (Nielson and Garg, 2016). Drilling using diamond coring is less affected of loss

circulation zone compared to rotary drilling, while rotary drilling is normally having higher drilling rate.

Table 1. Comparison of typical rotary drilling rig and coring drill rig for slim hole application.

Rotary Drill Coring drill

Crew 5 plus toolpusher 2 or 3 with or w/o toolpusher

Mud logger Yes No

Mud engineer Yes Sometimes

Company man Yes Sometimes

Drill site 400 ft by 250 ft

Large pits (1,200 bbl) and sump

200 ft by 150 ft or smaller

Small pits (25 bbl) and sumps

Rig size 150 ft high

40 or more truck loads

30 ft high base (large BOP, flowlines)

Big mud pumps (600 GPM / 3,000 psi)

60 ft high

2 to 5 loads

11 ft high base (smaller stacks)

Small pumps (45 GPM / 1,100 psi)

Slim hole drilling using coring rig is commonly used in mining industry, which can drill up to 1,000 m depth, although rigs that can core

up to 2,000 m are also available and can be adapted to geothermal service. As coring rig is significantly smaller in size than

conventional drilling rig in oil and gas or geothermal, coring rig has mobility advantage and require smaller well pads, infrastructure,

and produce less environmental disturbance. This make slim hole coring rig suitable for geothermal exploration in Indonesia where the

savings in infrastructure (roads, well pad) and transportation cost can be substantial.

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2.3. Drilling Fluid (mud)

The purpose of drilling fluid is to remove / lift cutting to the surface, keep the hole stable, and lubricate and cool the drill bit. Diamond

coring rig usually uses circulation rate of around 10 gallons per minute (GPM), while rotary drilling requires hundreds of gallons per

minute (Nielson and Garg, 2016). However, using hybrid drilling strategy (combining rotary and coring) may require more than 10

GPM at some hole section.

2.4. Drilling Tubular

Diamond coring uses drilling rod with flush joints instead of drill pipe with large tool joints. The rod sizes are as follow:

P = 4.5” x 4”

H = 3.5” x 3.063”

N = 2.75” x 2.375”

The annular space between the hole and the drilling rod is small, ranging from 0.25” to 0.5”.

2.5. Advantages and Drawbacks of Slim Hole Drilling

Some of the advantages of slim hole well drilling are smaller rig and wellpad area requirement, and sometimes require minimum access

road improvement or construction compared to standard hole or big hole wells (Purba, Dimwani, & Adityatama, 2018).

On the other side, there are also several disadvantages or concerns when applying slim hole drilling (Delahunty, Nielson, & Shervais,

2012; Garg & Combs, 1993; Nielson & Garg, 2016; Þórhallsson, 2012):

More complicated to reach desired drilling depth due to smaller annulus clearance compared to standard-sized well.

Slow penetration rate compared to standard size hole.

Developer does not want to invest money in slim hole due to it is less likely to contribute to production even if it succeeds.

Require further investigation to predict discharge characteristic of large-diameter well based on slim hole wells discharge

characteristic.

3. CASE STUDY

3.1. Slim Hole Drilling for Geothermal Exploration in Indonesia

In 2016, a geothermal company in Indonesia conducted 2 slim hole drilling as part of exploration activities. Slim hole well offers

significant decrease of risk at lower cost compared to standard hole by reducing the exploration upfront cost while still be able to

accomplish resource estimation goal (Finger, 1998; Nielson & Garg, 2016; White et al., 2010).

Slim Hole IJN-01 is the first exploration well drilled the company. The well is located near the center of the Kendeng ancestral caldera

where the possible hydrological upflow was inferred from the results of geophysical surveys. This well was programmed to penetrate

and case off the interpreted thick clay cap and reach a maximum depth of 2,000 vertical depth, where hot, neutral chloride water

hypothetically exist.

The objectives of Ijen-01 were:

1. To assess the geothermal potential of the Blawan-Ijen geothermal prospect.

2. To gain an understanding of the reservoir characteristic such as thermal gradient, reservoir rock and capping mechanism (as

inferred from the MT survey), fluid chemistry and permeability.

3. To intersect and test the permeability of NE trending structure located immediately east of the proposed well.

3.2. Drilling Program and Casing Plan

3.3. Drilling Rig Specification

Ijen-01 was drilled using KWL-1600H multitasking rig with specification shown in the Table 1.

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Table 2: Rig KWL-1600H general specification

Figure 9 shows rig KWL-1600H at wellpad D during Ijen-01 slimhole drilling operation.

Figure 2: Rig KWL1600H operated by PT Golden Asia Petroleum drilling Ijen-01 at Blawan-Ijen geothermal field

EQUIPMENT SPECIFICATIONS

General Specification

Rig with capability to Open Hole and Coring Drilling completed with Engine 440 HP @ 1800 rpm. Rig also completed with Top Drive.

Mast and capacity

Total length is 12.43m 11m Haul Winch Travel with Hoisting capacity 34,100 lb (15.5 TON), Hoisting speed is 1.1 m/sec, Pull Load capacity is 65 TON.

Retract Force capacity is 40,700 lb (18.5 TON).

Wire Line Coring

Drum capacity capable for accommodate min 2200 m with 6 mm rope (0.24”), hoisting capacity min 3,307 lb (1.5 TON) bare drum or 1,774 lb (0.850 ton) at full drum, 4 m / sec hoisting speed.

Top Drive Head Traverse Length is 7.5 m (24.61 feet).

Retract Force (@5000 psi) 40,700 lb (18.5 TON).

Pull Down Force is 21,206 LB (9.6 TON).

Coring Pump Capable to deliver until 65 gpm (264 Liter per minute), working pressure 1500 psi

Triplex Mud Pump

Triplex Mud Pump 440 HP with Engine. Min Pressure 1776 – 3300 psi with liner size 3 – 4-1/2”. Flow rate 200 – 300 gpm.

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Figure 10: Wellpad layout of Ijen-01

Rig KWL-1600H only require a wellpad with dimension 50 x 35 m2 as shown in Figure 10.

Figure 8: Ijen-01 well design

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4. DRILLING RESULT

Ijen-01 was spudded on Jan 16th, 2016. First partial circulation was experienced at 15 m-MD. Drilling depth progress relative to

budgeted time is shown in Figure 11.

The time breakdown of Ijen-01 drilling is shown in Figure 13. The most time consuming is (drilling/coring/standby??) activity which

equal to xx% of total days.

Figure 3: Time breakdown of Ijen-01 drilling

Several operational challenges that are experienced in Ijen-01 drilling are:

Figure 11: Drilling day vs depth graph of Ijen-01

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Need to discuss with MCG drilling

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Regarding sub-surface information, Ijen-01 successfully obtained downhole information as follows:

Lithological log from surface to total depth (TD), from 12mVD – 2000 mVD

Lost circulation depth

??? need to discuss with MCG subsurface team on this section

4.1. Lessons Learned

From the Ijen-01 drilling operation, there were several difficulties and problems that were faced by drilling team:

4.1.1. Well design

Drilling down to 2000mMD TD was planned to be utilized mainly HQ and NQ pipe, and there was no contingency drill string to

anticipate any hole problem occurrence which prevent drilling to TD (loss circulation, pipe stuck, etc). The rig itself has capability to

utilize superior grade drill string of PQ down to 1000m-MD.

Enlarging hole from 3.75” bit size was proven to be difficult and also time consuming as the rig wasn’t designed (and equipped) with

sufficient pump and proper swivel to do so. (please check if it is really swivel?).

4.1.2. Equipment

There are some standard geothermal-required rig components that are not necessarily available in slim hole drilling rig, for example the

Blow Out Preventer (BOP). As the consequences, modification or addition of supporting substructures and loading arm is required.

Besides that, some modifications to make mud (or gas) flow from wellbore to surface comply with current practice used in geothermal

drilling, as it is not regulated in mining industry.

Slim hole drilling use length range 1 (R1, 18-22 feet) drilling pipe and drilling collar which is very uncommon in Indonesia (the most

common is R3 range).

As geothermal (exploration) are commonly located in remote are, back up equipment, spare part and consumables (including mud and

cement material) is need to be sufficiently prepared and provided.

4.1.3. Personnel

The difference in industry standard and regulation means that there is a high chance for the drilling rig crews for not having required

certification. The difference in standard and practice also implies that the common and critical practice in geothermal drilling is not

known by drilling rig crew, for example the blow out prevention procedure, hence requires additional training to familiarize the drilling

rig crew with those procedure. On top of those certification, geothermal-experience and equipment-handling skill is something need to

be consider as the key element on selecting the crews.

4.1.4. Proposed solutions for improvement

Several actions that might be considered to be taken on the future slimhole drilling operation are:

1. An extensive and detailed preparation is required especially in drill pipe and drill collar tubular procurement.

2. It is advised to do open hole from the very beginning (from the surface).

3. It will be better if there is an assurance of slimhole project continuity, as it will reduce the initial capital investment, whether it is

for rig crew training or for the procurement and modifications for drilling equipment that are currently unavailable in slimhole

drilling rig in Indonesia.

5. REFERENCES

Delahunty, C., Nielson, D. L., & Shervais, J. W. (2012). Deep core drilling of three slim geothermal holes, snake river

plains, idaho. Geothermal Resources Council Transactions, 36, 641-647.

EBTKE. (2016). Buku statistik EBTKE 2016. Jakarta: Direktorat Jenderal Energi Baru, Terbarukan dan Konservasi Energi.

EBTKE. (2017). Potensi panas bumi indonesia. Jakarta: Direktorat Jenderal Energi Baru, Terbarukan dan Konservasi

Energi.

Finger, J. T. (1998). Slimhole drilling for geothermal exploration. Reservoir Technology,

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Garg, S. K., & Combs, J. (1993). Use of slim holes for geothermal exploration and reservoir

assessment: A preliminary report on japanese experience.

Nielson, L. D., & Garg, S. K. (2016). (2016). Slim hole reservoir characterization for risk reduction. Paper presented at the

Proceeding of the 41st Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California,

Poernomo, A., Satar, S., Effendi, P., Kusuma, A., Azimudin, T., & Sudarwo, S. (2015). (2015). An overview of indonesia

geothermal Development–Current status and its challenges. Paper presented at the Proceedings World Geothermal

Congress, Melbourne, Australia,

Purba, D. P., Dimwani, W., & Adityatama, D. W.Basic consideration in minimizing the uncertainty during developing

geothermal exploration drilling strategy in indonesia. ITB International Geothermal Workshop 2018,

White, P., Ussher, G., Lovelock, B., Charroy, J., Alexander, K., & Clotworthy, A. (2010). (2010). Mita, a newly

discovered geothermal system in guatemala. Paper presented at the World Geothermal Congress, Bali, Indonesia,

2010, Proceedings, , 6

Þórhallsson, S. (2012). Slim wells for geothermal exploration. 001226914,