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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

Make-up wells drilling cost in financial model for a

geothermal project

Fitri Oktaviani Purwaningsih1, Ruly Husnie2, Waldy Afuar1, Gugun

Abdurrahman1
1
Study Program of Geothermal Engineering, Faculty of Mining and Petroleum,
Bandung Institute of Technology, Bandung, Indonesia
2
PT Geo Dipa Energy, Jakarta, Indonesia
Email : Feetre@gmail.com

Abstract. After commissioning of a power plant, geothermal reservoir will encounter pressure
decline, which will affect wells productivity. Therefore, further drilling is carried out to enhance
steam production. Make-up wells are production wells drilled inside an already confirmed
reservoir to maintain steam production in a certain level. Based on Sanyal (2004), geothermal
power cost consists of three components, those are capital cost, O&M cost and make-up drilling
cost. The make-up drilling cost component is a major part of power cost which will give big
influence in a whole economical value of the project.

The objective of this paper it to analyse the make-up wells drilling cost component in financial
model of a geothermal power project. The research will calculate make-up wells requirements,
drilling costs as a function of time and how they influence the financial model and affect the
power cost. The best scenario in determining make-up wells strategy in relation with the project
financial model would be the result of this research.

1. Life cycle of a geothermal field

A geothermal field has a life cycle that is different with other energy resource. It consists of four periods:
(1) Developing, (2) Sustaining, (3) Declining and (4) Renewable [2]. In developing period, the
increments of a plant capacity come in line in steps. In sustaining period, the output of the field remains
steady over an extended period of time. The declining period begins when the underlying declines in the
capacity of individual wells become evident in the actual output of the entire facility. The renewable
period would achieved when the field reach a state in which net mass withdrawals in production stage
and the operations are still profitable at this level.
A geothermal power plant is built to generate a specific capacity that requires a certain supply
of geothermal steam. One of the consequence in a continuing operation of a geothermal field is
decreasing in production of steam. A lowering production to below the limit would cause a reduction in
plant output. A continued reduced output would reducing the economic value of the project along with
inability to meet previous contract with the electricity buyer.
Typically, a developer will drill enough wells at the early period of the project to be able to meet
plant capacity requirement, even set a limit of minimal surplus capacity at the wellhead to anticipate
declines in well productivity. Later, when previous production wells have decreasing production,
constant output is sustained by drilling make-up wells [2].

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd 1
6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

2. Flowchart of this paper

The objective of this paper is to analyze make-up well cost component in a financial model of a
geothermal power project. In this research, synthetic data is used to calculate make-up wells requirement
to maintain sustainability electricity production of 55 MW plant with project lifetime 30 years. The data
is obtained from sample case in a geothermal field in Indonesia. The project is assumed to starts in 2017
with four years allocation time for exploration, feasibility study stage and plant development. The field
is estimated to start producing electricity by 2021. The complete flow chart of this research is in figure
1.

Start

· Well Data,
· Surface Facility,
· Plant Capacity,

Calculate Required
Steam Supply

Calculate Minimum Numbers of

Production Wells

Assumptions:
· Inflation,
· Depreciation,
· Success Ratio
· Decline Rate

Lifetime Project,
Drilling Campaign

Calculate Required Numbers

of Make-up Wells

NO

Financial Model

NPV > 0
IRR > 16%

YES

Finish

Figure 1. Working flowchart of the research

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

Data collection is the first step done from this research. The required data is:
1. Well data retrieved from exploration and production wells
2. Surface facilities data, that include separator pressure, turbine characteristics and other surface
facilities components
Those data is required to determine required minimum steam supply to generate total capacity of 55
MW and accommodate 10% of excess steam supply and to calculate required number of production
wells. Then the data is used to calculate required number of make-up wells. The number of make-up
wells and its drilling cost then put into a geothermal financial model.
The financial model will calculate make-up well cost together with other parameters, including
exploration, well drilling, O&M and other costs, as well as revenue, tax, interest and all parameters that
built the model. The model then will used to evaluate value of the project using some financial
parameters as NPV (Net Present Value), IRR (Internal Rate of Return) and PBP (Payback Period).
This research will study make-up wells cost as one of component in a financial model. Evaluation
of the project with NPV, IRR and PI calculation would be done only to describe variation of those
financial indicators related with different strategy in make-up wells drilling. Calculation will be done
using basic financial model with expected result NPV > 0 and IRR > 16%. The IRR number is retrieved
from Abadi Purnomo (2007) stated that ideal IRR of a geothermal project is 16%.

3. Required parameters to calculate make-up wells

Miller stated that the term ‘make-up’ wells are applied to all wells drilled after plant start-up [3]. The
timing of the drilling of make-up wells and the number of those wells can make the difference between
project economic success and failure. The required number for make-up wells and the timing of drilling
those wells is directly dependent on some aspects as: plant capacity, life time of the project, capacity of
production wells, decline rates of the field and the minimum steam allowance.
Make-up wells are drilled to accommodate steam required by the geothermal plant to be able to
produce certain capacity. There are some parameters that need to be determined to calculate the number
of make-up wells required in a geothermal field for a specific lifetime project. The most important
parameters are plant capacity, lifetime of a project, average flow rate of geothermal well and steam
consumption rate to produce electricity. All calculation in this research use data from a synthetic field
of a steam dominated geothermal plant in Indonesia.

Table 1. Parameters of the field and calculation of number of wells required as production
wells in the beginning of the project.
No. Parameter Value Simbol
1 Plant Capacity 55 MWe
2 Lifetime project 30 year
3 Turbin Input Pressure 6.5 bar
4 Turbin Efficiency 85% Ƞt
5 Steam flow rate 371 ton/h
6 Specific steam consumption 6.7 ton/h/MW
7 Average well production rate 20 kg/s
8 Steam fraction in well 90%
9 Steam flow rate from a well 18 kg/s
10 Total mass flow rate (required) 114.6 kg/s
11 Well drilling success ratio 80%
12 Average well capacity 9.6 MW
13 Required production wells 8 unit
Make-up wells calculation parameters

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

No. Parameter Value Simbol

14 Excess Steam 10%
15 Minimum Steam Supply 60.5 MW
16 Decline Rate 3% Per Year
17 Average Production Capacity 9.6 MW/well

Some parameters in table 1 determined based on data on synthetic field in a steam dominated
geothermal field in Indonesia. Some others are calculated based on previous data. Calculation in this
paper use specific steam consumption 6.7 ton/hour/MW and plant capacity 55 MW. Based on the
calculation, the requirement of productions wells that have to be drilled in the beginning of the project
is 8 wells with each well have flow rate 20 kg/s or able to produce 9.6 MW in average. It means that the
geothermal plant need steam supply similar to 114.6 kg/s to be able to produce 55 MW in electricity.
The requirement is fulfilled by drilling 8 production wells with flow rate 18 kg/s and turbine efficiency
85%, or capacity 9.6 MW/well.
At initial production, there are 8 production wells that produce steam to fulfil plant
requirements. Concomitant with time, the field will have production decline due to lowering pressure,
scaling or other factor and the production will decline. Then the make-up wells need to be drilled to
fulfil steam production requirement.

4. Calculating make-up wells

To be able to calculate the number of make-up wells, there are some other parameters required. Those
are: excess steam, minimum steam supply and decline rate. Excess steam is a level of surplus in steam
production to maintain supply to geothermal plant above minimum requirement. In this research the
calculation for make-up wells will use excess steam 10%.Minimum steam supply is a number of
minimum steam production supplies to geothermal power plant. With excess steam 10% then the
minimum supplies value is 110% of plant requirement.
Decline rate is directly related to wells capacity to supply steam to geothermal power plant.
Well productivity decline rates may vary widely in different geothermal fields. In this paper decline
value is determined in 3% per year as stated in Adiprana (2015). Those parameters are calculated in
order to determine the required time to drill make-up wells and number of wells to be drilled. Specific
values of each parameter are in table 1.
Based on table 1, minimum flow rate to be applied to supply a 55 MW geothermal plant is 114.6
kg/s or equal to 55 MW. But ideal requirement is about 60.5 MW considering 10% excess steam
requirement. Thus, when production rate almost reach 60.5 MW or below, the field needs addition
supply, which can be accommodated by drilling make-up wells.
However, there are some limitations that need to be considered in make-up wells drilling
procedure as:
1. The real decline rate is vary through the year and wells, but in this research we use
similar decline rate for the field to forecast the production decline in whole period of
plant (30 years)
2. The minimum number of wells drilled in a drilling campaign period is 2 or 3,
considering the high rig mobilization cost. This research will use different number of
minimum wells drilled in a drilling campaign period as part of determining make-up
wells strategy.
3. Success rate of well make-up well drilling will affect the number of required well
drilling in each drilling campaign period. Calculation in this paper will use success rate
80%.

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

Figure 2. Make-up calculation of Scenario 1

Figure 2 contains make-up wells strategy for the whole 30 years of project. The field needs 8
make-up wells to maintain production level. Those wells need to be drilled in two drilling campaigns,
four wells each, considering the 80% success ratio. The drilling time would be in year 8 (2029) and year
25 (2046) with assumption that the project start in 2017 (for exploration and feasibility stage) and
electricity production start in 2021.

5. Make-up well cost in financial model

Make-up well drilling is treated as an occasional operating cost allocated to the years in which is
anticipated that additional drilling will be required [1]. Inserting the make-up wells drilling into financial
model must consider some important parameters, as inflation and depreciation.
Inflation is defined as a sustained increase in the general level of prices and usually measured
as an annual percentage increase (Investopedia). In make-up wells cost component, the cost is increasing
with a certain degree of inflation. The escalation cost is influenced by initial drilling cost in the start of
the project, inflation rate and year, the longer drilling time from year start of the project and the higher
inflation rate, the more costly the drilling price would be.
The calculation of make-up well cost is using initial cost of make-up well drilling as USD 8
million per well in start year of the project (2017), and inflation rate based on US PPI (United States
Producer Price Index) 2.5% in January 2017.
The more detailed calculation of present make-up well cost is in APPENDIX A. Based on
previous calculation, the additional make-up wells are required in year 8 (2029) and year 25 (2046) of
the project. Considering inflation, the required cost to fund 4 make-up wells drilling in 2029 is USD
38.98 million and USD 59.33 million in 2046. The total cost to drill make-up wells for the whole project
is USD 98.3 million.

6. Geothermal financial model

Every geothermal project is different with others because each of them has their own resource
characteristics with different risks and opportunity. The cost driver also behaves differently under
different conditions. Sanyal stated that geothermal power cost consists of three components, those are
capital cost, O&M cost and make-up drilling cost [5]. The make-up drilling cost component is a major
part of power cost because it requires big amount of money that must be spent during the plant project.

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

A financial model is anything that is used to calculate, forecast or estimate financial numbers.
A valuable financial model integrates accurately all the costs throughout all phases of development and
presents the resulting information in a manner to help various users make the appropriate decision
(Ngugi, 2014). One of the objectives of this paper is to provide analysis on technical aspect of a
geothermal project, focusing on make-up wells cost to apply in a proper financial model of the project.
The research will not focus on detailed data on specific process and determined cost in a
financial model, yet it will explain about how to put the make-up well cost properly in the structure of
general geothermal financial model.

6.1. Make-up well cost in geothermal financial model

A financial model in general is designed to meet the information need of investor and decision maker
with objective to consider whether the investment for the project is viable. The model is consists of four
financial statements that prepared annually in most cases, present the result and financial position of a
project (or a company) by a certain date (Ngugi, 2014). Those are:
1. Income statements
The objective of income statement is to establish the profitability of the project. The main
elements in this statement are revenue, expenses, depreciation, interest and tax.
2. Balance sheets
This statement detailed financial position and consists of two major grouping, the assets and the
equity and liability.
3. Cash flow statement
This statement mainly account for the cash availability to the company or project owner.
Amount of the cash accounted will define how much it can meet its financial obligation. The
statement is divided into three major segments: cash flow from operation, cash flow from
investment activities and cash flow from financing activities.
4. Statement of owner’s equity

The make-well costs are included in a geothermal financial model in some statements as:
· Operating expenses in Cash Flow Statements, that encompass all cost directly and indirectly
associated with the generation of the sold energy
· Depreciation expenses, because the make-up well cost is applied depreciation with a straight
line or fixed percentage deduction or other depreciation method applied

6.2. Make-Up Well Cost in Cash Flow Statement

Make-up wells cost is occasional operating cost which allocation years and amount is retrieved from
previous calculation in table 2. Forecasting the year of required drilling time needs technical calculation
while the amount of required cost would be affected by inflation rate. If there is more than one power
plant unit in the project, calculating the requirement of make-up well and forecasting their cost must be
done for each unit.

6.3. Make-up Well Cost in Depreciation Expense

Depreciation is an accounting method of allocating the cost of tangible asset over its useful life. A
business financial model will depreciate long-term asset for both tax and accounting purposes. For tax
purpose, businesses can deduct the cost of the tangible assets as business expense. For accounting
purpose, depreciation expense does not represent a cash transaction but it indicates how much of an
asset’s value has been used up over time.
Calculation depreciation of make-up well cost will provide better basis for investment decision.
The life of a make-up well would be calculated based on the time it was drilled. There are some methods
to calculate depreciation: the Straight Line and the Double Declining method. Besides choosing the
method, calculating depreciation also must consider the depreciation period time.

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

6.4. Make-Up Well Cost in Balance Sheet

Make-up Wells are considered as fix asset with book value calculated by taking the cost minus the
accumulated depreciation. Book values of make-up wells cost are carried in the balance sheet with other
fix assets as exploration book value and power plant.

7. Make-up well cost in determining investment feasibility of a project

Make-Up well cost will determine the feasibility of a geothermal project. Make-up wells drilling cost is
varying with range USD 3.2 Million (SKM, 2009) to 6 Million (Sudarman, 2012). The cost will increase
along the time and will be allocated as operating cost for the whole project. The huge cost of make-up
wells drilling along the project would affect the investment feasibility.
Some financial indicators as Nett Present Value (NPV), Internal Rate of Return (IRR) and
Payback Period (PBP) determine investment feasibility of a geothermal project.

7.1. Nett Present Value (NPV)

NPV is equal to present value of cash flow substract with initial investment, which represented by
following formula:
𝐶
𝑁𝑃𝑉 = ∑𝑛𝑡=1 (1+𝑟)
𝑡
𝑡 − 𝐶0 (1)

With r is internal rate of return (IRR), C0 is initial investment, Ct is cash flow in Year t and n is
period of investment in year. Positive NPV indicate that the present value of cash flow is more than
initial investment. Investor would need positive NPV that represent they have benefit from their
investment.

7.2. Internal Rate of Return (IRR)

IRR is the discount rate that makes the present value of investment’s income stream total to zero.
Following formula represents the IRR:
(𝐶𝐹 )
∑𝑁 𝑡
𝑡=0 (1+𝐼𝑅𝑅)𝑡 = 0 (2)

With CFt is cash flow at year t. The IRR is expected rate of return of the project that can be used to
forecast margin. When the IRR is larger than the cost of capital, the project will generate a positive
margin for the shareholder. Therefore a project with IRR greater than the cost of capital will increase
the project worth.

7.3. Payback Period (PBP)

The payback period is the length of time required to recover the cost of an investment. This indicator is
important to determine whether to undertake the position or project, as longer payback periods are
usually not desirable for investment position. The amount of make-up wells cost along the whole time
of the project will affect those financial feasibility projects. The huge cost that allocated for some periods
of the project will influence NPV, IRR and Payback Period.
Calculation of those financial indicators would not be done in this research. That calculation
would need further research and more complex parameters and project strategy that would not cover in
this paper.
However, there are some factors that influence make-up wells calculation in cash flow that will
generate different value of IRR, NPV and Payback Period. Those are:
1. Amount of initial well drilling cost
2. Inflation rate
3. Minimum well drilled in a drilling campaign period
4. Make-up well drilling success ratio

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IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

5. Allocated drilling time

Considering the time value of money, the bigger cost in the beginning of the project will
generate smaller NPV and IRR and longer PBP. Further research must be done to find out how different
strategy of make-up wells drilling strategy influences those indicators. The sensitivity analysis also
needs to be done to determine how different strategy of make-up well drilling impact NPV, IRR and
PBP.

8. Make-up well cost affect project financial

Determining when and how much make-up wells need to be drilled is one of production strategy. In
technical view, the make-up wells should be drilled soon after production rate decreasing to the
minimum requirement. In financial consideration make-up wells should be drilled in cost efficient
strategy.
In this paper, we only present four scenarios. Actually, in the real world there are many possible
scenarios regarding the number of required make-up wells to be drilled and when they are drilled. The
research choose only four possible scenarios only based on minimum steam requirement and minimum
wells drilled at one drilling period. In this research the limitation of minimum wells drilled in one drilling
period is 3 wells, based on economical reason and experience in real case in geothermal field.
To be able to know how big make-up well cost affects the feasibility of a geothermal project,
this paper will compare financial indicators from four different make-up wells strategy. Both are in the
same field and use similar data as previously calculated. The differences between those scenarios are
drilling time and the number of wells drilled in a drilling campaign period.
All the scenarios were analysed using basic geothermal financial model with constant cost and
general assumption. The objective of this analysis is to compare the result of NPV, IRR and PBP
indicator for different make-up wells strategies

8.1. Scenario 1
Based on figure 2, total make-up wells = 8 wells (4 wells in 2029 and 4 wells in 2046), total make-up
well cost = USD 98.3 Million (APPENDIX A).

8.2. Scenario 2
Based on scenario 2 in figure 3, total make-up wells = 8 wells, all drilled in 2029, total make-up well
cost = USD 77.97 Million (APPENDIX B).

Figure 3. Make-up calculation of Scenario 2

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

8.3. Strategy 3
Based on scenario 3 in figure 4, total make-up wells = 8 wells (4 wells in 2029 and 4 wells in 2038),
total make-up well cost = USD 87.68 Million (APPENDIX C).

Figure 4. Make-up calculation of Scenario 3

8.4. Strategy 4
Based on scenario 4 in figure 5, total make-up wells = 6 wells (3 wells in 2029 and 4 wells in 2042),
total make-up well cost = USD 69.55 Million (APPENDIX D).

Figure 5. Make-up calculation of Scenario 4

Using scenario 1, 2, 3 and 4, financial indicators of the project are description in table 2.

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

Table 2: NPV and IRR and Payback.

Scenario NPV IRR Payback Period
1 74 Million 17.5% 11 years
2 66 Million 16.5% 11 years
3 72 Million 17.4% 11 years
4 77 Million 17.8% 11 years

9. Resume of 4 make-up wells strategies

Resume of analyses from previous make-up wells drilling strategies are as in Table below:

Table 3. Resume of financial model for all scenario

Resume Drilling Time Total Make-Up Wells NPV IRR (%) Payback
Cost (USD Million) (USD Period
Million) (Year)
Strategy 1 4 Wells 2029 98.32 74 17.5 11
4 Wells 2046
Strategy 2 8 Wells 2029 77.98 66 16.5 11
Strategy 3 4 Wells 2029 87.68 72 17.4 11
4 Wells 2038
Strategy 4 3 Wells 2029 69.55 77 17.8 11

Based on calculation that had been done using financial model for 4 different make-up wells drilling
strategies, there are some conclusions as:
· Different strategies in drilling make-up wells will require different cost
· Different time of drilling and number of make-up wells drilled will affect NPV and IRR of the
project, but not significant
· Degree of IRR difference is between 0.1 to 6% or 0.16 to 1.2 point for average IRR Project
17.3%
· Amount of NPV difference is between 2.7 to 6% or USD 2 to 5 Million for average NPV project
USD 73.75 Million
· The Payback Period is similar for all strategies

10. Conclusion
From this study, there are some points that need to be considered in calculating make-up wells
requirement in a geothermal project and development. Calculations make-up well was not only based
on required power plant and decline rate, but also financial factors of the developer and drilling vendor
into the calculations that the optimum strategy. All calculation in those scenarios using electricity price
USD 12 ¢/kWh
• The best IRR (17.76%) is obtained with make-up wells strategy that used fewest make-up wells
• The best NPV USD 98.32 Million is obtained with make-up wells strategy that used 8 wells in
2 drilling
Based on technical requirement, the make-up wells strategy could vary as long as the steam
production not below the minimal production level. Then applying a make-up wells strategy should
consider not only technical requirement, but also the financial consideration.
Suggestion for further work is to develop more accurate model to find the most optimum strategy
of make-up wells scenario in a geothermal field.

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IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

11. References
[1] Randle, James B. (2005), "Financial Modelling of Geothermal Project,” Proceeding World
Geothermal Congress, Antalya, Turki
[2] Lovekin, James W. (1998), "Sustainable Geothermal Power: The Life Cycle of a Geothermal
Field,” Geothermal Resource Council Transaction, Vol. 22
[3] Miller, Richard J. (1982), "The Influence of Declines Rates and Pressure Interference Effects of
the Economic Viability of Vapor-Phase Geothermal Reservoir Development,” New Zealand
Geothermal Workshop
[4] Sanyal, Subir K. (2004), "Cost of Geothermal Power and Factors that Affect it,” Proceeding
Twenty-Ninth Workshop on Geothermal Reservoir Engineering, Stanford University,
California
[5] Sanyal, Subir K. and Morrow, James W. (2004), "An Investigation of Drilling Success in
Geothermal Exploration, Development and Operation,” GRC Transaction, Vol. 35

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

APPENDIX A. Calculation of required time, which is anticipated that additional make-up wells
drilling is required and make-up well (Scenario 1)

Available Additional Available

Production Well Capacity Required Make Up Well
before make Make-up After Make-
Year (MW) (MW) Cost (USD)
up (MW) Wells up (MW)

2021 55 60.5 76.82 0 76.82 0

2022 55 60.5 74.51 0 74.51 0
2023 55 60.5 72.28 0 72.28 0
2024 55 60.5 70.11 0 70.11 0
2025 55 60.5 68.01 0 68.01 0
2026 55 60.5 65.97 0 65.97 0
2027 55 60.5 63.99 0 63.99 0
2028 55 60.5 62.07 0 62.07 0
2029 55 60.5 60.21 4 98.62 38,988,893
2030 55 60.5 95.66 0 95.66 0
2031 55 60.5 92.79 0 92.79 0
2032 55 60.5 90.00 0 90.00 0
2033 55 60.5 87.30 0 87.30 0
2034 55 60.5 84.68 0 84.68 0
2035 55 60.5 82.14 0 82.14 0
2036 55 60.5 79.68 0 79.68 0
2037 55 60.5 77.29 0 77.29 0
2038 55 60.5 74.97 0 74.97 0
2039 55 60.5 72.72 0 72.72 0
2040 55 60.5 70.54 0 70.54 0
2041 55 60.5 68.42 0 68.42 0
2042 55 60.5 66.37 0 66.37 0
2043 55 60.5 64.38 0 64.38 0
2044 55 60.5 62.45 0 62.45 0
2045 55 60.5 60.57 0 60.57 0
2046 55 60.5 58.76 4 97.17 59,326,211
2047 55 60.5 94.25 0 94.25 0
2048 55 60.5 91.42 0 91.42 0
2049 55 60.5 88.68 0 88.68 0
2050 55 60.5 86.02 0 86.02 0
2051 55 60.5 83.44 0 83.44 0
Total 8 98,315,104

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6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

APPENDIX B. Calculation of required time, which is anticipated that additional make-up wells
drilling is required and make-up well (Scenario 2)
Well Available Additional Available
Production Required Make Up Well
Capacity before make Make-up After Make-
Year (MW) Cost (USD)
(MW) up (MW) Wells up (MW)

2021 55 60.5 76.82 0 76.82 0

2022 55 60.5 74.51 0 74.51 0
2023 55 60.5 72.28 0 72.28 0
2024 55 60.5 70.11 0 70.11 0
2025 55 60.5 68.01 0 68.01 0
2026 55 60.5 65.97 0 65.97 0
2027 55 60.5 63.99 0 63.99 0
2028 55 60.5 62.07 0 62.07 0
2029 55 60.5 60.21 8 137.02 77,977,785
2030 55 60.5 132.91 0 132.91 0
2031 55 60.5 128.93 0 128.93 0
2032 55 60.5 125.06 0 125.06 0
2033 55 60.5 121.31 0 121.31 0
2034 55 60.5 117.67 0 117.67 0
2035 55 60.5 114.14 0 114.14 0
2036 55 60.5 110.71 0 110.71 0
2037 55 60.5 107.39 0 107.39 0
2038 55 60.5 104.17 0 104.17 0
2039 55 60.5 101.05 0 101.05 0
2040 55 60.5 98.01 0 98.01 0
2041 55 60.5 95.07 0 95.07 0
2042 55 60.5 92.22 0 92.22 0
2043 55 60.5 89.45 0 89.45 0
2044 55 60.5 86.77 0 86.77 0
2045 55 60.5 84.17 0 84.17 0
2046 55 60.5 81.64 0 81.64 0
2047 55 60.5 79.19 0 79.19 0
2048 55 60.5 76.82 0 76.82 0
2049 55 60.5 74.51 0 74.51 0
2050 55 60.5 72.28 0 72.28 0
2051 55 60.5 70.11 0 70.11 0
Total 8 77,977,785

13
6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

APPENDIX C. Calculation of required time, which is anticipated that additional make-up wells
drilling is required and make-up well (Scenario 3)
Available
Well Additional Available After
Production Required before Make Up Well
Capacity Make-up Make-up
Year (MW) make up Cost (USD)
(MW) Wells (MW)
(MW)

2021 55 60.5 76.82 0 76.82 0

2022 55 60.5 74.51 0 74.51 0
2023 55 60.5 72.28 0 72.28 0
2024 55 60.5 70.11 0 70.11 0
2025 55 60.5 68.01 0 68.01 0
2026 55 60.5 65.97 0 65.97 0
2027 55 60.5 63.99 0 63.99 0
2028 55 60.5 62.07 0 62.07 0
2029 55 60.5 60.21 4 98.62 38,988,893
2030 55 60.5 95.66 0 95.66 0
2031 55 60.5 92.79 0 92.79 0
2032 55 60.5 90.00 0 90.00 0
2033 55 60.5 87.30 0 87.30 0
2034 55 60.5 84.68 0 84.68 0
2035 55 60.5 82.14 0 82.14 0
2036 55 60.5 79.68 0 79.68 0
2037 55 60.5 77.29 0 77.29 0
2038 55 60.5 74.97 4 113.38 48,691,784
2039 55 60.5 109.98 0 109.98 0
2040 55 60.5 106.68 0 106.68 0
2041 55 60.5 103.48 0 103.48 0
2042 55 60.5 100.37 0 100.37 0
2043 55 60.5 97.36 0 97.36 0
2044 55 60.5 94.44 0 94.44 0
2045 55 60.5 91.61 0 91.61 0
2046 55 60.5 88.86 0 88.86 0
2047 55 60.5 86.19 0 86.19 0
2048 55 60.5 83.61 0 83.61 0
2049 55 60.5 81.10 0 81.10 0
2050 55 60.5 78.67 0 78.67 0
2051 55 60.5 76.31 0 76.31 0
Total 8 87,680,677

14
6th ITB International Geothermal Workshop (IIGW2017) IOP Publishing
IOP Conf. Series: Earth and Environmental Science1234567890
103 (2018) 012010 doi:10.1088/1755-1315/103/1/012010

APPENDIX D. Calculation of required time, which is anticipated that additional make-up wells
drilling is required and make-up well (Scenario 4)
Available
Well Additional Available After
Production Required before Make Up Well
Capacity Make-up Make-up
Year (MW) make up Cost (USD)
(MW) Wells (MW)
(MW)

2021 55 60.5 76.82 0 76.82 0

2022 55 60.5 74.51 0 74.51 0
2023 55 60.5 72.28 0 72.28 0
2024 55 60.5 70.11 0 70.11 0
2025 55 60.5 68.01 0 68.01 0
2026 55 60.5 65.97 0 65.97 0
2027 55 60.5 63.99 0 63.99 0
2028 55 60.5 62.07 0 62.07 0
2029 55 60.5 60.21 3 89.01 29,241,670
2030 55 60.5 86.34 0 86.34 0
2031 55 60.5 83.75 0 83.75 0
2032 55 60.5 81.24 0 81.24 0
2033 55 60.5 78.80 0 78.80 0
2034 55 60.5 76.44 0 76.44 0
2035 55 60.5 74.15 0 74.15 0
2036 55 60.5 71.92 0 71.92 0
2037 55 60.5 69.76 0 69.76 0
2038 55 60.5 67.67 0 67.67 0
2039 55 60.5 65.64 0 65.64 0
2040 55 60.5 63.67 0 63.67 0
2041 55 60.5 61.76 0 61.76 0
2042 55 60.5 59.91 3 88.72 40,309,964
2043 55 60.5 86.05 0 86.05 0
2044 55 60.5 83.47 0 83.47 0
2045 55 60.5 80.97 0 80.97 0
2046 55 60.5 78.54 0 78.54 0
2047 55 60.5 76.18 0 76.18 0
2048 55 60.5 73.90 0 73.90 0
2049 55 60.5 71.68 0 71.68 0
2050 55 60.5 69.53 0 69.53 0
2051 55 60.5 67.44 0 67.44 0
Total 6 69,551,634

15