1 1. INTRODUCTION This report is a partial contribution from Iceland to the International Energy Agency Geothermal Implementing Agreement (IEA-GIA), Annexes VII and XI and the Lower Cost Drilling Annex of the International Partner- ship for Geothermal Technology (IPGT). After successful development of the Nesjavellir Field in the Hengill Geothermal Area (first plant commissioned in 1990, generating 120 MW e of electricity and 300 MW th of hot water), Reykjavik Energy decided to ex- plore other prospects in the area for new plants. In the period 2001-2011 the company drilled 55 exploration and production wells as well as 17 reinjection wells in the Hengill Area, and 5 make-up wells in the Nesjavellir Field. The Hellisheidi Geothermal Plant, about 20 km east of Reykjavík, was commissioned in four stages 2006-2011. It generates now 303 MW e of electricity in two units and 133 MW th of hot water for district heating. This intensive drilling activity in the same geothermal area provided a unique source of data to obtain statistical estimates of the cost and effectiveness of geothermal drilling. The number of working days to complete each of four depth sections of the well (surface, anchor, pro- duction casings and the slotted liner) was analyzed and the results were grouped according to which well design was used and technology applied. Cost calculations in this study are based on assumed market prices for servic- es and material, as the real cost was not made available. The drilling was done by Iceland Drilling Ltd. after int- ernational tendering. The majority of the wells were dr- illed with modern drill rigs, up to four at the same time. The rigs are all-hydraulic with a top-drive and the large ones with automatic pipe handling. The time breakdown in this study was worked out from the geological daily reports prepared by Iceland GeoSurvey as the daily rig reports are confidential. Former efforts to analyze this data were undertaken by Sveinbjornsson and Thorhallsson 1,2,3. This paper re- ports selected topics from these references, with emphas- is on the frequency of problems which led to excessive additional cost. The study compares workdays in drilling holes with two different casing diameter programs, for vertical and directional drilling. Completion tests at the end of drilling provide an Injectivity Index which serves as a preliminary estimate of future flow rates from the well. Such tests are of importance for deciding whether to drill deeper, drill a sidetrack or to apply well stimula- tion before moving the drilling rig off the well. To ob- tain reliable predictions of steam mass flow on the basis of the Injectivity Index one must, however, consider re- servoir conditions and enthalpy of the expected inflow ARMA 13-386 Drilling performance and productivity of geothermal wells - Case history from Hengill Geothermal Area in Iceland Sveinbjornsson, B.M. Iceland GeoSurvey, Reykjavik, Iceland Thorhallsson, S. Iceland GeoSurvey, Reykjavik, Iceland Copyright 2013 ARMA, American Rock Mechanics Association This paper was prepared for presentation at the 47 th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 23-26 June 2013. This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented. ABSTRACT: Drilling performance of 77 production and reinjection wells in the Hengill Geothermal Area in Iceland is analyzed. The study compares workdays, and time spent on seven different drilling activities, in drilling holes with two casing diameter pro- grams, both vertical and directional. The Monte Carlo method was applied to obtain a statistical estimate of the number of work- days and the cost of a 2235 m deep directional reference hole. On average this hole needed 45 workdays with a standard deviation of 7.2 days. The cost was inferred from time and usage data as the actual costs were not available for the study. The average cost for a large diameter reference well is 4.3 million USD or about 2000 USD/m. Drilling problems due to loss of circulation or coll- apsing geological formations led to higher drilling costs for 24 wells but the majority of holes were drilled according to the origin- al schedule. The risk of drilling such holes in terms of cost and output is not as high as often alledged. To predict steam mass flow on the basis of the Injectivity Index, determined at the end of drilling, one must consider reservoir conditions and enthalpy of the expected inflow into wells. About 80% of the drilled wells are productive. The average generating capacity is ~5.9 MW e per drill- ed well. Injection of waste water from the power plant resulted in thousands of earthquakes, of magnitude up to M l 4.
10
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1
1. INTRODUCTION
This report is a partial contribution from Iceland to the
International Energy Agency Geothermal Implementing
Agreement (IEA-GIA), Annexes VII and XI and the
Lower Cost Drilling Annex of the International Partner-
ship for Geothermal Technology (IPGT).
After successful development of the Nesjavellir Field in
the Hengill Geothermal Area (first plant commissioned
in 1990, generating 120 MWe of electricity and 300
MWth of hot water), Reykjavik Energy decided to ex-
plore other prospects in the area for new plants. In the
period 2001-2011 the company drilled 55 exploration
and production wells as well as 17 reinjection wells in
the Hengill Area, and 5 make-up wells in the Nesjavellir
Field. The Hellisheidi Geothermal Plant, about 20 km
east of Reykjavík, was commissioned in four stages
2006-2011. It generates now 303 MWe of electricity in
two units and 133 MWth of hot water for district heating.
This intensive drilling activity in the same geothermal
area provided a unique source of data to obtain statistical
estimates of the cost and effectiveness of geothermal
drilling. The number of working days to complete each
of four depth sections of the well (surface, anchor, pro-
duction casings and the slotted liner) was analyzed and
the results were grouped according to which well design
was used and technology applied. Cost calculations in
this study are based on assumed market prices for servic-
es and material, as the real cost was not made available.
The drilling was done by Iceland Drilling Ltd. after int-
ernational tendering. The majority of the wells were dr-
illed with modern drill rigs, up to four at the same time.
The rigs are all-hydraulic with a top-drive and the large
ones with automatic pipe handling. The time breakdown
in this study was worked out from the geological daily
reports prepared by Iceland GeoSurvey as the daily rig
reports are confidential.
Former efforts to analyze this data were undertaken by
Sveinbjornssonand Thorhallsson 1,2,3. This paper re-
ports selected topics from these references, with emphas-
is on the frequency of problems which led to excessive
additional cost. The study compares workdays in drilling
holes with two different casing diameter programs, for
vertical and directional drilling. Completion tests at the
end of drilling provide an Injectivity Index which serves
as a preliminary estimate of future flow rates from the
well. Such tests are of importance for deciding whether
to drill deeper, drill a sidetrack or to apply well stimula-
tion before moving the drilling rig off the well. To ob-
tain reliable predictions of steam mass flow on the basis
of the Injectivity Index one must, however, consider re-
servoir conditions and enthalpy of the expected inflow
ARMA 13-386
Drilling performance and productivity of geothermal wells - Case history from Hengill Geothermal Area in Iceland
Sveinbjornsson, B.M.
Iceland GeoSurvey, Reykjavik, Iceland
Thorhallsson, S.
Iceland GeoSurvey, Reykjavik, Iceland
Copyright 2013 ARMA, American Rock Mechanics Association
This paper was prepared for presentation at the 47th US Rock Mechanics / Geomechanics Symposium held in San Francisco, CA, USA, 23-26
June 2013.
This paper was selected for presentation at the symposium by an ARMA Technical Program Committee based on a technical and critical review of the paper by a minimum of two technical reviewers. The material, as presented, does not necessarily reflect any position of ARMA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of ARMA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 200 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgement of where and by whom the paper was presented.
ABSTRACT: Drilling performance of 77 production and reinjection wells in the Hengill Geothermal Area in Iceland is analyzed.
The study compares workdays, and time spent on seven different drilling activities, in drilling holes with two casing diameter pro-
grams, both vertical and directional. The Monte Carlo method was applied to obtain a statistical estimate of the number of work-
days and the cost of a 2235 m deep directional reference hole. On average this hole needed 45 workdays with a standard deviation
of 7.2 days. The cost was inferred from time and usage data as the actual costs were not available for the study. The average cost
for a large diameter reference well is 4.3 million USD or about 2000 USD/m. Drilling problems due to loss of circulation or coll-
apsing geological formations led to higher drilling costs for 24 wells but the majority of holes were drilled according to the origin-
al schedule. The risk of drilling such holes in terms of cost and output is not as high as often alledged. To predict steam mass flow
on the basis of the Injectivity Index, determined at the end of drilling, one must consider reservoir conditions and enthalpy of the
expected inflow into wells. About 80% of the drilled wells are productive. The average generating capacity is ~5.9 MWe per drill-
ed well. Injection of waste water from the power plant resulted in thousands of earthquakes, of magnitude up to Ml 4.
2
into wells. About 80% of the drilled production and
make-up wells turned out to be productive. The power
output of the large diameter wells is 30-40% higher than
that of the regular diameter wells.
Injection of waste water from the Hellisheidi power
plant led to an intense swarm of induced earthquakes
which gradually declined in frequency and magnitude.
While the results obtained in the Hengill field may not
apply in fields with different geology and reservoir char-
acteristics, the approach in this paper could lead to valu-
able comparisons.
2. DRILLING PERFORMANCE
2.1 Drilling in the Hengill Area A recent description of the conceptual model of the
Hengill geothermal system was given by Franzson et al.
4. The drill fields are shown in Fig. 1.
Fig. 1. Prospective fields in Hengill Geothermal Area. Most of
the wells drilled in the years 2001-2011 were in the Hellis-
heidi, Grauhnukar and Hverahlid Fields. Figure from Iceland
GeoSurvey.
Fig. 2 shows the location and trajectories of wells and
the formation temperature in the drill fields.
Two types of casing designs for high temperature wells
were used. The wells of both programs were either drill-
ed vertical or directional. The most common type is a
directional well with a “large diameter” casing program.
The pre-drilling (Section 0) is done by a small rig with a
26" bit down to 90 m for a 22½" surface casing, follow-
ed by Section 1 drilled by a larger rig with a 21" bit to
300 m for the 18⅝" anchor casing. Inclined drilling
starts with a kick-off point (KOP) in Section 2, where
the inclination is gradually built up by 2.5-3.0° per 30 m.
The section is drilled with a 17½" bit to 800 m for 13⅜"
production casing. The open hole in Section 3 is drilled
with a 12¼" bit to a depth of 1800 to maximum of 3300
m for 9⅝" slotted production liner. The other design is
narrower, each casing one size smaller, and called the
“regular diameter” casing program. The sections are the
same but the diameters 18⅝" of the surface casing, 13⅜"
anchor casing, 9⅝" production casing, and 7" slotted
production liner. Fig. 3 shows examples of the design of
a vertical well of regular diameter and a directional well
of large diameter.
Fig. 2. Formation temperature in the Hellisheidi, Grauhnukar
and Hverahlid Fields of the Hengill Area at 1000 m below sea
level, about 1400 m depth. Blue dots and lines indicate well-
heads and trajectories of directional wells. A red star on the
trajectory indicates where the well reaches the depth of the
map. Figure from Bessason et al. 5 .
Fig. 3. Examples of the design of a vertical well of regular dia-
meter and a directional large diameter well. On left, the four
depth intervals for the reference well, Sections 0, 1, 2 and 3.
The same section numbers are used in Tables 1 to 4. Figure
from Iceland GeoSurvey.
3
The pre-drilling to about 90 m was performed with air
hammer and foam, or with tricone insert bits, using mud
and water as circulation fluids. Rotary drilling techniq-
ues with tricone insert bits were applied in Section 1, but
in Section 2 a mud motor was used to rotate the bit and a
Measurement While Drilling (MWD) tool inserted in the
drill string to monitor direction (azimuth) and inclination
(degrees) of the well. In Section 3 until total depth no
mud was used but drilling was carried out with only wat-
er as long as there were no circulation losses. After loss-
es were encountered the circulation was in most wells
switched over to aerated water by compressed air for
pressure balance drilling.
Seven drill rigs were used to carry out the drilling. Two
small rigs with a hook-load capacity of 50 tonnes were
often used for the pre-drilling to 90 m depth. An inter-
mediate rig (100 t) was sometimes used for the sections
to 300 m to meet the tight schedule and fully utilize the
rig fleet. The four large rigs (180, 200, 300 t) were used
in all sections, and always in the deepest one (Fig. 4).
Fig. 4. A drilling rig with 200 tonnes hook-load capacity on
site at Hellisheidi field. Photo from Iceland Drilling Ltd.
2.2. Time Analysis of Drilling Data To compare the drilling time for different wells, the re-
spective numbers of workdays were normalized for a
reference well of that design and the average depth of
the group which was 2235 m. The frequency distribution
of workdays for each section was found to be asymmetr-
ic with the most frequent value lower than the average.
An example of this distribution is presented in Fig. 5 for
the workdays in drilling Section 3 from 800-2235 m in
46 large diameter directional wells. The data is best fitt-
ed by a Beta-PERT distribution, defined by the lowest,
most likely and the highest value observed.
Table 1 shows the lowest, most likely and highest values
of normalized workdays in drilling the four sections of
directional large diameter wells. Average and standard
deviation are calculated for the respective Beta-PERT
distribution. The number of wells analyzed for each sect-
ion varies as fewer reports were available from Section 0
of pre-drilling and Section 1 drilling for the anchor cas-
ing. The most complete set of geological reports is avail-
able from drilling Section 2 for the production casing
and Section 3 drilling of the open hole.
Fig. 5. Frequency distribution of normalized workdays in drill-
ing Section 3 from 800-2235 m in 46 large diameter direct-
ionally drilled wells.
Table 1. Normalized workdays for directional, large diameter
reference wells.
Section Drilled Number Lowest Most likely Highest Average(m) (days) (s) (%)