summary of new drilling technologies - iea...
TRANSCRIPT
Summary of New Drilling Technologies
Manuela Richter
August 2017
Disclaimer
IEA Geothermal do not warrant the validity of any information or the
views and findings expressed by the authors in this report. Neither IEA
Geothermal (IEA-GIA) nor IEA shall be held liable, in any way, for use
of, or reliance on, any information contained in this report.
Manuela Richter1, Summary of Geothermal Drilling Technologies, IEA
Geothermal, August 2017.
1 Projektträger Jülich, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
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Table of Contents
1. Introduction ...................................................................................................................................... 0
1.1 Background ............................................................................................................................................................. 1
1.2 Requirements for new Drilling Technologies .............................................................................................. 1
1.3 Examples of new Drilling Technologies ........................................................................................................ 1
2. Examples of Innovative Drilling Research Efforts ................................................................ 2
2.1 Development of Innovative Drilling and Completion Technologies to Realize Supercritical
Geothermal Developments ........................................................................................................................................... 2
2.1.1 Collaborators ........................................................................................................................................... 2
2.1.2 Funding ...................................................................................................................................................... 2
2.1.3 Project Summary ..................................................................................................................................... 2
2.2 Deep Drilling in Hard Rocks with Lightning ................................................................................................. 3
2.2.1 Collaborators ............................................................................................................................................ 3
2.2.2 Funding ....................................................................................................................................................... 4
2.2.3 Introduction of the Electro Impulse Technology (EIT) ................................................................. 4
2.2.4 Concept ..................................................................................................................................................... 5
2.2.5 Projects ...................................................................................................................................................... 6
2.2.6 Saving Potential ...................................................................................................................................... 6
2.3 LaserJetDrilling ..................................................................................................................................................... 8
2.3.1 Collaborators ........................................................................................................................................... 8
2.3.2 Funding ...................................................................................................................................................... 8
2.3.3 Scope of the Project .............................................................................................................................. 8
2.3.4 Status of the Project .............................................................................................................................. 9
2.3.5 Expected Savings in Time and Costs While Drilling ................................................................. 10
2.3.6 Expected Benefits and Future Objectives of Technology ...................................................... 10
2.4 Percussion Drilling: Hydraulic Down-the-Hole Hammer Development ............................................ 11
2.4.1 Introduction............................................................................................................................................... 11
2.4.2 Water Powered Percussion Drilling ................................................................................................ 12
2.4.3 Hydraulic Percussion Mechanisms ................................................................................................. 12
2.4.4 Conclusions for the Development of DTH Hammers ............................................................... 16
3. Conclusions .................................................................................................................................... 17
3.1 State of the Art ..................................................................................................................................................... 17
3.2 Expected Impacts ............................................................................................................................................... 17
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4. References ..................................................................................................................................... 19
List of Figures
Figure 1: principle structure of the EIT process area .......................................................................................................... 4
Figure 2: principle design of the EIT system and its technical parameters .............................................................. 5
Figure 3: design of the test facility ........................................................................................................................................... 6
Figure 4: saving potential of the EIT......................................................................................................................................... 7
Figure 5: schematic diagram of the LaserJet ...................................................................................................................... 9
Figure 6: Hydraulic ram hammer (direct acting variant using compression springs) operation. On the left
hand, flow through for hole cleaning is shown. Pushing the bit into the hammer, the hammer gets activated.
............................................................................................................................................................................................................. 13
Figure 7: Schematic and operation cycle of the differential pressure hammer percussion mechanism. ...... 14
Figure 8: GZB prototype hydraulic ram hammer with accumulator piston ............................................................... 15
Acknowledgements
Thanks to the team of Task B: Drilling Technology of the IEA Geothermal Working Group
13 - Emerging Geothermal Technologies. I would like to especially thank the authors who
provided summaries of the selected projects. Thanks for your research work; drilling
costs for geothermal wells will be reduced in the future.
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Executive summary
Innovation is defined as a "new idea, device, or method".
Geothermal resources tend to be located in deeper and harder geologic formations than
the typical hydrocarbon reservoirs. The exploration and production of high enthalpy
geothermal resources for district heating and power generation usually implies
development of Engineered/Enhanced Geothermal Systems or EGS (in Central Europe
EGS is also described by the adjective petrothermal). These systems represent more than
85 % of the total geothermal potential in Central Europe and other countries. The
remaining 15 % is attributed to hydrothermal systems (e.g. in the Bavarian Molasse Basin)
and to tectonically modified parts of the upper crust (e.g. in the Upper Rhine Valley).
Engineered/Enhanced Geothermal systems need to be explored and developed by
innovative methods, in general derived from the oil & gas drilling technology sector.
Without further development of these systems, an important source of renewable energy
will remain untapped in many countries.
Current manufacturing methods and the technology development of deep drilling and
completion equipment is geared towards the requirements of the oil and gas industry.
Oil and gas wells need to be reliable and have robust links between a reservoir at depth
and the surface installation for the duration of production. Fundamentally, this also applies
to geothermal wells, in particular for hydrothermal wells with final depths of 3,000 - 4,000
m. However, geothermal wells and stimulation technologies for EGS Systems aimed at
extracting heat stored in nominally “dry, hard” rock have significantly different functional
specifications when compared to oil and natural gas wells:
• the average temperature is greater
• the ultimate goal is not a relatively weak reservoir rock but hard rock, for example
a volcanic rock. for the sustainable operation of an EGS, large surface areas are
required that consist of both naturally existing fractures that act as heat
exchangers, and engineered “heat exchangers” or fractures
• to minimize the hydraulic resistance and impedance during production or
injection, large cross-sectional flows are required
• the average depth is greater
New answers must be found to handle these unfavorable conditions and challenges. At
the same time, however, costs have to be reduced: on average the oil and gas industry
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drills 5000 m deep wells at a cost of about €2.5-3.0 million per 1000 m. Owing to the
high energy density of oil and gas when compared to hot water, the specific unit cost of
a geothermal well (€/MWh) is significantly higher and geothermal systems must
frequently have an associated re-injection well into the same hydrologic unit. The aim of
the task is therefore to provide scientifically high-quality contributions to:
• reduce drilling costs;
• make drilling technology safer for use in hard and hot rocks; and
• reduce the risk of discovery.
Some of the new developments in the field of innovative drilling technologies are
presented below. The examples illustrate that it is possible to save costs for deep drilling
or that there are ways to keep the costs constant, irrespective of the type of rock
encountered in the subsurface.
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1. Introduction
Geothermal energy is an important
component of the future worldwide
energy supply, offering a wide range of
possible applications and having a
great development potential in many
countries. To ensure that geothermal
energy can play its optimal role in the
future energy supply, it is essential to
address strategic groups of political
decision makers, potential investors
and the public to address possible
concerns that may block an increased
use of geothermal technologies. One
such concern is the high specific well
cost (€/MWh) of drilling to large depths
-- the highest cost factor in the
construction of a geothermal plant.
Specific well costs are highly
dependent on the local conditions. The main cost drivers are the nature of the
underground rock and the geothermal temperature gradient. The higher the gradient,
the less depth is required to reach the temperature for the particular application.
Depending on the characteristics of the subsurface, the drilling costs will be in the range
of 1000 to 2000 euros per borehole meter, including the costs for the drill site, equipment
rental, surveying, development, staff and energy. For example, in mature regions of the
Bavarian Molasse Basin, the cost for a 3500 m deep well amounts to 5,250,000 euros
assuming costs of € 1500 per meter drilled. These well costs amount to 70% of the total
investment costs in this region. Thus, further development in this technology area can
significantly reduce the overall cost of deep geothermal energy projects.
Researchers develop innovative drilling technologies and strategies that increase the life
of drilling tools, reduce energy for and material consumption during drilling, and thus
minimize the cost and risk of deep drilling.
In order to develop and greatly expand the use of geothermal energy in the future, new
drilling methods and equipment are needed to penetrate hard, abrasive rock, to provide
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wellbore stabilization and to deal with very high temperatures and other extreme
conditions encountered in geothermal wells.
1.1 Background
The drilling methods listed below are only a few highlights. Wells are drilled all over the
world and the utilization of geothermal energy is a growing market. Worldwide electricity
production from geothermal is increasing, but geothermal drilling activity is minuscule
compared to oil and gas. If we consider a typical capacity of a geothermal well as 6-10
MWe, along with injection wells numbering one third the number of producers, this
represents a total of only 1700 to 2800 active wells globally. This number is somewhat
misleading because many more wells have been drilled than are currently active. There
are exploratory wells that once were used to identify and evaluate geothermal prospects;
there are many former production or injection wells that have been plugged and
abandoned; and many well workovers are executed each year in active geothermal fields
to remedy the consequences of the corrosive and solids-laden brines produced to the
surface. Nevertheless, the overall market is still so small that few drilling contractors or
service companies can stay in business by focusing solely on geothermal activities.
1.2 Requirements for new Drilling Technologies
Reaching the following targets is crucial for the successful development and deployment
of new deep drilling technologies:
• vertical or inclined well bores up to a true vertical depth of 10 km and more have to be routinely possible
• large diameter wellbores – multiple times (up to 5x) larger than oil and gas wells at the final drilling depth
• casing while drilling and/or monobore cased with very long stretches of expandable tubulars
• ultimate goal in economic terms: the specific unit well cost (€/MWh) needs to increase linearly with depth.
1.3 Examples of new Drilling Technologies
There are more than 20 research efforts dealing with a wide variety of innovative drilling
technologies such as: enhanced rotary, laser, spallation, plasma, electron beam, electric
spark and discharge, electric arc, water jet erosion, ultrasonic, chemical, induction,
nuclear, forced flame explosive, turbine, high frequency, microwave, hammer and several
others. Some promising approaches are presented in the following sections.
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2. Examples of Innovative Drilling Research Efforts
This chapter describes current innovative drilling research projects from Japan and
Germany.
2.1 Development of Innovative Drilling and Completion Technologies to Realize Supercritical Geothermal Developments
2.1.1 Collaborators
The University of Tokyo Tohoku University National Institute of Advanced Industrial Science and Technology (AIST) Geothermal Engineering Co., Ltd. (Geo-E) Geothermal Energy Research & Development Co., Ltd. (GERD)
2.1.2 Funding
“Advanced Research Program for Energy and Environmental Technologies”
New Energy and Industrial Technology Development Organization (NEDO)
October 2015–October 2017
USD 2 million
2.1.3 Project Summary
The development of a supercritical geothermal system located in the Japan Trench
subduction zone is expected to supply terawatt-scale energy and to solve various
problems existing in conventional geothermal systems at a single stroke such as site-
dependency, sustainability, capacity, scale inhibition, coexistence with national parks and
hot springs, and induced seismicity. Technologies to effectively drill into deep ductile
formations with temperatures in excess of 400°C and to complete wells with sufficient
integrity are indispensable for supercritical geothermal system development, and no one
has ever experienced resource developments in such a harsh and hostile environment.
This research project focuses on the development of a new innovative drilling method
using thermal-shock or thermal-stress failure of rock induced by rapid cooling.
The research project consists of the following four elements:
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2.1.3.1 Development of a new drilling system using thermal-shock failure of rock induced by decompression, boiling and cooling of drilling fluid bottom hole
Based on the previously developed decompression drilling concept, Tohoku University
has conducted a couple of hydrothermal experiments to obtain various data for modeling
the behavior and mechanism of thermal-shock rock failures in the vicinity of the wellbore.
A University of Tokyo team studied existing downhole tools and simulation technologies
to create new methods to locally decompress and cool the bottom hole.
2.1.3.2 Development of wellbore hydrothermal simulation technology applicable to supercritical condition
The Geothermal Energy Research & Development (GERD) group evaluated several
existing hydrothermal simulators to specify implementation schemes to couple them with
a newly developed supercritical calculation module.
2.1.3.3 Development a of “Thermal Expansion Packer”
The Geothermal Engineering (Geo-E) and the Teiseki Drilling (TDC) groups designed and
constructed small-size prototypes of thermal expansion packers for evaluation tests
which will be conducted next fiscal year (2018).
2.1.3.4 Research and development of acid- and corrosion-resistant materials against supercritical environment
National Institute of Advanced Industrial Science and Technology (AIST) group reviewed
selection guides of oil country tubular goods (OCTGs) for corrosion environments, and
vast numbers of material evaluation research reports from a past national project for
development of new energy technologies, the “Sunshine Project.”
2.2 Deep Drilling in Hard Rocks with Lightning
2.2.1 Collaborators
• Technische Universität Dresden • BAUER Maschinen GmbH • Thomas Werner Industrielle Elektronik e. Kfm. • BITSz electronics GmbH • Baker Hughes INTEQ GmbH • ILEAG e.V. Institut für leichte elektrische Antriebe und Generatoren
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2.2.2 Funding
Germany´s “6th Energy Research Programme of the Federal Government”; Research for
an environmentally-friendly, reliable and affordable energy supply
January 2015 – December 2017
Approx. EUR 2.6 million
2.2.3 Introduction of the Electro Impulse Technology (EIT)
Drilling through hard rock continues to be a major challenge for conventional drilling
technology. The main problem of mechanical drilling technologies is high wear of the
drilling head, which is caused by the mechanical interaction with the rock. In addition, the
drilling rate is massively limited by the high rock strength. Presently a rate of penetration
between 1 and 2 m/h and a service life of approximately 50 to 60 hours with conventional
tools is considered realistic.
Recent drill bit developments have indeed shown higher rates of penetration but only in
the case of a new bit and significantly higher tool prices.
Therefore, there is a need to establish new processes for drilling through hard rock, which
will compensate for those disadvantages and complement the range of drill tools in a fit-
for-purpose manner.
A potentially suitable method is electric impulse technology (EIT). Here, high electric
voltage impulses impinge on and pulverize rock (Figure 1).
Figure 1: principle structure of the EIT process area
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The basic principle rests on having optimal impulse parameters which include the
generation of transient voltages in the range of up to 600 kV with a rise time of less than
120 to 150 ns.
If parameters are complied with, the discharged voltage causes stresses large enough
to surpass the strength of the rock rather than that of the drilling mud present. EIT does
not require the application of a mechanical load as such on the drill string other than the
electrodes being in loose contact with the rock.
2.2.4 Concept
In order to use the EIT drilling head assembly in conventional drill rigs, one has to develop
an assembly capable of being integrated into existing drilling / bottomhole assembly
systems. Therefore, the fabrication concept aims at delivery of an EIT drill system that is
deployed in much the same way as a conventional drill bit. Hence, the drill rig does not
require any modification or additional equipment.
Figure 2 shows the EIT assembly with its essential components.
Figure 2: principle design of the EIT system and its technical parameters
It is driven by a mud motor, which powers an electrical generator which in turn provides
the required electrical energy. Via a transformer and a rectifier the charging voltage for
the surge voltage source is generated. Controlled by the trigger, the source generates
the high voltage impulses which are then transmitted to the mining electrode.
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2.2.5 Projects
An element of the feasibility study included a review of the technical implementation. A
technical realization of the system is possible with conventional oil-based mud systems
under downhole conditions (pressures up to 1000 bar and temperatures up to 200°C).
Laboratory tests have proven that the process can be operated at pressures of up to 500
bar.
Based on the feasibility study a drilling head, comprising a surge voltage source and a
mining electrode for a 12 ¼" well, was developed and successfully tested in the laboratory
in a new research project (Figure 3). During the tests, drilling rates were between 0.5 and
1 m/h. Rates can be further enhanced if cuttings removal is more efficient. In addition, the
laboratory tests show that water-based drilling mud can be used as well.
Based on the development of the drilling head, a power supply is currently being
engineered. The overall system is designed for a borehole diameter of 12 ¼",
temperatures up to 200 °C and pressures up to 1000 bar.
Figure 3: design of the test facility
2.2.6 Saving Potential
The key driver for an EIT drilling system is the minimization in NPT (non-productive time)
due to faster tool changes. Calculations show that the trip times have a very large impact
on drilling costs, since a large part of the drilling costs are time-dependent. Based on the
experiments, the lifetime of the system is estimated to be 350 h. This represents an
increase of up to seven times when compared to conventional drill bits leading to a
reduction in the number of tool changes. Scenarios suggest that the expected cost
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savings are approximately 20 to 30% when compared to use of mechanical tools (Figure
4).
In addition to cost savings there are additional technological advantages to using EIT.
Since virtually no weight–on–bit it is needed, one may strongly extend the reach of
horizontal sections. Extended reach horizontal wells in turn open up a potentially vast
choice in the design of subsurface heat exchangers required to optimize geothermal
heat production to surface.
Because of the high compatibility of the EIT system with conventional systems, it is
possible to respond better to subsurface drilling conditions without changing the
operations of the drill rig itself. Thus, advantages of EIT in combination with conventional
drilling technology offer a much-improved value proposition as well as a reduced
ecological footprint.
Figure 4: saving potential of the EIT
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2.3 LaserJetDrilling
2.3.1 Collaborators
• Herrenknecht Vertical GmbH • International Geothermal Center GZB, Bochum • IPG Laser GmbH, Burbach • KAMAT Pumpen GmbH & Co. KG, Witten • Fraunhofer Institute for Production Technology IPT, Aachen • Associated: Synova S.A., Ecublens, Swizerland
2.3.2 Funding
Germany’s “6th Energy Research Programme of the Federal Government”; Research for
an environmentally-friendly, reliable and affordable energy supply
December 2014- November 2017
Project budget: approx. EUR 3 million
2.3.3 Scope of the Project
The overall objective of the LaserJetDrilling project is to enable the use of geothermal
energy in Germany by significantly reducing both time and cost of the drilling process.
Current capital costs of geothermal power generation projects are mainly due to the
drilling process itself (approx. 70%). A novel, high-potential, mechanically assisted laser
drilling process may significantly lower the unit technical well cost (€/MWh), thus lowering
the high upfront capital investment. This is one of the highest barriers to realizing deep
geothermal projects. When drilling down to depths of more than 5000 m for power
generation projects, high unit technical costs are mainly related to low rates of
penetration (ROP) in the typically hard rock (compressive strength of greater than
200 MPa). Combined with a low service life of currently deployed drilling tools such as
polycrystalline diamond PCD bits resulting from crushing of impregnated bits, drilling
operations face frequent time-consuming and hence expensive round trips. In general,
one expects well cost to increase exponentially with depth.
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Figure 5: schematic diagram of the LaserJet
The system and process technology under development in the LaserJetDrilling project
aims at increasing both the ROP and the service life of the drilling tools, by use of
innovative laser technology. A high-power industrial laser source of up to 30 kW transfers
additional energy to the bit face. The addition of a thermal load to the mechanical load
improves the process of rock pulverization because a sufficiently high thermal load will
cause thermal stresses large enough for the rock to spall. Thermal stresses are induced
by guiding the laser beam onto the rock’s surface, using a water jet to protect laser optics.
In tandem, a mechanical drill bit can more easily crush and remove the rock and
remaining particles. With less weight-on-bit (WOB) and torque on the drill string, a higher
ROP and less tool wear are possible.
2.3.4 Status of the Project
Following the concept development and design, the consortium is currently building a
scaled laboratory rig at the GZB in Bochum which will be operative in 2017. The setup
consists of a newly designed drill bit with an outer diameter of 6” containing a laser head
developed by the Fraunhofer Institute for Production Technology (IPT). The supply with
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the required fluids will be realized through a new multi-pipe-in-pipe drill string. For the
initial drill tests at the surface, the consortium aims at a net drilling depth of 2 m. The
objective of this phase is to investigate the fundamental interactions of the new drilling
technology, especially the interaction of photonic and mechanical rock crushing
processes. Secondly, a high-pressure high-temperature simulator (HPHT) at GZB will be
used to investigate the influence of in-situ reservoir conditions on the drilling process and
the system technology.
2.3.5 Expected Savings in Time and Costs While Drilling
Precise figures concerning the effectiveness of the LaserJetDrilling technology will be
available after the initial tests in 2017; however, the technology has the potential to
significantly increase the overall efficiency of the drilling process. Theoretical estimates
suggest that the ROP in hard rock may increase up to 10 m/h by the use of laser
technology. Compared to the state of the art drilling processes with approximate ROPs
of less than 1.5 m/h, the net drilling time to a target depth may be cut by a factor of around
7. Due to the lower weight on bit required to pulverize the rock, the lifetime of the tool is
significantly increased which in return requires fewer round trips leading to a reduced
overall drilling time, less non-productive times, and lower maintenance requirements for
the tools. Even if additional laser components require a higher capital expenditure
compared to conventional drilling techniques, the overall operational costs of the
LaserJetDrilling system are expected to be lower through cost savings. When compared
to state-of-the-art drilling processes, the cost savings are expected to range around 10-
20% depending on the well design.
Additional preliminary figures for savings in time and costs will be given by the end of
2017, once the drilling tests have been completed and a more complete database has
been generated.
2.3.6 Expected Benefits and Future Objectives of Technology
The LaserJetDrilling technology will benefit geothermal power generation by lowering
the unit technical cost of wells (€/MWh). Future development aims at an adaptive drilling
process. Intelligent sensor technology will assist in the identification of the rock at and
ahead of the bit-rock interface and thus enable an optimal and energy-efficient rock
pulverization process. Overall, the LaserJetDrilling technology will greatly improve the
overall footprint of geothermal energy utilization.
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2.4 Percussion Drilling: Hydraulic Down-the-Hole Hammer Development
Internationales GeothermieZentrum Bochum (GZB)
2.4.1 Introduction
Geothermal reservoirs are generally in deeper and harder geologic formations than
typical hydrocarbon reservoirs. The exploration and development of these high enthalpy
geothermal reservoirs for heating and power generation usually means development of
petrothermal or engineered geothermal systems (EGS). These reservoirs represent over
85% of the total geothermal potential in Central Europe. Engineered/Enhanced
Geothermal reservoirs need to be explored and developed by innovative exploration
methods derived from the oil & gas drilling technology sector. Without further advances
in the development of Engineered/Enhanced Geothermal reservoirs, Europe and
countries elsewhere might miss out on an important source for renewable energies.
One example for the development and application of new drilling technology is hydraulic
DTH (down the hole) percussion hammers powered by drill mud.
Air powered percussion drilling is a wide-spread technology used for drilling in hard rock
with a high ROP (rate of penetration). Limitations arise from using air as a drilling fluid. The
fluid powering a DTH hammer exits the drill bit and serves as a drilling fluid in the annulus,
transporting the cuttings to surface. Groundwater bearing formations can limit the
operating depth of such hammers as lifting water by air can only be achieved down to
certain depths.
Hydraulically powered percussive DTH hammers do not have such a depth limitation, but
to date are only available as water powered DTH hammers. There have been many
attempts to build an operative mud hammer. A percussion tool for assisting regular drilling
tools was tested in the late 1960s and used with regular tricone bits (Vincent & Wilder
1969). Novatek designed a hammer assisted jet-steerable drill bit (Pixton & Hall 2002)
and tested various percussion hammers under depth conditions. TU Clausthal also
performed research on percussion hammers for powering them with drill mud.
Unfortunately, none of these mud hammers achieved market readiness.
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2.4.2 Water Powered Percussion Drilling
Powering the downhole hammer with water instead of air addresses the groundwater
issues. Water powered downhole hammers are available today from Wassara and Hanjin,
neither of them compatible with drill mud. In 2012, using a 7-¼ inch drill and a 6-inch
Wassara W150 percussion hammer, GZB in Bochum drilled more than 20 wells, each 200
m deep, with Wassara water hammer technology. The hammer was equipped with a
modified bit, designed by GZB and Karnebogen, to have the same weight as the hammer
striking piston to optimize momentum transfer. Most of the tool wear was visible at the
spline shaft at the drill bit and inside the hammer there was cavitation damage.
The ROP was high, with values of around 45 m/h in sandstone and claystone layers, while
most of the water was actually used for hole cleaning. In Sweden, LKAB drilled granite
and leptite with a ROP of about 20 m/h (Tuomas 2004).
2.4.3 Hydraulic Percussion Mechanisms
The key to building a hydraulic down-the-hole percussion drill is the percussion
mechanism, which converts hydraulic energy to a percussive movement. While air
powered DTH hammers make explicit use of the gas compressibility, hydraulic powered
hammers use incompressible fluids. This leads to more difficult and complex
mechanisms. There have been attempts to categorize the working principle of such
hammers (Melame et al. 1997); the GZB distinguishes four main categories of working
mechanisms. Differential pressure mechanisms and hydraulic ram hammers, using drill
mud to accelerate the striking piston, are described below.
Fluid hammers make use of a fluidic amplifier to create an oscillating movement and use
only one moving part. During the China Continental Scientific Drilling Project (CCSD), such
a percussion mechanism was used in prototype hammer drills (Wang et al. 2015). A recent
research project at TU Freiberg has used a mud driven PDM (positive displacement
motor) downhole drive to power a percussion mechanism, which has no contact to the
drill mud. The percussion mechanism can be a hydraulic or mechanical drive (Lehmann
& Reich 2015).
2.4.3.1 Hydraulic Ram Hammer
The hydraulic ram hammer was invented in the 1930s (Zublin 1932) and makes use of the
hydraulic inertia in the drill string. The mechanism achieves a momentum transfer from
the hydraulic fluid to the striking piston by interrupting the fluid flow. The hammers can
13
be divided into three groups (direct acting, reverse acting and double acting) depending
on the direction of the hydraulic momentum transfer or how the compression springs are
used (Ясов 1977). The mechanism described as direct acting hydraulic ram hammer, is
shown in Figure 6.
Figure 6: Hydraulic ram hammer (direct acting variant using compression springs) operation. On the left hand, flow through for hole cleaning is shown. Pushing the bit into the hammer, the hammer gets
activated.
In the initial state (left of Figure 6) the hydraulic fluid flows freely through the hammer
mechanism. Then a valve (top of assembly) closes this path, causing the hydraulic fluid to
accelerate the striking piston. By lifting the striking piston toward the valve, the flow path
closes. The striking piston´s velocity is zero or even in the opposite direction to the
hydraulic fluid flow as the valve closes. The hydraulic fluid will continue to flow due to the
hydraulic inertia; this builds up a high pressure which triggers a momentum transfer
between hammer and piston. The volume displaced by the accelerated striking piston
may be larger than the integrated hydraulic volume flow (Nickel et al. 1993). Once
accelerated, the striking piston also re-opens the flow path through the hammer, the
pressure drops and flow will recommence, building up hydraulic inertia for the next strike.
These hammers have been the subject of various research projects at the Technical
University of Clausthal; one such percussion hammer was developed for use in the KTB
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drilling project (Engeser 1996). Later the ZW-1 prototype mud hammer was developed
(Teodoriu 2011) which uses a hydraulic piston return mechanism instead of compression
springs. It delivers 440 J of impact energy at 27 Hz frequency using 1.2 m3 liters of drill
mud per minute. Numerical techniques were developed to simulate the hydraulic
mechanisms inside the hammer during the development of the ZW-1 (Zhao 1998).
2.4.3.2 Differential Pressure Hammer
Also known for more than 100 years is a commonly used mechanism, the differential
pressure hammer. The hydraulic fluid has to steadily displace a mass to achieve flow
through the hammer. This mass can be the striking piston and the valve.
Figure 7: Schematic and operation cycle of the differential pressure hammer percussion mechanism.
The valve controls whether the striking piston is pushed upwards or downwards by
connecting one side of the piston to the pressure inlet and the other side to the outlet,
15
usually to the bit. On the upper and lower positions, the striking piston will switch the
valve position (mostly by opening control channels), which then reverses the hydraulic
forces on the striking piston. Figure 7 schematically shows the valve and piston during
one cycle.
Both commercially available water-powered hammers from Wassara and Hanjin can be
described by this mechanism. The Wassara water hammer uses different hydraulic areas
on the striking piston. The lower piston chamber is always connected to the pressure
inlet, and the valve switches the upper piston chamber between the pressure inlet and
outlet to the drill bit (Tuomas 2004). This feature simplifies the valve design. The Hanjin
water hammer uses a valve assembly around the piston to modify the effective areas and
distribute pressure and outlet.
GZB Mud Hammer Prototype
Research performed by GZB on hydraulic ram hammers first resulted in the development
of a working prototype based on the hydraulic ram hammer. This concept continues to
hold much promise because it comprises only a few, easily manufactured components.
The concept of the hydraulic ram hammer is modified by creating a compression spring-
based accumulator inside the piston as shown in Figure 8. Field tests and measurements
show that this modification significantly reduces pressure peaks during the momentum
transfer. As soon as the valve is closed, the acceleration process extends to a longer
duration and maximum pressures are lowered.
Figure 8: GZB prototype hydraulic ram hammer with accumulator piston
During the measurement campaign, position sensors measured displacements of the
striking piston and valve. Fluid pressures were measured at five locations inside the
hammer. To record pressure peaks, a high frequency pressure transmitter was used.
16
During tests with and without piston accumulator, pressure peaks were significantly
reduced. Pressure peaks appear when the valve closes and cavitation occurs following
the momentum transfer. Without the accumulator, the momentum transfer pressure was
105 bar when operating with 20 bar supply pressure. The subsequent cavitation pressure
peak had pressures of more than 110 bar, which is more than the initial pressure peak
caused by the valve closing. With the accumulator and under the same conditions (20
bar supply pressure) the pressure peak was reduced to 50 bars. The cavitation pressure
peak was reduced to 40 bar.
2.4.4 Conclusions for the Development of DTH Hammers
Percussion drilling through hard rock is an established drilling technology, either using
air DTH hammers or hydraulic driven top hammers. For drilling deep geothermal wells, a
hydraulic driven percussion drill needs to have certain characteristics. So far, only water
powered percussion drills have been available. Using recirculated drill mud, the
percussion drill must withstand more severe conditions; minimizing cavitation and
reducing pressure peaks will reduce overall wear and component stress.
Modeling the percussion mechanism as a physical model suggested the incorporation of
an elastic element into the hammer. An accumulator system was subsequently
developed for the first prototype hammer, which helped reduce damaging pressure
peaks and cavitation effects
The next development centers on a new mechanism that reduces pressure peaks and
avoids all cavitation. Physical modelling helps to understand the interaction between
hydraulics and mechanics and allows us to find optimized parameters prior to
manufacturing a prototype. The GZB test site allows field testing of prototypes while
collecting measurements necessary for validating the simulation tools.
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3. Conclusions
3.1 State of the Art
It is well established that drilling technology plays a major role in both the exploration and
production phases of reservoir development. For the purposes of this summary and to
characterize the general situation for drilling, the following major factors influence the unit
technical cost of wells:
- High temperature - Hard, abrasive rock - Corrosive fluids and gases - Higher cost of non-productive time - Large depths and larger diameter holes
In the past, it has been amply demonstrated that naturally occurring liquid-water and
vapor-dominated geothermal reservoirs can be developed by rotary drilling methods –
often adapting to methods from the oil and gas industry. Low well costs (€/MWh) will
significantly lower the barrier to wide-spread utilization of geothermal energy sources, to
a level where they will contribute substantially to nation’s energy supply.
3.2 Expected Impacts
Research of geothermal energy technologies is performed in many places all over the
world. There is an urgent need to significantly intensify, coordinate and focus the
research of enabling technologies for power generation, such as innovative deep drilling
in hard rocks.
Research and development of deep drilling technology aims at effective and efficient
drilling through hard rock, ultimately with unit cost rising at most linearly with depth.
Deploying novel drilling technologies will enable the development of deep geothermal
systems located in previously costly and thus unsuitable sites. Unlike traditional
geothermal systems, which rely on natural anomalies with high hydrothermal potential,
Engineered/Enhanced geothermal systems use the hot deep rock to heat water in
artificially created reservoirs as a heat exchanger almost anywhere – vastly increasing
the technically exploitable resource base.
For regions without access to prime geothermal resources and with average geothermal
gradients, the key driver for developing innovative drilling technologies is the unit cost of
drilling wells to depths of 5 to 7 km where the temperatures for power production are
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optimal. Current demand for drilling of geothermal wells in exceptionally high quality
resources is met with conventional drilling technology. However, factors like greater
depths, high temperature, high pressure, hard fractured rock and corrosion increase the
cost of average geothermal wells two to five times compared to equally deep oil and gas
wells, while the energy yield of an average geothermal well is orders of magnitudes less
than that of oil and gas wells. An expanded and accelerated geothermal energy
development effort can benefit from improvements in drilling technology which will lower
costs.
Longer term R&D on new drilling methods and advanced drilling technologies are risky,
but can potentially pay off in greatly expanded national geothermal energy development
programs. Future exploration and extraction efforts for other minerals and fuels will also
benefit from successful development of advanced drilling systems. New drilling
technologies will play a major role in lowering unit costs (€/MWh) of drilling into deeper,
very hot geothermal energy formations and reservoirs.
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IEA Geothermal
Executive Secretary
IEA Geothermal
C/ - GNS Science
Wairakei Research Centre
Ph: +64 7 374 8211