Geothermal Well Test Analysis

Fundamentals, Applications and Advanced Techniques

Sadiq J. Zarrouk, Katie McLean

• Publisher: Academic Press
• Year: 2019
• ISBN:
  9780128149478
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Geothermal Well Test Analysis: Fundamentals, Applications and Advanced Techniques provides a comprehensive review of the geothermal pressure transient analysis methodology and its similarities and differences with petroleum and groundwater well test analysis. Also discussed are the different tests undertaken in geothermal wells during completion testing, output/production testing, and the interpretation of data. In addition, the book focuses on pressure transient analysis by numerical simulation and inverse methods, also covering the familiar pressure derivative plot. Finally, non-standard geothermal pressure transient behaviors are analyzed and interpreted by numerical techniques for cases beyond the limit of existing analytical techniques.

• Provides a guide on the analysis of well test data in geothermal wells, including pressure transient analysis, completion testing and output testing
• Presents practical information on how to avoid common issues with data collection in geothermal wells
• Uses SI units, converting existing equations and models found in literature to this unit system instead of oilfield units

Chapter 1

Introduction

Abstract

Geothermal energy is simply heat from Earth’s interior generated by the radioactive decay of heavy nuclei, which then flows to the ground surface. This vast renewable energy source can be utilised for both electricity generation and direct use applications. It has the potential to provide the world’s energy needs for future generations. This energy is accessed through drilling and testing of geothermal wells. The historic use, growth and challenges facing the geothermal industry will be discussed in this chapter.

Keywords

Geothermal energy; thermal gradient; conventional geothermal systems; EGS; sedimentary aquifers; power production; direct use; well testing

Contents

• 1.1 Background 2
• 1.2 Geothermal energy 3
• 1.3 Power production 5
• 1.4 Direct use of geothermal energy 7
• 1.5 Scope of this book 9

Earth is a large powerhouse continuously generating approximately 46±3 TW of thermal power (Jaupart et al., 2007), by the radioactive decay of heavy nuclei ²³⁸U, ²³²Th and ⁴⁰K inside the crust and mantle (Dickson and Fanelli, 2003). This energy manifests itself at the surface from time to time through seismic and volcanic activities mainly along tectonic plate boundaries. Earth also has a massive stored thermal energy (inertia) estimated around 12.6×10²⁴ MJ. Of these, 5.4×10²¹ MJ (1.5×10¹² TW h) of energy is in the Earth’s crust (Armstead, 1978). Knowing that the total world energy consumption in 2012 was 154,795 TW h (USEIA, 2017), geothermal energy can effectively provide all of humanity’s energy needs for many generations to come. Theoretically, the geothermal energy stored and generated underground is more than all other (fossil and renewable) energy sources combined. However, the technology needed to harness geothermal energy faces many technical and commercial challenges. The main challenge is the high cost and commercial risks associated with drilling deep geothermal wells to produce this energy.

Geothermal energy developments are known for their high availability and independence from weather conditions compared to the other renewable sources (Zarrouk and Moon, 2015). Unlike solar or wind energy, geothermal energy does not need to be integrated with energy storage systems because the geothermal energy is naturally stored underground and can be directly accessed when needed through the geothermal well. On the other (down) side, unlike other renewable energy sources (such as solar and wind), geothermal energy can be site specific, require longer development time and involve high upfront cost and risk associated with drilling into permeable hot fluid targets.

The historic trends in geothermal power development since the 1950s show that the growth in geothermal power development is highly affected by the fluctuation in the price of oil; and since the late 2000s, geothermal energy has been challenged by low-cost solar energy (Zarrouk, 2017). However, geothermal energy will always have a role to play as the world moves toward a low-carbon economy by reducing greenhouse gas emissions and phasing out fossil-fuelled thermal and thermal–nuclear plants. Geothermal energy is an integral part of the strategy in many countries to achieve energy independence and reduce reliance on fossil fuels.

Geothermal wells are the veins and arteries of any geothermal development, allowing both the production of hot geothermal fluid to the surface and the reinjection of the utilised fluids back into the reservoir. Geothermal projects become commercially viable (bankable) only after the drilling and testing of large and deep wells and when the power potential/output of each well is measured and quantified. The behaviour of geothermal wells can also change with time; generally, the power output of production wells reduces with time, which makes it necessary to drill make-up wells. Reinjection wells can suffer from reduction in their injectivity, which will require intervention. Well testing can help identify the reasons for changes in well behaviour and help guide the reservoir engineers to the potential solution or well intervention.

1.1 Background

The motivation for this book came through our observation that the worldwide boom in the applications and research in geothermal energy have mainly focussed on enhanced geothermal systems (EGS), above ground geothermal technology (e.g. Organic Rankin Cycles), low temperature direct use and ground source heat pump applications. However, there is not much published work or research on testing, assessing and understanding the behaviour of geothermal wells, despite the fact they are critical for any geothermal development and are a major investment. One reason is the lack of understanding of – and appreciation for – the importance of geothermal well testing by researchers from different backgrounds venturing into geothermal energy.

Geothermal energy training is very specialised with only a few established institutions in the world offering internationally recognised academic training (Zarrouk, 2017). Only a handful of these courses cover geothermal well test analysis in some detail, since well test analysis practices differ between regions depending on the types of geothermal systems being dealt with. In addition the well test data are normally commercially sensitive (confidential) and only available to geothermal reservoir engineers working in the industry. Therefore unlike the petroleum industry, geothermal well test expertise and skills are not commonly found in research institutions or academia.

Geothermal well test analysis has sprung from analytical methods developed by the petroleum and groundwater industries. Geothermal well test data largely do not satisfy the fundamental assumptions upon which these techniques were developed. Therefore it is common that geothermal well test analysis leads to incorrect interpretations or behaviour that is difficult to interpret. For this reason, there is low confidence in well test analysis and it is common for reservoir engineers not to report well test results.

From our experience in analysing data from a host of geothermal wells from around the world, it became obvious that testing and analysing the well test data should be carried out differently from petroleum and groundwater wells. The two-phase condition (steam and water) and high temperature of the geothermal fluid can lead to false effects when using techniques developed for single-phase isothermal conditions (McLean and Zarrouk, 2015a). This undermines the accuracy and the findings of the transient geothermal well test analysis. As a result, geothermal well test analysis is to some extent perceived as a black art. Young geothermal engineers and scientists find it difficult to understand and master well testing without making mistakes on the way as they try to develop their skills in the absence of specialised training or experienced mentors.

1.2 Geothermal energy

The thermal power that is generated in Earth’s mantle travels to the ground surface through the rock formations of the crust by thermal conduction. This generates an average conductive temperature gradient between 20 and 30°C/km (Armstead, 1978), which results in a heat flux of about 40–60 mW/m². In some parts of the world the local thermal gradient is higher than 30°C/km, for example the measured thermal gradient of Huntly, New Zealand, ranges between 52 and 55°C/km (Zarrouk and Moore, 2007). The geothermal gradient is also affected by the thermal conductivity of the different rock formations that it passes through, following Fourier’s law of thermal conduction.

The thermal gradient is often thought of as linear, though in reality a higher thermal gradient is expected through less conductive rock, and a lower gradient expected through rock that is more conductive. For example the deep EGS well of the Habanero project in Australia has a local thermal gradient that ranges between 32.3 and 63.3°C/km depending on the rock type (Fig. 1.1). It is known that coal, coal measures and rocks bearing hydrocarbons are less conductive than other rock types and can act as thermal insulators, trapping heat underneath. In addition, some deep volcanic rocks (e.g. granite) generate heat by radioactive decay which can also result in an above average thermal gradient through these rock types. Natural state numerical modelling shows that the temperature gradient of Fig. 1.1 can be reproduced with natural heat flux of 125 mW/m² and heat generation of 10 µW/m³ (Llanos et al., 2015).

Figure 1.1 The geothermal gradient of the H01 Habanero well. Data source from Llanos, E.M., Zarrouk, S.J., Hogarth, R., 2015. Simulation of the habanero geothermal reservoir, Australia. Geothermics 53, 308–319.

In areas along Earth’s plate tectonic boundaries the natural thermal gradient can be as high as 100°C/km. When there is reasonable permeability in the surrounding rocks and good natural supply of water (meteoric or seawater), the thermal gradient will become unstable giving way to convective heat transfer through water movement, which carries much more thermal energy than thermal conduction. Natural thermal convection can result in significantly elevated temperatures close to the ground surface (Fig. 1.2), and this high energy density can be accessed by drilling into these convective upflows of fluid. It is at plate boundaries that most geothermal heat manifests itself in the form of thermal springs, hot pools, geysers, steaming ground and bubbling mud pools. These are referred to as conventional geothermal systems and have been extensively studied and commercially developed for energy production in many parts of the world. Most of the geothermal power generated around the world comes from conventional systems. Effectively these systems are much easier to develop (low-hanging fruit) than the nonconventional systems (e.g. EGS, geopressured systems) that will be discussed later in this book.

Figure 1.2 Cross section through a convective geothermal system utilised for power production. From Brian Lovelock, Jacobs Ltd. with kind permission.

1.3 Power production

Geothermal electric power generation first commenced in 1904 at Larderello, Italy. The first generation used reciprocating steam engines which soon failed due to corrosion problems, after which clean steam was generated in heat exchangers. Development of new technology and materials enabled the heat exchanger to be dispensed with, and a 250 kWe power station was put into operation in 1913. By 1940, 130 MWe was feeding the Italian railway system. This was later destroyed in the Second World War but has since been rebuilt and is still generating successfully.

It was not until the early 1950s that New Zealand started to plan a geothermal plant. The first geothermal electricity in New Zealand was at the Spa Hotel in Taupo on 13 February 1952. The steam engine did not run for long due to the deposition of mineral scale carried by the wet steam. In 1958 the first power unit was commissioned at Wairakei. Then the United States was next to produce geothermal power in 1960 at The Geysers in California. Many countries have followed in geothermal power development, which was reported in 26 countries around the world in 2015 with a total installed capacity of more than 12.7 GWe, with a forecast of 21.0 GWe in 51 countries by 2020 (Bertani, 2016).

Wairakei, New Zealand, was the first low enthalpy liquid-dominated geothermal system to be developed, as wells at Larderello and The Geysers produced dry steam. Two-phase fluid produced from wells at Wairakei required the development of new technology for separating steam from water and disposing of the separated water (brine).

Thermal power generation requires satisfying the second law of thermodynamics, which limits the maximum theoretical power conversion efficiency to the Carnot cycle. The efficiency for geothermal power plants are significantly lower than other thermal power plants as they operate at much lower ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• About the authors
• Preface
• Acknowledgements
• Chapter 1. Introduction
• Chapter 2. Geothermal systems
• Chapter 3. Geothermal wells
• Chapter 4. Introduction to pressure-transient analysis
• Chapter 5. Advanced analytical pressure-transient analysis relevant to geothermal wells
• Chapter 6. Completion and output testing
• Chapter 7. Downhole tools and other practical considerations
• Chapter 8. Numerical pressure-transient analysis modelling framework
• Chapter 9. Operation and management of geothermal wells
• Chapter 10. Field studies
• Appendix 1. Quick reference guide to characteristic pressure transient features in geothermal wells
• Appendix 2. Glossary of common terms used in geothermal energy technology
• List of symbols
• References
• Index