Applied Gaseous Fluid Drilling Engineering: Design and Field Case Studies

By Boyun Guo, Yingfeng Meng and Na Wei

Applied Gaseous Fluid Drilling Engineering: Design and Field Case Studies provides an introduction on the benefits of using gaseous fluid drilling engineering. In addition, the book describes the multi-phase systems needed, along with discussions on stability control. Safety and economic considerations are also included, as well as key components of surface equipment needed and how to properly select equipment depending on the type of fluid system. Rounding out with proven case studies that demonstrate good practices and lessons from failures, this book delivers a practical tool for understanding the guidelines and mitigations needed to utilize this valuable process and technology.

• Helps readers gain a framework of understanding regarding the basic processes, technology and equipment needed for gaseous fluid drilling operations
• Highlights benefits and challenges using drilling flow charts, photos of relevant equipment, and table comparisons of available fluid systems
• Presents multiple case studies involving successful and unsuccessful operations

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 23, 2021
• ISBN: 9780323903288

Boyun Guo

Boyun Guo is a Professor at the University of Louisiana at Lafayette in the Petroleum Engineering Department and Director of the Center for Optimization of Petroleum Systems (COPS) of the Energy Institute of Louisiana (EIL). He has 40 years of work experience in the oil and gas industry and academia. He is the principal author of 11 books and author/coauthor of over 150 research papers. He holds a BS degree in Engineering Science from Daqing Petroleum Institute in China, MS degree in Petroleum Engineering from Montana College of Mineral Science and Technology, and a PhD degree in Petroleum Engineering from New Mexico Institute of Mining and Technology.

Book preview

Preface

Boyun Guo, University of Louisiana at Lafayette, Lafayette, LA, United States

Gaseous fluid drilling is referred to as an engineering process of drilling oil and gas wells with multiphase fluids as circulation media. Existing books in this area present too much theory and equations that are not needed by drilling engineers; do not address major problems encountered in the industry such as water influx, borehole stability, and pipe sticking; and do not provide computer software to generate quick solutions. This book is complementary to the existing books. It focuses on the application side of the gaseous fluid drilling, targets major problems encountered in the industry, and provides computer software needed to generate solutions.

This book is written for well-design engineers and field-drilling engineers who need to build their knowledge to solve their problems in performing their daily jobs in gaseous fluid drilling. The audience of the book also includes graduating college students at undergraduate and graduate levels to learn fundamentals in designing gaseous fluid drilling operations in their jobs to come.

This book is intended to cover the full scope of the gaseous fluid drilling engineering. Following a sequence from basic to advanced topics, this book presents its contents in nine chapters:

• Chapter 1 presents an introduction to gaseous fluid drilling applications.

• Chapter 2 describes gaseous drilling fluid systems.

• Chapter 3 discusses hydrology models for multiphase drilling fluids.

• Chapter 4 introduces surface and downhole equipment used in the industry.

• Chapter 5 describes method of gas compressor selection.

• Chapter 6 covers hydraulics design for gaseous fluid drilling.

• Chapter 7 presents operation procedures used in the industry.

• Chapter 8 outlines unconventional gas drilling systems.

• Chapter 9 presents field cases of gaseous fluid drilling applications.

Because the substance of this book is virtually boundless in depth, knowing what to omit was the greatest difficulty with its editing. The authors believe that it would require many books to fully cover the basics of gaseous fluid drilling engineering. To counter any deficiency that might arise from space limitations, the book contains a list of reference books and papers at the end of each chapter, so that readers would not experience little difficulty in pursuing each topic beyond the presented scope.

Regarding presentation, this book focuses on presenting and illustrating engineering principles and simple equations used in gaseous fluid drilling, rather than covering in-depth theories. The derivation of the equations is beyond the scope of this book. Applications are illustrated by solving sample problems using computer spreadsheet programs except for very simple problems. All the computer programs are provided with the book.

This book is based on numerous documents, including reports and papers accumulated through years of industry and academic work by the authors. We are grateful to the University of Louisiana at Lafayette and the Southwest Petroleum University for permission to publish their materials. Special thanks are due to Chevron Corporation for providing Chevron I and Chevron II professorships during the editing of this book. Our thanks are due to Ms. Tamaralayefa Ayemidi Timiyan of the University of Louisiana at Lafayette who made a thorough review and editing of this book manuscript. On the basis of collective experience, the authors expect this book to be of value to well-drilling engineers in the petroleum industry and undergraduate and graduate students in universities.

Chapter 1

Introduction

Abstract

Gaseous fluid drilling can be used for solving problems encountered in drilling operations and improving well productivity by reducing formation damage. This chapter presents definitions of gaseous drilling fluids on the basis of International Association of Drilling Contractors (IADC) classification; outlines major applications of three types of gaseous drilling fluids, namely, gas, aqueous foam, and gasified liquid; summarizes benefits and limitations of each type of gaseous drilling fluids; and reviews IADC guidelines to underbalanced drilling using gaseous drilling fluids.

Keywords

Gaseous; fluids; underbalanced; drilling; IADC; guidelines

1.1 Fluid systems

International Association of Drilling Contractors (IADC) classifies drilling fluids in five categories, namely (Guimerans et al., 2001; IADC, 2005):

1. Gas—gas as the fluid medium. No liquid intentionally added.

2. Mist—fluid medium with liquid entrained in a continuous gaseous phase. Typical mist systems have less than 2.5% liquid content.

3. Foam—two-phase fluid medium with a continuous liquid phase generated from the addition of liquid, surfactant, and gas. Typical foams range from 55% to 97.5% gas.

4. Gasified liquid—fluid medium with a gas entrained in a liquid phase.

5. Liquid—fluid medium with a single liquid phase.

The first four types of drilling fluids are gaseous fluids. Because gas and mist are similar in physical properties and most importantly in applications, they are considered as one type called gas in this book.

The gaseous fluid drilling, which is the subject of this book, is referred to as drilling processes, with gases, foams, and gasified liquids are the circulation fluids. Since all of these processes involve drilling boreholes with underbalanced formation pressure, they are also referred to as underbalanced drilling (UBD) techniques.

1.2 Major applications

Gas drilling is defined as drilling processes where air, nitrogen gas, natural gas, and sometimes carbon dioxide (CO2), is utilized to drill geotechnical boreholes, mining boreholes, oil and gas recovery wells, and water wells. The primary purpose of using this drilling technique is to increase rate of penetration in drilling dry and hard formations for cost reduction. The fast rate of penetration is achieved using the low-density nature of gas, which exerts low pressure and thus low stress in the bottom-hole rock, increasing rock drillability.

Foam drilling is referred to as drilling processes where stable aqueous foams are employed to drill oil, gas, and water wells. Foam is made up of water and gas with surfactants as foaming agents. Major applications of foam drilling are found to reduce drilling complications associated with loss of circulation during drilling low-pressure or naturally fractured formations. Lost circulation is mitigated using foam’s properties of low density to reduce borehole pressure and high viscosity to seal formation pores and fractures. In many cases, gas drilling is converted to foam drilling for better control of borehole collapse and excessive formation water influx.

Gasified liquid drilling is a drilling process where liquid-gas two-phase mixture is used to drill oil and gas wells. It is different from foam drilling in that foaming agent is not added in the gasified liquids. Gasified liquid drilling is a major technique used in UBD for reducing drilling fluid damage to the oil and gas pay zones. However, depending on the UBD pressure differential used, gasified liquid drilling does not completely eliminate formation damage due to capillary pressure effect.

1.3 Benefits and limitations

Gaseous fluid drilling is usually used for UBD. It avoids overbalanced pressure to the rock below the drill bit. Removing this confining pressure makes the rock easier for the bit teeth to cut and frees the generated cuttings from the bottom of the hole. This helps bottom-hole cleaning and increases the rate of penetration (ROP). ROP can be increased as much as 10 times over that for mud drilling in equivalent formations. An ROP as high as 120 ft./h can easily be achieved in air and gas drilling.

Gaseous fluid drilling is an effective means of minimizing lost-circulation problems in drilling naturally fractured petroleum reservoirs and pressure-depleted reservoirs. Related benefits in deep well drilling include the abilities to drill within the narrow margin between the formation pore pressure gradient and the fracturing gradient and to adjust the equivalent circulating density of the drilling fluid during drilling.

Gaseous fluid drilling as a UBD technology is demanded by oil and gas producers because it minimizes formation damage during drilling. Although this technology can be more expensive than overbalanced drilling in certain areas, it reduces stimulation requirements and saves the cost of well-stimulation treatments in some marginal reservoirs. Gaseous fluid drilling can eliminate drilling fluid and solid invasions that change rock wettability and relative permeabilities and plug rock pores. These changes reduce the effective permeability of the desired fluid (oil or gas) in the reservoir.

Gaseous fluid drilling can prolong drill bit life. Rock compressive strength increases due to overbalanced pressure from the drilling fluid. This pressure confinement does not exist during gaseous fluid drilling. Therefore the rock can be fragmented by bit teeth easier in gaseous fluid drilling than in conventional liquid drilling. Another explanation is that removing the pressure confinement lets generated cuttings become entrained more easily in the drilling fluid, which minimizes regrinding actions to the cuttings by the bit teeth.

Gaseous fluid drilling can minimize differential-pressure-type pipe sticking. This type of pipe sticking is believed to be associated with mud cake formed against permeable zones during overbalanced drilling. Stuck pipe occurs when the tool or equipment in the hole cannot be pulled out without exceeding the working load of the equipment. There is no filter cake during gaseous fluid drilling. Thus the pressure differential pipe-sticking problem does not exist.

Gaseous fluid drilling can improve formation evaluation. It provides a means to immediately detect hydrocarbon zones by directly observing the returned drilling fluid. These productive zones may otherwise be bypassed if the well is drilled with liquid. Because of fast return of drilling fluid carrying cuttings and reservoir fluids, hydrocarbon pay zones can be identified more accurately in depth during gaseous fluid drilling. In addition, the reduction or elimination of drilling fluid invasion into the formation that results from gaseous fluid drilling also improves the interpretation of open-hole logs and pressure transient tests.

Gaseous fluid drilling yields earlier oil production. With suitable surface equipment available, oil can be collected as soon as a productive zone is opened during gaseous fluid drilling. While drilling ahead to penetrate more zones, the produced oil is accumulated. It is possible for gaseous fluid-drilled wells to be paid for by the oil produced during the drilling stage.

Gaseous fluid drilling benefits the environment. Gas drilling eliminates potential pollution of drilling mud to environments during and after drilling. Chemicals used in mist and foam drilling are normally benign, biodegradable surfactants that do not pose significant environmental concerns. Of course, formation fluids produced during gaseous fluid drilling need to be handled with closed surface systems to minimize the potential for environmental contamination.

The technology to deal with reservoir protection is still evolving, but it is clear from the literature and field experience that gaseous fluid drilling using current technology will not solve all problems of low well productivity. It is not a production enhancement technique or a panacea for all problems. If a reservoir will not produce without fracturing, gaseous fluid drilling is probably not a terminal technology for well productivity improvement. It can only solve skin damage and fracture plugging.

Wellbore pressures are lower in gaseous fluid drilling operations than in conventional liquid-drilling operations. This may cause wellbore instability due to mechanical borehole collapse. The lower wellbore pressure also increases the tendency of tight holes due to yielding of some formations. Large shale fragments are often observed in foam and gasified liquid drilling. It is believed that these large shale fragments are not from the cutting action of drill bit at the bottom of the hole but from caving in, or sloughing, of the wellbore wall. This type of wellbore instability problem may occur when drilling formations have significant amounts of water-sensitive clays.

Liquid influx is always a problem in gaseous fluid drilling. Liquid influx includes water inflow and oil production. Although oil production during drilling is somewhat favorable, it requires that the surface equipment be able to handle the maximum rate of oil production safely at certain pressure. Under most circumstances, suitable surface equipment can be used to handle the produced oil during drilling. But if the oil production rate is too high to handle, the gaseous fluid drilling should be converted to liquid drilling.

There are some directional drilling difficulties in gaseous fluid drilling. Mud pulse telemetry measurement while drilling (MWD) tools cannot operate with compressible fluids used in gaseous fluid drilling. This is because the pressure pulses generated by the MWD tools do not propagate through compressible fluids to the surface with detectable amplitude. Electromagnetic MWD tools are required for drilling directional wells with compressible fluids. Conventional downhole motors used in directional drilling operate on incompressible fluids. Their performance deteriorates when they are run with compressible fluids.

Safety is always an issue in gaseous fluid drilling. Downhole fires/explosions occur under certain conditions during air drilling. Although they are rare, the consequences are severe. The bottom-hole assembly can melt or burn away. However, the probability of downhole fire occurrence can be minimized by using mist or foam drilling with sacrificed penetration rate. Vibration and noise are also issues of safety in gas drilling. While vibration can cause drill string failure and personnel injury, the high noise level is detrimental to human health.

Economic consideration hinders gaseous fluid drilling in some regions. In many locations, environmental restrictions make disposal of produced water expensive. The savings from the increased penetration rate may not compensate the cost for liquid handling. The gain in well productivity due to UBD may not always justify the drilling cost. This is especially true if a hydraulic fracturing treatment is still required after drilling the well underbalanced. In addition, local equipment availability and local logistics are important factors that should be considered when planning gaseous fluid drilling project.

New gaseous fluid drilling projects are often undertaken in wells where there is very little chance for success and therefore little risk of damaging the reservoir or incurring extraordinary costs. A poor well will never become a winner and, in the end, poor results or poor production will detract from the potential of a promising technology. The following is only a summary partial screening process for drilling a candidate well. If any of these conditions appear, they must not be in open-hole section for gaseous fluid drilling, but behind casing. Thus, by definition, these require that gaseous fluid drilling be halted in that open-hole interval. However, very little is absolute and local conditions and practices may modify this screening process.

Do not drill with gaseous fluids if there exist (1) weak formations, (2) dipping fractured formations, (3) thick coal beds, and (4) young geopressured shales. It is possible, but drill it and case it quickly, if there are (1) thick shale sections or older geopressured shales and (2) hard and thin salt beds. Gaseous fluid drilling is expensive in areas where there are high pressure water flows and/or hydrogen sulfide.

The benefits and problems of using three types of gaseous fluids, that is, gas, foam, and gasified liquid, are further outlined as follows.

The benefits of using gas drilling include (Guo and Liu, 2011):

• Significant increase in rate of penetration in hard and dry formations;

• Reduced water need in some dry areas;

• Minimized impact of liquid drilling to the environments;

• Timely finding of hydrocarbon reservoirs in explorations wells; and

• The fast rate of penetration is achieved using the low-density nature of gas, which exerts low pressure and thus low stress in the bottom-hole rock, increasing rock drillability.

The rate of penetration can be improved by more than 10-folds with gas drilling due to reduced bottom rock stress. The process avoids the use of large amount of water as drilling fluid and thus presents an advantage in dry areas where water is precious. Using gas as drilling fluid does not require much chemicals for fluid property control and therefore can significantly reduce the pollution of chemicals to the environment. Since the borehole pressure is less than the formation pore pressure, oil/gas-bearing formations are timely indicated from the returned fluid stream and undamaged drill cuttings. Some advantages of gas drilling are outlined in Fig. 1.1.

Figure 1.1 Some advantages of gas drilling.

There are some limitations to the use of gas drilling, depending strongly on the characteristics of formations to drill. Because of the low density and low viscosity of fluid, gas drilling is only employed for drilling long sections of hard and dry formations with low water flow. The disadvantages of gas drilling include (Guo and Ghalambor, 2002):

• Borehole collapse problems;

• Downhole fire/explosion if not properly depressed with misting water;

• Pipe sticking due to inadequate hole cleaning and mud ringing;

• Drill string corrosion/rust if air is not used;

• Casing erosion;

• Severe vibration of drill stem; and

• Life-threatening is H2S areas.

Some disadvantages of gas drilling are illustrated in Fig. 1.2. When the formation is damp from water or oil, the cuttings form a mud that is deposited against the side of the hole. This tends to form rings of mud that, as they grow larger, restrict gas flow and cause the pressure to increase. Mud rings cause high friction that can result in downhole burn-offs (fires) when air is used as well as stuck pipe. Mud rings can be cut with detergent additions to the drilling fluid. A light mist often will not cut mud rings. Tight hole problems appear to be related to mud-ring problems or floating beds. The important considerations are to not turn off the gas and to keep working the pipe. If the pipe is pulled up too hard it may stick tight. Gas drilling has problems with bit balling for the same reasons bits ball with mud. There are too much solids and not enough gas. Reservoirs, and other low-permeability formations will weep fluid. This leads to bit balling and mud rings. Nitrogen and natural gas, since they are so dry, are particularly effective at drying a damp or weeping formation. Since gas drilling is typically done in hard-rock dipping formations, key seating, while not common, does occur. A key-seated pipe can usually be driven down and out of the seat. Dropped pipe in a gas hole is often a catastrophe since there is no buffering from the drilling mud. In an open gas drilling hole, sloughing or broken ledges can drop large pieces of rock on top of the collars or bit. This can also occur with a floating bed or by not cleaning the hole before stopping circulation. Coal, especially broken coals, will slump into the hole until it reaches its critical angle of repose. This is true with mud as well as gas. In gas drilling, washouts reduce gas velocity and cause a floating bed