Practical Onshore Gas Field Engineering

David Simpson

• Publisher: Gulf Professional Publishing
• Year: 2017
• Print ISBN: 9780128130223
• eBook ISBN: 9780128130230
• Topic: Technology & Engineering
• Subtopic: Energy
• Index: Technology & Engineering

Practical Onshore Gas Field Engineering delivers the necessary framework to help engineers understand the needs of the reservoir, including sections on early transmission and during the life of the well. Written from a reservoir perspective, this reference includes methods and equipment from gas reservoirs, covering the gathering stage at the gas facility for transportation and processing. Loaded with real-world case studies and examples, the book offers a variety of different types of gas fields that demonstrate how surface systems can work through each scenario. Users will gain an increased understanding of today's gas system aspects, along with tactics on how to optimize bottom line revenue.As reservoir and production engineers face many challenges in getting gas from the reservoir to the final sales point, especially as a result of the shale boom, a new demand for more facility engineers now exists in the market. This book addresses new challenges in the market and brings new tactics to the forefront.- Presents the full lifecycle of the gas surface facility, from reservoir to gathering and transmission- Helps users gain experience through case studies that explain successes and failures on a variety of gas fields, including unconventional and shale- Teaches how the surface gas facility system and equipment work individually, and as an integrated system

Chapter One

Gas Reservoirs

Abstract

The source of value in Oil & Gas is hydrocarbons within reservoirs. The conditions that must exist to cause naturally occurring hydrocarbons to be trapped within these reservoirs, the methods that the reservoir fluids migrate within the reservoir, and the characteristics of storage and transport within the various types of reservoirs are discussed. The chapter goes on to discuss difference between gas reservoirs and oil reservoirs, between conventional reservoirs and unconventional reservoirs, and some of the key characteristics of each. Key reservoir parameters are defined and discussed.

Keywords

Permeability; porosity; hydrocarbon traps; reservoir pressure; original gas in place (OGIP); conventional gas; unconventional gas; tight gas; coalbed methane (CBM); coal seam gas (CSG)

Simpson’s First Postulate: Every activity, joint of pipe, piece of equipment, and facility should have the goal of maximizing reservoir profitability—any activity which ignores that goal is going to result in sub-optimum performance.

Contents

1.1 Source of Hydrocarbons 46

1.1.1 Recoverable hydrocarbons explained 46

1.1.2 Biotic hydrocarbons 47

1.1.3 Abiotic hydrocarbons 49

1.1.4 Do abiotic hydrocarbons matter to the oil & gas industry? 49

1.2 Reservoir Rocks 50

1.2.1 Porosity 50

1.2.2 Permeability 52

1.2.3 Hydrocarbon traps 53

1.3 Reservoir Concepts 55

1.3.1 Reservoir temperature 55

1.3.2 Reservoir pressure 56

1.3.3 Original gas in place 57

1.3.4 Reservoir pressure versus gas in place overview 57

1.4 Primary Gas-Field Distinctions 58

1.5 Conventional Gas Fields 58

1.5.1 Reservoir pressure versus OGIP conventional 59

1.5.2 Conventional gas 60

1.5.3 Conventional reservoir materials 61

1.6 Unconventional Fields 62

1.6.1 Tight gas 63

1.6.2 Coalbed methane 64

1.6.3 Shale 74

1.7 Reservoir Development 78

1.7.1 Types of resources 79

1.8 Conclusion 80

References 81

Further Reading 82

Nomenclature 82

Exercises 84

1.1 Source of Hydrocarbons

The Oil & Gas industry has always considered hydrocarbons as being biogenic (i.e., produced by living organisms) and it is certain that some proportion of the hydrocarbons recovered from wells is biogenic.

However, the idea that hydrocarbons can be formed in the core of the earth and migrate through the mantle has been gaining a resurgence in acceptance. This abiotic (i.e., not derived from organisms) gas and oil would provide some unknown quantity of supplemental hydrocarbons that would have a potential for recovery and commercialization.

Exactly what that quantity would be requires some in-depth analysis.

1.1.1 Recoverable hydrocarbons explained

Recoverable hydrocarbons must first be hydrocarbons, then they must be located in a rock stratum that allows them to be stored. Finally, there must be some sort of containment to keep them from leaking out of the storage strata.

In the Oil & Gas industry, we call these entities “source rock,” “reservoir rock,” and “cap rock” (Fig. 1.1). Once a reservoir is delineated by having a source, a reservoir, and a containment, it requires a flow path to an outlet. Most of the time this flow path takes the hydrocarbons around the edges of the cap rock and they leak to the surface of the earth.

Figure 1.1Source rock, reservoir rock, and cap rock.

Occasionally, the cap rock has an effective seal on the reservoir. When the Oil & Gas industry comes along and drills a well and stimulates the reservoir, the flow path becomes a commercial transaction—this is the reason that the Oil & Gas industry exists.

1.1.2 Biotic hydrocarbons

The total amount of carbon that is tied up within living biological material on the earth is about 4 trillion metric tons (tonne, 4.4 trillion US tons). Something like 105 billion tonne of carbon are discarded by their living host each year (Wikipedia, 1, 2016)—the leaves of deciduous trees fall to the ground each autumn, some number of animals, plants, bacteria, and viruses die each year, all organisms emit some amount of solid, liquid, and gaseous waste, and so on.

This carbon will eventually become CO₂ and water vapor in an aerobic environment and mostly CH₄ in an anaerobic environment. The discarded biomass is approximately equally divided between land and sea.

Aerobic decomposition is exothermic and tends to be fairly rapid (i.e., from the onset of decomposition to a sterile, carbon-free mass takes weeks or months) and is largely free of the worst odors. Aerobic decomposition converts the carbon in the waste material to CO₂ and H₂O.

Anaerobic decomposition is endothermic, much slower and very smelly. It can take hundreds or thousands of years for an undisturbed anaerobic process to run to completion. The time required is largely a function of the energy input to the process. Anaerobic decomposition converts the carbon in the waste material to CH₄ and amounts of CO₂, and H₂O limited by available oxygen.

It is common for decomposing organic material to accumulate on the seafloor away from thermal vents and begin the decomposition process at very low energy input. While the mass is decomposing it sometimes happens that a storm event or a seismic event will cover the biomass with sand. Above the sand over many years, more organic material will collect that can eventually turn into shale which is one of the most common cap rocks.

Now we have a source, a reservoir, and a containment. Over millions of years, the methane product of decomposition can be converted through the application of heat and pressure into heavier hydrocarbons. The sealed volume can move upward or downward or it can tip (usually allowing the hydrocarbons to spill out of the reservoir, but not always) due to tectonic and seismic activity.

There is no competent theory to allow prediction of the proportion of biomass that will be subject to anaerobic decomposition. We can make some (arbitrary, but conservative) assumptions about the proportions:

• At least 1% of the biomass on land undergoes anaerobic decomposition (this includes human sanitary landfills, lakes, and wetlands, and in the stomachs of many animals and insects). One percent of half of 105 billion tonne per year is 0.525 billion tonne of carbon per year subjected to anaerobic decompo...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter Zero. Introduction
• Chapter One. Gas Reservoirs
• Chapter Two. Well-Bore Construction (Drilling and Completions)
• Chapter Three. Well Dynamics
• Chapter Four. Surface Engineering Concepts
• Chapter Five. Well-Site Equipment
• Chapter Six. Gas Gathering Systems
• Chapter Seven. Water Collection and Disposal
• Chapter Eight. Gas Compression
• Chapter Nine. Interface to Plants
• Chapter Ten. Integration of Concepts
• Appendix A. Acronyms and Glossary
• Appendix B. Unit Conversions
• Appendix C. Valve Summary
• Index