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NATURAL GAS HYDRATES IN FLOW ASSURANCE
By DENDY SLOAN CAROLYN KOH AMADEU K. SUM ADAM L. BALLARD JEFFERSON CREEK MICHAEL EATON JASON LACHANCE NORM MCMULLEN THIERRY PALERMO GEORGE SHOUP LARRY TALLEY
Gulf Professional Publishing
Copyright © 2011 Dendy Sloan, Carolyn Ann Koh, Amadeu K. Sum, Norman D. McMullen, George Shoup, Adam L. Ballard, and Thierry Palermo
All right reserved. ISBN: 978-1-85617-946-1
Chapter One
Introduction Dendy Sloan
Contents
1.1 Why Are Hydrates Important? 1 1.2 What Are Hydrates? 2 1.2.1 Hydrate Crystal Structures 2 1.3 Four Rules of Thumb Arising from Crystal Structure 4 1.4 Chapter Summary Application: Methane Hydrate Formation on an Emulsified Water Droplet 9 References 11
1.1 WHY ARE HYDRATES IMPORTANT?
Oil and gas facilities are expensive, but offshore oil and gas facilities are very expensive. Frequently the cost of an offshore project exceeds US$ 1billion, with wells and flowlines comprising 39% and 38%, respectively, of the total cost (Forsdyke, 2000, p. 4). It is rare for a single company to assume the total risk of such a development, so partnerships are the norm. Field and project partners change from one project to the next, taking information with them to new projects, so that a common body of flow assurance knowledge is becoming available.
In a survey of 110 energy companies, flow assurance was listed as the major technical problem in offshore energy development (Macintosh, 2000, citing Welling and Associates, 1999). Beginning a September 24, 2003 flow assurance forum, James Brill (University of Tulsa) discussed the need for a new academic discipline called "flow assurance." Such a question, presented to an audience of 289 flow assurance engineers, would not have been considered a decade previously, when the flow assurance community totaled a few dozen people. The term "flow assurance" was not coined until 1995, with a modification of the term "Procap" used by Petrobras. Yet the statement of Brill's question indicates the importance of flow assurance.
Safety is perhaps a more important reason for understanding hydrate blockages. Every few years, somewhere in the world a major injury occurs and/or major equipment damage is done, due to hydrates. After the first two chapters on fundamental structures, and how hydrate plugs form, Chapter 3 deals with hydrate safety, providing guidelines and case studies. The remainder of the book deals with hydrate industrial practices and safe removal of plugs.
The above two areas, safety and flow assurance, are the major topics of this book. The principle ofthe book is that experience is a better guide to reality than is theory, as reflected by most ofthe coauthors' careers and the case studies herein.
1.2 WHAT ARE HYDRATES?
Natural gas hydrates are termed "clathrates" or inclusion compounds. This means that there is a network of cages of water molecules that can trap small paraffin guest molecules, such as methane, ethane, and propane.
Of the three common hydrates structures (known as structures I, II, and H), only structures I and II are typically found in oil and gas production and processing. Yet the principles presented in this book for structures I and II apply equally well to structure H. A detailed discussion of structure is given elsewhere (Sloan and Koh, 2008). The following four structure rules of thumb in this introductory chapter are applied in safety and flow assurance:
• Fit of the guest molecule within the host water cage determines the crystal structure.
• Guest molecules are concentrated in the hydrate, by a factor as high as 180.
• Guest:cage size ratio controls formation pressure and temperature.
• Because hydrates are 85 mole % water and 15 mole % gas, gas-water interfacial formation dominates.
At the chapter conclusion, the previous principles are illustrated showing the concept of how hydrates form on a water droplet.
1.2.1 Hydrate Crystal Structures
Figure 1.1 shows the three hydrate unit crystal structures, the smallest crystal unit that repeats itself in space. It is important to review these structures to obtain a basic understanding at the microscopic level, which impacts macroscopic hydrate plugs.
The three rightmost structures in Figure 1.1 are composed of cages, but particularly a basic cage, the 512, forms as a building block for all three. The 512 cage is composed of 12 pentagonal faces, formed by water molecules that are hydrogen-bonded to each other, with an oxygen at each vertex. Inside the 512 free diameter (5.1 Å) is a hydrocarbon molecule like methane (4.36 Å diameter), which effectively props the cage open. There are no chemical bonds between a cage and a guest molecule; rather the presence of the guest keeps the cage open. Without most of the cages filled, hydrogen-bonded hydrate structures collapse and do not exist in water.
When the 512 cage is connected to others like it via the vertices, a body-centered cubic crystal of 512 cages forms, called hydrate structure I, which exists primarily outside the pipeline, in nature. However, because the 512 cavities alone cannot fill space without strain on the hydrogen bonds, the bond strain is relieved by the inclusion of hexagonal faces to form connecting 51262 cages, with both the 12 original pentagonal faces and two additional hexagonal, strain-relieving faces.
The free diameter of the 51262 cage is somewhat larger (5.86 Å) and can contain molecules the size of ethane (5.5 Å diameter), typically the second most common component of natural gas. Methane can fit in the 51262 cage also, when hydrates are formed from pure methane gas. But methane is too small to prop open the 51262 effectively, so when mixtures of methane and ethane form structure I (sI), the ethane molecules reside in the 51262 cages because ethane is too large for the 512 cage. In mixtures of methane and ethane, methane resides mostly in the 512 cages and a small number of the 51262 cages. In some circumstances, methane and ethane can combine to form structure II (sII) (Sloan and Koh, 2008, chapter 2).
In sum, two 512 cages and six 51262 cages with 46 water molecules, comprise the sI repeating unit crystal shown in Figure 1.1. Structure I is found mostly in nature because methane is the major component of most hydrates found outside the pipeline. Figure 1.1 shows the sI unit crystal fits a cube 12 Å on a side.
When a larger hydrocarbon, such as propane (6.3 Å diameter), is present in a gas, the propane molecule is too large to be contained in the 51262 cage, so a larger 51264 cage (6.66 Å free diameter) forms around larger molecules, such as propane and i-butane (6.5 Å diameter). The 51264 cage, with twelve pentagonal and four hexagonal faces, is the large cage that relieves hydrogen bond strain when the 512 basic building blocks are connected to each other via their faces. Again the 512 cages cannot fill space, when the 512 are connected to each other, but this time by their pentagonal faces.
The combination of 16 small 512 cages with 8 large 51264 cages forms the sII unit crystal shown in Figure 1.1, incorporating 136 water molecules in this smallest repeating structure. The sII hydrates are typically found in gas and oil operations and processes, and will be the major concern of this book. The diamond lattice of sII is in a cubic framework that is 17.1 Å on a side.
Still larger molecules such as normal pentane (9.3 Å diameter) cannot fit in any sI or sII cages and are excluded from the hydrate structures of concern to us. The structure H crystals are seldom found in artificial or in natural processes, so we will not deal with them here.
1.3 FOUR RULES OF THUMB ARISING FROM CRYSTAL STRUCTURE
The above discussion leads to the first rule of thumb of hydrate structures:
1. The fit of the guest molecule within the water cage determines the crystal structure.
Consider Table 1.1 that shows the size ratios (or the fit) of the first five common hydrocarbon gases (CH4, C2 H6, C3 H8, i-C4 H10, and n-C4 H10) in the two cages of sI (512 and 512 62) and the two cages of sII (512 and 512 64).
Note that in Table 1.1, the superscripted symbol "F" indicates the cage occupied by a pure gas, so that pure CH4 and C2 H6 are indicated to form sI, while pure C3 H8 and i-C4 H10 form sII. Table 1.1 provides three guidelines for hydrate structure and stability:
a. Molecules that are too large for a cage will not form in that cage as a single guest. This principle is illustrated by the case of n-C4H10, which is too large (guest:cage size ratios above 1.0) for every cage and so forms neither sI nor sII as a single guest. However, smaller hydrocarbons will fit in at least one of the four cages, with a fit that determines the structure formed. Ethane, propane, and i-butane fit the larger cages of their structures; pure ethane forms in the 512 62 cage of sI, while propane and i-butane each form in the 512 64 cage of sII, leaving the smaller cages vacant in each case.
b. The second guideline from Table 1.1 is the optimal size for the guest:cage diameter ratio is between 0.86 and 0.98. For ratios less than 0.8 the guest does not lend much repulsive stability to the cage. For example, methane props open the 512 62 (size ratio of 0.74) better than it does the 512 64 (size ratio of 0.66), while the ratio is almost the same for methane in the 512 of sI and sII (0.86 and 0.87, respectively), so sI is the stable structure for pure methane hydrate.
c. Due to the similar ratios for methane in the 512 of sI and sII, the controlling factor is the fit of the large cavity, as shown in item 1b above. However, if natural gases contain any amount of C3 H8 and/ or i-C4 H10, those larger molecules will only be able to fit into the 512 64 of sII. Since the size ratio of methane in the 5 of sI and sII is almost identical, the presence of any amount of a larger, common molecule (e.g., C3 H8 or i-C4 H10) will convert the hydrate structure to sII in almost all instances. It is relatively rare to find sI hydrates in oil and gas production due to the common presence of larger molecules.
2. Hydrates concentrate energy equivalent to a compressed gas.
If all cages were filled in sI or sII, the guest molecules would be much closer together than in the gas phase at ambient conditions. In fact, hydrate concentrates the gas volume by as much as a factor of 180, relative to the gas volume at 273 K and 1 atmosphere. This concentration has the energy density of a compressed gas, but hydrates have only 42% of the energy density of liquefied methane.
3. The guest:cage size ratio controls hydrate formation pressure and temperature.
Figure 1.2 shows the hydrate formation pressure at 273 K as a function of the guest:cage size ratio, for the optimum cage size stabilized by the first four hydrocarbons in Table 1.1 (512 for CH4, 512 62 for C2 H6, 512 64 for C3 H8, and 512 64 for i-C4 H10). As a molecule better fits the cage, the formation pressure decreases. Methane is a relatively poor fit (0.86) in the 512, so methane's hydrate formation pressure is high (2.56 MPa). In contrast, i-C4 H10 is a good fit (0.98) in the 512 64 cage yielding a low formation pressure (0.133 MPa).
The above rules of thumb connect the pressure–temperature stability of hydrates to their crystal structure. Normally, however, natural gases are composed of a number of components, including acid gases CO2 and H2S, relative to the simple one- and two-component gases for which the above rules of thumb apply.
To determine the hydrate stability conditions for more complex gases, the reader is encouraged to use a commercial computer program, such as Multiflash®,PVTSim®, DBRHydrate®, HWHyd, or CSMGem, to determine the hydrate formation conditions of pressure and temperature. Simpler hand calculation methods, together with CSMGem and a user's manual are provided in Sloan and Koh (2008).
4. Because hydrates are 85 mole % water and 15 mole %gas, hydrate usually forms at the gas-water interface.
Hydrates contain much higher concentrations of hydrocarbon and water than are normally found in a single phase. At ambient conditions, every 10,000 molecules of liquid water dissolve only eight molecules from a gaseous methane atmosphere. Similarly, 1000 molecules of gaseous methane will only have a single molecule of liquid water vaporized into the gas. If we consider the solubility rule of thumb, "like dissolves like," we must conclude that methane and water are very dissimilar molecules due to their mutual insolubility.
In all three hydrate crystal structures, however, the molecular ratio of hydrocarbon to water is very high (15:85) when all cages are filled. These relative concentrations—in hydrate, in the gas, and in the water phases—are shown in Figure 1.3.
Hydrates will likely form at the methane–water interface. The low solubility of methane in water suggests that only a small amount of hydrate cages will form in the body of the liquid, as there are insufficient methane molecules to occupy many hydrate cages in the liquid water. Similarly the small amount of water vaporized in the methane gas provides only a few hydrate cages forming in the body of the gas; there are too few water molecules to make significant amounts of the host crystal structure in the bulk gas.
Hydrates normally form at the hydrocarbon–water interface, as a consequence of the mismatch between (1) the requirement of high concentrations of both hydrate components and (2) the mutual insolubility (low concentrations) of hydrocarbons and water. Hydrates form at the interface between the two phases, rather than the body of liquid water or the body of gaseous methane.
When one looks down through a quartz glass window into a methane-pressurized cell of water when hydrates form, one sees through the clear methane gas, giving a picture like that shown in the inset of Figure 1.4. Hydrate, the dimpled phase in the photo insert of Figure 1.4, forms from left to right across the clear gray water interface, shown on the right.
With a fast camera, one can take time-lapse pictures, and plot the linear progress of the film at time and distance increments (Figure 1.4). Regression of the distance with time, shows that the hydrate film grows fairly rapidly, at about 1 mm every 3 sec (Freer, 2000).
The hydrate film grows across the water–gas interface very rapidly. The initial thickness of the hydrate film is small, between 5 and 30 µm(Taylor et al., 2007). The solid phase forms a barrier between the methane gas and the water phases, so that the initial hydrate film prevents further contact of the gas and water, and the hydrate film thickens very slowly. The solid hydrate film formed at the vapor–liquid interface controls the subsequent rates of hydrate formation.
(Continues...)
Excerpted from NATURAL GAS HYDRATES IN FLOW ASSURANCE by DENDY SLOAN CAROLYN KOH AMADEU K. SUM ADAM L. BALLARD JEFFERSON CREEK MICHAEL EATON JASON LACHANCE NORM MCMULLEN THIERRY PALERMO GEORGE SHOUP LARRY TALLEY Copyright © 2011 by Dendy Sloan, Carolyn Ann Koh, Amadeu K. Sum, Norman D. McMullen, George Shoup, Adam L. Ballard, and Thierry Palermo . Excerpted by permission of Gulf Professional Publishing. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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