HIDRATOS DE GAS

GAS HYDRATES AND THEIR PREVENTION

Gas hydrates are representatives of a class of compounds known as clathrates or inclusion compounds.

Natural gas and crude oil normally reside in reservoir in contact with connate water. Water can

combine with low-molecular weight natural gases to form a solid, hydrate, even if the temperature is

above water freezing point.

Hydrates are considered as nuisance because they block transmission lines, plug blowout preventers,

jeopardize the foundation of deepwater platforms and pipelines, cause tubing and casing collapses, and

foul process heat exchangers, valves, and expanders.

Hydrates act to concentrate hydrocarbons; 1 cuft of hydrates may contain as much as 180 SCF of gas.

Large natural reserves of hydrocarbons exist in hydrated form, both in deep oceans and in the

permafrost. Evaluation of these reserves is highly uncertain, yet even conservative estimates indicate

that there is perhaps twice as much energy in the hydrated form as in all other hydrocarbon sources

combined.

This chapter is intended to provide the basic information needed for engineering purposes about

hydrates.

4.1 WATER CONTENT OF NATURAL GAS

When natural gas leaves the reservoir, it flows up the wellbore, which has lower temperatures than the

reservoir, and hence the gas saturated with water will tend to yield a liquid water condensate. Pressure

drop also affects the water content, but this effect during flow up a wellbore is smaller than that of

temperature.

Figure 4.1 shows a chart giving the water content of natural gases saturated with water vapor. Plotted

on this graph is an equilibrium curve of hydrate formation, which should be a function of gas

composition.

Determination of water content by this chart produces an error not exceeding 4%, which is acceptable

for engineering purposes.

As seen in Figure 4.1, the water content of a natural gas increases with the increase in temperature and

decreases with increase in pressure. Moreover, the water content of natural gases drops with an

increase in their molecular weight and with an increase in the water salinity. The two auxiliary graphs

shown in Figure 4.1 are for finding the correction factors for the molecular weight (gas density), Cg and

water salinity (Cs).

Let us now look at analytical methods, which allow us to find the water content of natural gases in a

selected range of pressures and temperatures.

The most suitable analytical method is Bukacek's method1 permitting the determination of water

content within a pressure range from 1 to 700 kgf/cm2 and of temperature range from -40 to 230 0C.

The error by this method does not exceed 4%.

The following expression is used for finding gas water content:

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where

A, B = coefficients, function of temperature. The values of A and B are given in Table 4.1.

P = gas pressure, kg/cm2

Table 4.1 Coefficients for Equation 4.1

In view of the fact that Equation 4.1 was obtained for natural gases with a specific gravity of 0.6 in

contact with fresh water. For finding the water content of gases with a different gravity and of gases in

contact with mineralized water, the Equation 4.1 takes the following form:

+B

P

A

W = (4.1)

+B)C C

P

A

W =( g s (4.2)

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Cg, Cs = Correction factors for gas gravity and salinity of water.

The correction factors Cg and Cs can be obtained from the insert graphs of Figure 4.1. It is also possible

to obtain the salinity correction, Csfrom Equation 4.32.

where S is the salinity in weight percent. Figure 4.3 agrees with existing graphical correlation to within

1%.

4.2 WHAT IS A GAS HYDRATE?

Natural gas hydrates are two or multi-component non-stoichiometric crystalline compounds where one

of the components is always water. Gas hydrates exist due to the ability of water molecules to form a

lattice structure, through hydrogen bonding, stabilized by small, non-polar gas molecules. By the

inclusion of the gaseous component, the structure, which alone is thermodynamically unstable,

becomes stabilized.

Gas molecules are physically enclosed in the cavities of the water lattice, and they are released from the

cavities only under appropriate circumstances, when the water lattice breaks down. Thus, the gas

components filling the cavities are not directly bonded to the water molecules of the framework. It is

for geometrical reasons that they cannot leave the hydrogen-bonded water molecule lattice until it

collapses.

Gas hydrates of interest to the hydrocarbon industry are composed of water and the following eight

molecules:

Methane Carbon dioxide

Ethane Nitrogen

Propane Hydrogen sulphide

iso-Butane normal-Butane

Hydrates normally form in one of the small, repeating crystal structures, shown in Figure 4.2. Structure

I (sI), a body-centered cubic structure and forms with natural gases containing molecules smaller than

propane. Structure II (sII), a diamond lattice within a cubic framework, forms when natural gases or

oils contain molecules larger than ethane but smaller than pentane. Structure H (sH) has been found

recently3. sH hydrates are unique since they form in the presence of a light gas such as methane and

molecules typically in oils and condensates4. Several sH formers such as methylcyclopentane,

methylcyclohexane, neohexane, and adamantane are indigenous to petroleum. Preliminary studies

demonstrate the possible occurrence of sH hydrate as the most stable hydrate structure for certain

multi-component systems.

Table 4.2 provides a hydrate structure summary for sI, sII and sH. The "small" cavities of all structures

are pentagonal dodecahedra (512) formed by the water molecules. The "large" cavities of sI are

tetradecahedra, formed by 2 opposing hexagons and 12 pentagons situated between them (51262); in sII

they are hexadecahedra, constructed from 4 hexagons and 12 pentagons (51264). Structure H has a

second dodecahedron, the 435663 cavity, which is built of three quadrates, six pentagons and three

hexagons. The largest cavity, the 51268 cavity, is built of twelve pentagons and eight hexagons. Figure

4.3 depicts the five cavities of hydrate structures5.

C 1 4.920 10 3 S 1.762 10 4 S 2 (4.3) s

= - ´ - - ´ -

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Each of the cavities in either structure may contain only one guest molecule. The cavity occupied is a

function of the size ratio of the guest molecule within the host cavity. Table 4.3 shows the size ratio of

several common gas molecules within each of the four cavities6. Note that a size ratio approximately

0.9 is necessary for the stability of a simple hydrate, given by the superscript "†". When the size ratio

exceeds unity, the molecule will not fit within the cavity and the structure will not form. When the ratio

is significantly less than 0.9 the molecule cannot lend stability to the cavity to cause formation. Table

4.3 indicates which of the gaseous compounds found in typical petroleum mixtures may enter hydrate

cavities.

Table 4.2 Some physical constants of hydrate structures.

Property Structure I Structure II Structure H

Number of H2O molecules/unit cell 46 136 34

Number of 512 cavity 2 16 3

Number of 51262 cavity 6 - -

Number of 51264 cavity - 8 -

Number of 435663 cavity - - 2

Number of 51268 cavity - - 1

Diameter of 512 cavity, Å 5.0 5.0 5.0

Diameter of 51262 cavity, Å 5.8 - -

Diameter of 51264 cavity, Å - 6.47 -

Diameter of 435663 cavity, Å - - » 5.0

Diameter of 51268 cavity, Å - - » 9.0

Table 4.3 Ratio of molecular diameters to cavity diameters for some molecules forming sI and sII

hydrates.

(Molecular diameter)/(Cavity diameter)

Cavity type Structure I Structure II

Molecule Guest diameter (Å) 512 51262 512 51264

Ne* 2.97 0.604 0.516 0.606 0.459

Ar 3.8 0.772 0.660 0.775† 0.599†

Kr 4.0 0.813 0.694 0.816† 0.619†

N2 4.1 0.833 0.712 0.836† 0.634†

O2 4.2 0.853 0.729 0.856† 0.649†

CH4 4.36 0.886† 0.757† 0.889 0.675

H2S 4.58 0.931† 0.795† 0.934 0.708

CO2 5.12 1.041 0.889† 1.044 0.792

C2H6 5.5 1.118 0.955† 1.122 0.851

C3H8 6.3 1.276 1.090 1.280 0.971†

i- C4H10 6.5 1.321 1.128 1.325 1.005†

n-C4H10

* 7.1 1.443 1.232 1.447 1.098

† Indicates the cavity occupied by the simple hydrate former.

* Indicates that a molecule will not form hydrates as a single component, because it is too small (or too large) to stabilize

a cavity.

Note: If a molecule enters the small cavities of a structure, it will also enter the large cavities.

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4.3 PHASE EQUILIBRIA

The hydrates of natural gases are solid compounds containing a number of gas components. The range

of existence and the properties of hydrates indicate that the nature of the gas, the state of the water, the

pressure and the temperature determine the initial conditions of gas hydrate formation. The formation

conditions

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