PEH Volume I Chapter 11: Phase Behavior

I've recently added a page to the Guidebook on Hydrates. Prior editions had the basics, but I found over time I wanted more detail on this complicated subject. 

C1-3: Math
C4: Fluid Sampling
C5: Gas Properties
C6: Oil Correlations
C7: Thermo/Phase
C8: Phase Diagrams
C9: Asphaltene/Wax
C10: Produced Water
C11: Phase Behavior
C12: Emulsions
C13: Rock Properties
C14: Permeability
C15: Relative Permeability
C16: Economics
C17-18: Law

Hydrates: the most common solid-phase flow-assurance problem.
H2O & HC typically have 2 separate phases…because H2O bonds >> strength than HC bonds.

Hydrates are found at Low Temperature and High Pressure.

    …OR in small-sized HC < n-pentane size (I-501).

3 hydrate structures common in HC yet some are unknown (I-508).

Intense variables: T, P, and compositions.

Gibbs phase rule (I-335).

F = C – P + 2 used for:

   …how many intensive variables important in phase equilibria.

   …for small number of components.

   …insight to max number phases that can form.

   …insight to number of intensive properties independently specified.

Example: 1 phase, 1 component: only 2 intensive properties can be specified (degree of freedom 2).

Example: 3 phases, 2 components: only 1 intensive property can be specified.

Gibbs P, T diagrams (I-512; semilog plots for nearly straight lines).

2-component system: “area”. 3-component system: “line”. 4-component system: “point”.

Single NG components Hydrate for 3-phase conditions (CH4, etc. Table 11.6).

Water Content (lbm) per HC wet gas (MMscf) 60 °F, 14.7psia; correct for salinity, gravity (I-502 Fig. 11.1).

4 Types of H2O-HC Equilibrium PT Diagrams that include hydrates (I-509-512).

1. gases or vapors (say CH4, N2).

2. gas + single condensate + water (HC may be vapor or liquid).

3. gas + mixed oil/condensate + water.

4. H2O-HC hydrate + inhibitors (MeOH, MEG, salts; note methanol most economical).

Hand Calculatable: 3-phase Lw-H-V system hydrate formation or wet-gas expansion through valves.

NOT hand calculable: Lw-H-Lh, I-H-V, 4-phase BUT a Lw-H-V hand calcs can check computer quality.

Hammerschmidt Expression for inhibitors finds ΔT (65-X °F) = 2,335W / (100M – MW) (I-516, 521).

ΔT = hydrate temperature depression and constant regardless of pressure (65 °F – T °F).

W = weight % of inhibitor (free-water phase) shifting Lw-H-V line left below lowest operating temperature.

M = molecular weight of inhibitor (M = 32 for MeOH, 62.07 for MEG).

Hammerschmidt calculation examples:

ΔT = 2,335W / 100M – MW = 2,335(25) / 100(32) – 32(25) = 24 °F.

W = 100MΔT / MΔT +2,335 = 100(32)24 / (32)24 +2,335 = 25 %wt (of water + MeOH lbm mix).

1. Find gas gravity, temperature, pressure (of hydrate formation conditions).

2. Find W from the Hammerschmidt expression.

3. Find mass of liquid water, from condensed and liquid water (lbm water/MMscf of gas).

4. Find rate of MeOH in the aqueous phase W = MeOH/(H2O+MeOH).

Hydrate Formation on Expansion Across Valve or Restriction (I-524-528).