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).
