Engineering lesson
Hydrate onset versus wax appearance in a flow assurance model
Hydrate onset and wax appearance describe different phase-equilibrium problems. Hydrates involve water and gas guests in a solid lattice; wax involves crystallizing heavy hydrocarbon material.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Use the appropriate feed and model
A hydrate calculation needs gas composition, pressure and water activity on the supported model basis. A wax calculation needs a heavy-hydrocarbon representation and solid–liquid thermodynamic assumptions.
A methane-rich gas input cannot automatically supply the heavy-fraction information needed for wax appearance.
Onset is an equilibrium boundary
The recorded hydrate case varies pressure with composition and water activity fixed. It reports the model's equilibrium onset temperature.
The result does not predict nucleation delay, plug growth, deposition rate or inhibitor kinetics. A temperature margin is conditional on the selected fluid and water model.
Keep uncertainty and operating history separate
Actual systems may exhibit thermal gradients, changing composition, water distribution and time-dependent effects. These require additional evidence.
Compare a local temperature profile with the relevant equilibrium boundary only after matching pressure, composition and property basis.
Explore the example
Original recorded synthetic cases solved with the site's mixed-gas-hydrate-onset engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Absolute gas pressure (MPa(a)) = 5. Hydrate onset temperature (K): 287.264214; Temperature minus hydrate onset (K difference): 2.73578582.
- Hydrate onset temperature (K)
Worked example
Mixed-Gas Hydrate Onset and Cage Occupancy Calculator
Fixed controls: Operating temperature = 290 K; Supplied liquid-water activity = 1 dimensionless; Methane guest-gas mole fraction = 0.9 mol/mol; Ethane guest-gas mole fraction = 0.05 mol/mol; Propane guest-gas mole fraction = 0.02 mol/mol; Carbon dioxide guest-gas mole fraction = 0.02 mol/mol; Nitrogen guest-gas mole fraction = 0.01 mol/mol.
Relation and unit basis
C_ic=A_ic/T exp(B_ic/T), in 1/bar; fi=zi*phi_i*p in bar. A/B are the five-guest NeqSim 3.26.0 COMP.csv subset, not Kihara-integral coefficients. Gas fugacity is calculated with classical PR and explicitly assumed zero binary interactions.; Use the pinned NeqSim PVTsim empty-lattice Gibbs/enthalpy/Cp/volume expression relative to liquid water. nu=(1/23,3/23) for sI and (2/17,1/17) for sII. Solve each bracketed 273.15-303.15 K onset; choose the higher temperature as the first structure to become stable on cooling.
Absolute gas pressure is varied within the supported input range. 273.15-303.15 K, 3-10 MPa(a), water activity 0.8-1; fixed CH4-rich gas only. Condensation, failed gas stability and unbracketed onset fail explicitly.
1. Define the baseline
Absolute gas pressure (MPa(a)) = 5
Fixed controls: Operating temperature = 290 K; Supplied liquid-water activity = 1 dimensionless; Methane guest-gas mole fraction = 0.9 mol/mol; Ethane guest-gas mole fraction = 0.05 mol/mol; Propane guest-gas mole fraction = 0.02 mol/mol; Carbon dioxide guest-gas mole fraction = 0.02 mol/mol; Nitrogen guest-gas mole fraction = 0.01 mol/mol.
2. Select the relation
C_ic=A_ic/T exp(B_ic/T), in 1/bar; fi=zi*phi_i*p in bar. A/B are the five-guest NeqSim 3.26.0 COMP.csv subset, not Kihara-integral coefficients. Gas fugacity is calculated with classical PR and explicitly assumed zero binary interactions.; Use the pinned NeqSim PVTsim empty-lattice Gibbs/enthalpy/Cp/volume expression relative to liquid water. nu=(1/23,3/23) for sI and (2/17,1/17) for sII. Solve each bracketed 273.15-303.15 K onset; choose the higher temperature as the first structure to become stable on cooling.
Absolute gas pressure is varied within the supported input range. 273.15-303.15 K, 3-10 MPa(a), water activity 0.8-1; fixed CH4-rich gas only. Condensation, failed gas stability and unbracketed onset fail explicitly.
3. Substitute the values
p = 5 MPa(a); with the fixed controls above, the workflow evaluates Hydrate onset temperature = 287.264214 K; Temperature minus hydrate onset = 2.73578582 K difference
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Hydrate onset temperature (K) = 287.264214; Temperature minus hydrate onset (K difference) = 2.73578582
Water activity is supplied, not derived from salinity, glycols, methanol, dissolved gas or reactive aqueous chemistry. The PR gas and empirical hydrate parameter combination needs measured validation for a particular feed. No ice competition, sH, hydrate quantity, kinetics, multiple simultaneous hydrate phases, condensate or joint VLLS equilibrium. This is a bounded onset screen, not the full NeqSim process simulator.
Result and interpretation
Hydrate onset temperature (K) = 287.264214; Temperature minus hydrate onset (K difference) = 2.73578582
Water activity is supplied, not derived from salinity, glycols, methanol, dissolved gas or reactive aqueous chemistry. The PR gas and empirical hydrate parameter combination needs measured validation for a particular feed. No ice competition, sH, hydrate quantity, kinetics, multiple simultaneous hydrate phases, condensate or joint VLLS equilibrium. This is a bounded onset screen, not the full NeqSim process simulator.
Pressure varies in the recorded hydrate model. No wax or deposition result is inferred from that sweep.
Exact scenario data
Original recorded synthetic cases solved with the site's mixed-gas-hydrate-onset engine. The full inputs, outputs and diagnostic rows are in the downloadable JSON. These are discrete solved states; the lesson does not interpolate a new solve.
| Absolute gas pressure (MPa(a)) | Hydrate onset temperature (K) | Temperature minus hydrate onset (K difference) |
|---|---|---|
| 3 | 283.3515 | 6.648468 |
| 4 | 285.605 | 4.395039 |
| 5 | 287.2642 | 2.735786 |
| 7 | 289.5627 | 0.4372863 |
| 10 | 291.6684 | -1.668437 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. Does being below a calculated wax appearance temperature determine deposition rate?
Show the worked answer
No. Equilibrium appearance and transport or deposition kinetics are different calculations.
2. Reproduce the baseline Hydrate onset temperature (K) without rounding intermediate values.
Show the worked answer
p = 5 MPa(a); with the fixed controls above, the workflow evaluates Hydrate onset temperature = 287.264214 K; Temperature minus hydrate onset = 2.73578582 K difference. Hydrate onset temperature (K) = 287.264214. Water activity is supplied, not derived from salinity, glycols, methanol, dissolved gas or reactive aqueous chemistry. The PR gas and empirical hydrate parameter combination needs measured validation for a particular feed. No ice competition, sH, hydrate quantity, kinetics, multiple simultaneous hydrate phases, condensate or joint VLLS equilibrium. This is a bounded onset screen, not the full NeqSim process simulator.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Absolute gas pressure (MPa(a)) changes from 3 to 10. Other controls remain as stated in the premise. Hydrate onset temperature (K) changes from 283.351532 to 291.668437.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. Absolute gas pressure is varied within the supported input range. 273.15-303.15 K, 3-10 MPa(a), water activity 0.8-1; fixed CH4-rich gas only. Condensation, failed gas stability and unbracketed onset fail explicitly. Water activity is supplied, not derived from salinity, glycols, methanol, dissolved gas or reactive aqueous chemistry. The PR gas and empirical hydrate parameter combination needs measured validation for a particular feed. No ice competition, sH, hydrate quantity, kinetics, multiple simultaneous hydrate phases, condensate or joint VLLS equilibrium. This is a bounded onset screen, not the full NeqSim process simulator.
Before using the result
- Use the correct solid-phase problem.
- Match pressure and composition.
- Keep equilibrium onset separate from plug or deposit growth.
Water activity is supplied, not derived from salinity, glycols, methanol, dissolved gas or reactive aqueous chemistry. The PR gas and empirical hydrate parameter combination needs measured validation for a particular feed. No ice competition, sH, hydrate quantity, kinetics, multiple simultaneous hydrate phases, condensate or joint VLLS equilibrium. This is a bounded onset screen, not the full NeqSim process simulator.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.