Gas Hydrates and Wax
Gas Hydrates
There are now a number of commercial software packages and plug-ins/add-ons available which offer hydrate prediction. However, many of these are restricted in their capabilities and reliability, often because developers have had to rely heavily on published literature studies for development, without access to the laboratory equipment capable of generating the data needed to tune/validate for systems where experimental studies are lacking. As a result, models may cope reasonably well with reasonably simple systems such as natural gas-water with small concentrations of aqueous salt and/or alcohols/glycols. However, when it comes to more challenging conditions, such as at high concentrations of salts and organic inhibitors, high-pressure oil systems above the bubble point, and low water content gases, predictions may not be possible or highly unreliable. In a number of cases, some compounds which can form gas hydrates under certain conditions are treated solely as inhibitors, potentially resulting in considerable deviation between the predicted and actual hydrate free zone.
Hydrate/aqueous and hydrocarbon PVTX thermodynamic modelling at Hydrafact dates back over 25 years. Numerous industry sponsored R&D projects undertaken by the Heriot-Watt Hydrate and Reservoir Fluids research groups during this time have led to the generation of a vast, largely unpublished in-house hydrate and hydrocarbon equilibria database which has been used to tune and validate our predictive software.
The well known commercial version of our predictive model, Hydrafact Heriot-Watt Hydrate (HWHYD) is currently used by a large number of major oil, gas and service companies. Comparative assessments of model predictions with those of other commercial hydrate programs by industrial clients have repeatedly demonstrated the superior versatility and reliability of the HWHYD model for simulating various production scenarios.
The software - details of which can be found on our software page - can carry out a wide range of PVT and hydrate calculations, including:
- Phase equilibrium (VLE, VLLE, LLE, VSE, VSLE, VSLLE, LSE, etc) and phase property calculations for systems containing hydrocarbons, water, salts and organic inhibitors
- Hydrate phase boundaries and hydrate composition
- Water content of natural gases
- Inhibitor (e.g. methanol) loss to hydrocarbon phases
- Salt solubility and salting-out
- Freezing (ice melting) points of aqueous salt and organic inhibitor solutions
The model currently contains over 80 components, including conventional, heavy and water-soluble hydrate formers, non-hydrate formers up to C20, 18 common electrolytes and 8 chemical inhibitors.
Experimental hydrate data and model predictions for simulated heavy oil subsea separation
Experimental hydrate data and model predictions for methanol-NaCl-methane-water systems
In addition to the above, our more advanced in-house models are capable of:
- Simulating of hydrate plug dissociation in pipelines
- Predicting hydrate phase equilibria in porous media, including the effects of pore size and water saturation on the hydrate stability zone
- Simulating hydrate formation in a batch reactor
- Optimising hydrate reactor design and volume for full scale continuous hydrate production
Wax Precipitation
The industry has directed considerable efforts towards generating reliable experimental data and developing thermodynamic models for estimating wax deposition conditions in hydrocarbon flowlines.
For wax, the cloud point temperature - or wax appearance temperature (WAT) - is commonly measured in laboratories, and this traditionally used in developing and/or validating wax models. However, the WAT is not an equilibrium point; wax appearance is a kinetically-controlled nucleation process, and as such can be influenced by many factors, including temperature, temperature gradients (e.g. wall cooling), cooling rates, and availability of nucleation sites (e.g. small particles). This means that quite different WATs may be observed depending on experimental procedures/equipment.
Furthermore, when determining the wax phase boundary at pipeline conditions, the common practice is to measure the wax phase boundary at atmospheric pressure, then apply the results to real pipeline pressure conditions. However, this neglects the effect of pressure and associated fluid thermophysical/compositional changes which can lead to unreliable results.
At Hydrafact, we have developed a number of novel techniques for the measurement of wax precipitation conditions in crude oil. In addition to investigating the parameters which affect WAT, we can determine reliable values for wax disappearance temperature (WDT) - the equilibrium condition where all hydrocarbon solids re-enter solution, i.e. the true wax phase boundary.
Using WDT for validation, we have developed a new thermodynamic model for wax which has proved very reliable for predicting wax phase
boundaries for binary and multi-component hydrocarbon systems. Using an in-house modified UNIQUAC approach, the model can predict:
- Wax precipitation conditions
- Composition of deposited wax
- The effect of pressure
Precipitated wax composition: Experimental data and model predictions
Model predictions compare well both in-house and with independent experimental data, demonstrating the reliability of the thermodynamic approach.
Wax modelling capabilities are currently offered as part of technical studies. In the future, our wax model will be incorporated into our commercial hydrate (HWHYD) package to provide a suite of flow assurance predictive tools for industry engineers.
