Precipitation and deposition of solids in pipelines are determined through a fine interplay between phase equilibria and flow conditions. Calsep offers simulation studies considering precipitation hydrates, wax, asphaltenes, and/or scale.
The projects are carried out using Calsep’ s PVT simulation package - PVTsim Nova. It relies on almost 30 years’ of experience in modeling of solids precipitation.
During pipeline transport of multiphase mixtures of reservoir fluids and formation water, conditions may promote formation of gas hydrates. Hydrate formation may be prevented by insulating or even heating the pipeline or by addition of hydrate inhibitors.
Calsep undertakes evaluations of the hydrate potential for whole pipeline systems, taking into account the
- Pressure drop
- Heat exchange with the surroundings
- Hydrate inhibition by salts and liquid inhibitors
- Loss of inhibitor to hydrocarbon phases
The worst-case-scenario hydrate potential can be evaluated under normal operation, shutdown, and start-up.
The applied hydrate model was developed and published by Calsep in the late 1980s. Calsep has further published a number of papers about the loss of hydrate inhibitor to hydrocarbon phases. All publications appear in peer-reviewed journals - a hallmark of all Calsep models.
The hydrate model accounts for hydrate structures I, II, and H as well as hydrocarbon and aqueous phases, solid salt phases, and ice. Calsep has verified the hydrate model using extensive published experimental data, including data with up to 60 weight percent liquid inhibitor and saturated salt solutions. Validation reports are available for clients.
Most reservoir fluids contain heavy paraffinic compounds that may precipitate as a solid or solid-like waxy material if the fluid is cooled down. Wax precipitation may cause operational problems at various points of hydrocarbon flow. Calsep has, in collaboration with clients and renowned experts, developed simulation models for:
- Wax precipitation from reservoir fluids
- Influence on viscosity of precipitated wax
- Effect of wax inhibitors
- Wax deposition in pipelines
Wax Deposition Simulations
Calsep undertakes wax deposition simulations for wells and pipelines. The simulations are carried out using the fully compositional wax deposition flow simulator, DepoWax, which is an integral part of PVTsim Nova. DepoWax considers wax deposition at the inner side of a pipeline. The deposition is assumed to take place by molecular diffusion. The viscosity effect of solid wax particles in the bulk oil phase is accounted for through a non-Newtonian viscosity model. It is further possible to consider the viscosity effect of a wax inhibitor.
With continued transport, the wax layer builds up and may eventually plug the pipeline if not mechanically removed. The simulation results include the development with time in thickness of the wax layer and how it influences the pressure drop. This may be used to decide on pigging frequency.
It is possible to consider pipelines with multiple inlets and different section of the pipeline may have different insulation (walls) and different ambient conditions.