A Practical Model for the Effect of Salinity on Gas Hydrate Formation

Abstract

This paper is based on a previously developed thermodynamic model for gas hydrates and hydrate inhibition. The model uses a cubic equation of state for the fluid phases and parameters have already been determined for the following inhibitors methanol, MEG, DEG and TEG. The paper describes the extension of the model to include the effect of salinity in produced water or sea water on hydrate formation. To ensure the model would be of practical value, it was designed with the following characteristics: it should be simple to use requiring as input no more than a typical ion analysis table from a laboratory report. It should be based on a cubic equation of state suitable for engineering calculations and the model should operate reliably at temperatures and pressures normally encountered in oil and gas production. The model represents the ionic components in water by a single salt pseudocomponent of the equation of state. The physical properties of the pseudocomponent were set by regressing them to experimental data for sodium chloride solutions. Results will be presented to show that the model can simultaneously represent the lowering of the hydrate dissociation temperatures, the depression of the freezing point of water and the reduction in the water vapour pressure (osmotic coefficient). As sodium chloride is usually the dominant component in produced water or sea water, other salts are handled on a sodium chloride equivalent basis, so that only one salt pseudocomponent is needed for practical calculations. Data for the effect of natural water supports the use of this approximation. In practice, a hydrate inhibitor may be added to the water phase so it is important that the inhibition model can give accurate predictions in the presence of saline solutions. This has been confirmed by investigating the salting-out effect for methanol and results are shown for salt-water-methanol mixtures. Practical ways of accessing and applying the model are summarized. These include using the model as a standalone computer program, accessing the model via a spreadsheet or using the model as an object code library or as a dynamic link library. Introduction Gas hydrates are a well appreciated hazard in oil and gas pipelines and processing equipment. They may also occur in certain conditions during drilling operations giving rise to gas kick. If operating conditions are such that hydrates may form, hydrate inhibitors such as methanol or glycols have to be injected into the hydrocarbon fluid. If produced water or sea water is in contact with the hydrocarbon fluid, the salinity of the water will itself inhibit hydrate formation. The object of this work is to devise a practical engineering method to predict the effect of salinity on hydrate formation in combination with added chemical inhibitors. Existing Hydrate Model. The authors have previously developed an original computer algorithm for solving multiphase equilibrium problems involving any number and combination of solid, liquid or gas phases1. The phases may have quite different properties calculated from different thermodynamic models. The altorighm can perform all the normal engineering flash calculations; it automatically determines which phases will be present under given conditions and returns the relative amounts, compositions and properties of the phases. To model the fluid phases, a commonly used equation of state was required. The choice was between the SRK2 or Peng-Robinson3 equations. The SRK equation was selected as it appears to give more accurate fugacities for natural gases4. The tendency of the SRK equation to give poor liquid densities was addressed by correcting the SRK densities using the Peneloux volume shift method5. Existing Hydrate Model. The authors have previously developed an original computer algorithm for solving multiphase equilibrium problems involving any number and combination of solid, liquid or gas phases1. The phases may have quite different properties calculated from different thermodynamic models. The altorighm can perform all the normal engineering flash calculations; it automatically determines which phases will be present under given conditions and returns the relative amounts, compositions and properties of the phases. To model the fluid phases, a commonly used equation of state was required. The choice was between the SRK2 or Peng-Robinson3 equations. The SRK equation was selected as it appears to give more accurate fugacities for natural gases4. The tendency of the SRK equation to give poor liquid densities was addressed by correcting the SRK densities using the Peneloux volume shift method5.

Thermodynamic Modeling of Natural Gas Systems Containing Water

“…CPA is used for the vapor, liquid, and ice phases while CPA together with the vdW-P theory is used for the hydrate phases. The hydrate structure H has not been considered as it is only relevant to iso-alkanes, cyclo-alkanes, and alkenes and is less stable than structure II. , Modeling asphaltene phase behavior has been omitted as beyond the scope of this work.…”

The Successful Use of Both Kinetic Hydrate and Paraffin Inhibitors Together in a Deepwater Pipeline with a High Water Cut in the Gulf of Mexico

“…25 It also allows the use of methanol along with the "salt component." This can be important since methanol can be "salted-out" by sodium chloride.…”

Enhancement of the performance of kinetic inhibitors in the presence of polyethylene oxide and polypropylene oxide for binary mixtures during gas hydrate formation in a flow mini‐loop apparatus

“…However, under certain conditions, the PEO–water system exhibits a closed solubility gap. PEO is weak thermodynamic inhibitors (Barker and Gomez, 1989; Edmonds et al, 1996). Furthermore, it has been considered to be unable to exhibit kinetic inhibitors by itself (Dholabhai et al, 1992).…”