Prediction of methanol content in natural gas with the GC-PR-CPA model

Produced reservoir fluids are principally composed of hydrocarbons but contain also impurities such as carbon dioxide, hydrogen sulphide and nitrogen. These fluids are saturated with the formation water at reservoir conditions. During production, transportation and processing ice and/or gas hydrates formation may occur. Gas hydrate and ice formation are a serious flow assurance and inherently security issues in natural gas production, processing and transport. Therefore, inhibitors are usually injected as a hydrate inhibitor and antifreeze. For example, methanol is often used for hydrate inhibition or in some cases during start up, shut down or pipeline plug removal. Therefore impurities, water and methanol usually end up in natural gas conditioning and fractionation units. These units produce end user pipeline gas subject to local specifications and natural gas liquids like ethane, LPG or heaviers. This is why the accurate knowledge of methanol content at different operating conditions is important. In this study, a group contribution model, the GC-PR-CPA EoS [1] (Group Contribution – Peng-Robinson – Cubic-Plus-Association), is successfully applied for hydrocarbons systems containing methanol. Predictions of phase envelopes of binary systems as well as partition coefficients of methanol in hydrocarbons mixtures are in good agreement with experimental data.