Methanol Losses to the Hydrocarbon Phases in Offshore Pipelines and its Effect on Economics

Abstract

Abstract Methanol is the most commonly used hydrate inhibitor in offshore pipelines. The amount of methanol that is lost to the hydrocarbon phases and the implications that this has on the flow assurance strategy and the economics of offshore developments are presented. Methanol losses to the hydrocarbon phases are greatest in pipelines with small amounts of water. For cases of high methanol losses, non-volatile or thermal inhibition methods may be more economical. The importance of accurate predictions of methanol losses is discussed along with the need for experimental data. An experimental technique for measuring methanol partitioning between aqueous and hydrocarbon phases is discussed. Comparisons between predictions and data are made for methanol partitioning between aqueous and hydrocarbon phases of live fluids. Introduction Methanol is the most widely used gas hydrate inhibitor in offshore pipelines because of its low cost and high thermodynamic effectiveness, on a weight basis. The actual world-wide offshore flowline usage of methanol is impossible to accurately determine at this time. This is because delivered offshore methanol is also used for other purposes and because offshore regulatory agencies do not compile flowline usage data. Based on inhibition needs for existing developments and sales estimates, it is apparent that the annual world-wide offshore methanol usage is in excess of 200 million pounds. One of the least precise areas concerning hydrate inhibition in offshore pipelines is methanol partitioning between the aqueous phase and the hydrocarbon gas and liquid phases. Since methanol is only effective as an inhibitor in the free water phase, any partitioning into the hydrocarbon phases is considered a loss and must be compensated with additional methanol injection. Sample Cases In order to quantitatively compare methanol usage and the effects on economics, four cases of methanol partitioning are considered. An example gas condensate flowline and an example black oil flowline are considered at both high and low pressures. Gas Condensate Flowline: 200 MMscfd gas, 300 BPD condensate liquid at 40–50 °API with a MW of 100 and 40 BPD water. The moles of hydrocarbons per mole of water is 680. Black Oil Flowline: 10,000 BOPD, 35 °API, MW of 200, GOR of 2000 scf/STB and a water cut of 5% (6.86 moles live oil per mole water). Further properties of the two flowlines are presented in Tables 1 and 2, which were constructed using realistic values for these fluids.