Estimation of Methanol-Pill Batch Size for Hydrate Prevention in Offshore Oil & Gas Subsea Operations

Estimation of Methanol-Pill Batch Size for Hydrate Prevention in Offshore Oil & Gas Subsea Operations

by R.Kini (Chevron), S.Cochran (Chevron) & J.J.Manzano-Ruiz (Bechtel)

ABSTRACT

The oil production from offshore reservoirs located in deep-water faces substantial risk of subsea-system plugging by hydrate formation during operations such as dead-oil displacement (DOD) from a dead-leg flowline into an uninhibited producing flowline.  A methanol pill is also used for dewatering gas carrying flowlines, between incompatible fluids during umbilical flushing and, during the flowline restart after an unplanned shutdown. Usually the operating company forgoes continuous injection of hydrate inhibitors (e.g. methanol, mono-ethylene glycol, LDHI) to avoid hydrocarbon product contamination and minimize HES risks and operating expenses. Without an inhibitor, a live-oil or a gas condensate stream could form hydrates during cooldown if it is exposed to an environment of high pressure and low temperature in the presence of produced water.

One common practice among deep-water oil producers is to run uninhibited production operations and, in case of an unplanned shutdown, the dead-oil displacement procedure is executed to displace live-oil with dead-oil within the subsea system. The dehydrated and gas-free dead-oil can withstand a long shutdown at any subsea pressure/temperature without the risk of hydrate formation and blockage. Once the shutdown contingency is over, the production of live-oil can be safely resumed into the subsea system containing dead oil. The DOD operation has to be executed soon after shutdown, typically within 8-12 hours after the flow stops, even in case of thermally insulated flowlines. During the DOD operation there is a risk of hydrate formation at the interface between the dead-oil and the cold and uninhibited live-oil due to the increase in pressure caused by pumping. To prevent a potential hydrate issue at this interface a batch of methanol (a "pill") is pumped ahead of the dead-oil to separate the two streams. The batch or pill size of methanol is a concern for the operator not only for cost but because it depletes the limited inventory onboard the topsides, and contaminates the separated gas and oil downstream to be out-of-spec with the exported hydrocarbons.

This paper focuses on establishing a criterion to size the methanol pill to avoid its oversizing while guaranteeing its effectiveness. The optimum size depends on the mechanism of methanol-phase dispersion by turbulent diffusion through the phases on both sides of the pill, and by methanol attrition through a thin-film shed on the pipe wall. This paper estimates the methanol pill size based on the turbulent mass-transfer and hydraulics. The basis of the mass-transfer analysis is the momentum- and mass-transfer Reynolds-analogy to derive a virtual turbulent-coefficient of diffusion “K”, following G.I.Taylor’s 1954 seminal paper.   

Examples of typical DOD operation with a methanol-pill between dead-oil and live-oil were analyzed. The mixing at the methanol/live-oil interface was modelled based on mass transfer and hydraulics for flowlines with diameter between 6 and 10 inches. The resulting error-function concentration distribution yielded mixing layers ranging between 250 and 600 feet long, depending on the prevailing Reynolds number. Similar length-scale results were also obtained for the mixing at the methanol/dead-oil interface. The mixed volume of methanol with dead- and live oil varies between 25 and 180 bbls in this analysis. The methanol film-thickness was estimated as a function of Reynolds number and shear stress. Typical results obtained for the film thickness were in the range of 1 – 6 mm depending on Reynolds number, resulting in an attrition volume of the order of 20 – 200 bbls.

The methodology presented in this paper offers a novel approach to estimate methanol pill sizes based on the fundamentals and operational parameters.