In cold weather, a wet, high-pressure gas line can plug itself with something that looks like ice but is not: a gas hydrate, a solid that forms when water and gas molecules combine under the right conditions and grows until it blocks the pipe. Methanol injection is the common defense - a controlled stream of methanol fed into the line shifts the conditions so hydrates cannot form. This guide explains why hydrates threaten wet, cold gathering lines, how methanol suppresses them, how injection rate is controlled against pressure and temperature, and how SCADA flags a forming plug or a failing injection pump before the line blocks.
Methanol Injection (Hydrate Control) in one line: Methanol injection is the continuous or intermittent addition of methanol into a wet, cold, high-pressure gas line to prevent gas hydrates from forming. Methanol is a thermodynamic inhibitor - it shifts the temperature and pressure conditions at which hydrates can exist, so at a given pressure the line must get colder before hydrates form, keeping it out of the hydrate region. The injection rate is set against the line's pressure and temperature and its water content, and SCADA monitors the injection pumps and line pressures to catch a failing pump or a forming plug.
Gas hydrates are ice-like crystalline solids that form when water and small gas molecules such as methane come together under high pressure and low temperature - the water molecules cage the gas molecules into a solid lattice. Crucially, hydrates can form at temperatures well above the freezing point of water, so a line does not have to be near freezing to be at risk; it only needs free water present, enough pressure, and a low enough temperature to enter the hydrate-forming region. Wet gas gathering lines meet these conditions readily, especially in cold weather or where the gas cools as it expands through a restriction.
The danger is that hydrates do not just form, they grow and agglomerate, building up on the pipe wall and coalescing until they restrict and eventually plug the line entirely. A hydrate plug stops flow, can trap dangerous differential pressure across it, and is slow and hazardous to remove - depressuring a line to melt a plug has to be done carefully because a plug can move like a projectile when the pressure behind it is released. Preventing hydrates from forming in the first place is far preferable to clearing them once formed.
Gathering lines are particularly exposed because they carry gas that is often still wet - not yet dehydrated - and they run across the surface or shallowly buried where they cool to ambient temperature. Points where gas expands and cools, such as chokes, control valves, and pressure lets-down, are especially prone because the temperature drop there can push the gas straight into the hydrate region. These are exactly the spots where inhibition is targeted.
Methanol works as a thermodynamic inhibitor: dissolved into the free water in the line, it lowers the temperature at which hydrates can form at a given pressure, shifting the whole hydrate boundary so that the line's actual operating temperature stays on the safe side of it. In effect it moves the goalposts - with enough methanol in the water, the gas would have to get substantially colder before hydrates could form, and the line simply never reaches those conditions. Glycol can be used the same way as an alternative inhibitor, but methanol is common in gathering service because it is inexpensive, effective, and easy to inject where needed.
The injection point is chosen to get methanol into the stream ahead of where hydrates would form - upstream of a choke or a cold section - so the methanol is present in the water phase before the gas cools into the risk region. A metering pump delivers a controlled rate, and that rate depends on how much free water is present and how far the operating conditions sit inside the hydrate region, which is a function of pressure and temperature. The colder and higher-pressure the line, and the more water it carries, the more methanol is needed to keep the water sufficiently inhibited.
Getting the rate right is a balance. Too little methanol leaves part of the water uninhibited and lets hydrates form, defeating the purpose; too much wastes an expensive chemical and can create issues downstream. Operators set the rate against the line's pressure and temperature and its water load, and adjust it as conditions change - raising injection as a cold snap drives temperatures down or as water production climbs, and easing it back when the risk subsides. Because the risk moves with the weather and the well conditions, the injection rate is not a set-and-forget value but one that tracks the line's state.
Two failure modes make methanol injection a natural thing to monitor remotely: the injection can fail, or a hydrate can start forming despite it, and both need to be caught early. On the injection side, the key signals are the pump's operation and delivery - whether the metering pump is running, its rate or stroke, and the level in the methanol supply tank. A pump that has stopped, lost prime, or is drawing from an empty tank means the line is running uninhibited even though nothing else looks wrong, which is exactly the kind of silent failure that leads to a plug hours later.
On the line side, pressure is the early indicator of a forming hydrate. As a hydrate builds and restricts the bore, differential pressure across the affected section rises for the same flow - the line has to push harder to move the gas past the growing obstruction - so a climbing pressure drop, or an upstream pressure creeping up while downstream falls, is a warning that a restriction is developing. A cloud SCADA such as Merobix brings the injection-pump status and tank level together with the line pressures and temperatures onto one screen, so an operator sees both the cause and the symptom in one place.
That combined view is what lets an operator act before a plug forms rather than after flow stops. A rule can flag an injection pump that has faulted or a supply tank running low, prompting a refill or repair while the line is still protected, and a rising differential pressure at a known cold spot can prompt increased injection or a temperature check before a partial restriction becomes a full block. For gathering systems spread across cold country with many injection points, this remote oversight is what keeps hydrate control reliable without a crew visiting every skid, turning both pump faults and forming plugs into alarms an operator can respond to in time.
No - gas hydrates can form at temperatures well above the freezing point of water. They form wherever there is free water, sufficient pressure, and a low enough temperature to enter the hydrate-forming region, and under high pressure that region extends above freezing. This is why a wet, high-pressure gathering line can plug with hydrates even when it is not near freezing, particularly where gas cools as it expands through a choke or valve.
Methanol acts as a thermodynamic inhibitor: dissolved into the free water in the line, it lowers the temperature at which hydrates can form at a given pressure, shifting the hydrate boundary so the line's actual temperature stays on the safe side of it. In effect the gas would have to get substantially colder before hydrates could form, and the line never reaches those conditions. It is injected upstream of the cold or restriction point so the methanol is present in the water before the gas cools into the risk region.
SCADA watches both the cause and the symptom. On the injection side it monitors the metering pump's operation and rate and the methanol tank level, so a stopped pump or an empty tank - a silent failure that leaves the line uninhibited - raises an alarm. On the line side, a rising differential pressure or an upstream pressure creeping up while downstream falls signals a restriction forming as a hydrate builds. Seeing both together lets an operator refill or repair the injection, or increase the rate, before a partial restriction becomes a full plug.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.