Chances of Wellhead freeze-off greater in fracturing wells
Two features of shale gas wells make the risk of freeze-off greater. The first is increased production from more profitable wet gas plays that contain higher quantities (GPM or gallons per MMcf) of natural gas liquids. The higher the liquids content of natural gas, the higher the risk of hydrates forming and causing freeze-offs. The second freeze-off risk factor is because hydraulic fracturing involves pumping thousands of gallons of water into the well - a proportion of which remains underground and rises to the surface with production (see Tales of the Tight Sand Laterals). The presence of that water increases the likelihood of freeze-offs in cold weather.
What does all this data tell us about freeze-off risks? Unexpected extended freeze events can and do occur – maybe not every year but they happen, just like hurricanes. Increased onshore natural gas production, declining offshore Gulf production (less susceptible to freeze offs) and increased dependence on natural gas for power generation mean that the risks of disruptions due to a freeze-off are increasing not decreasing. Those risks may also be increasing with the use of hydraulic fracturing and higher production of gas liquids. Gas producers and utilities should probably consider this development in their planning and risk mitigation processes.
So what causes wellhead freeze-off? The production stream from a gas well contains raw gas mixed with various amounts of water and oil condensates that have to be promptly separated before the gas can be placed in a gathering‐system pipeline and sent to a processing plant. The diagram above shows where common freeze‐off effects including blockage of gas flow can occur (the points below refer to the yellow numbers in the diagram):
Water in the pipe‐and‐valve “Christmas” tree on top of the wellhead freezes
Water freezes in the inlet scrubber or separator that splits out the gas, water and condensate streams
Natural gas liquids (NGLs) or hydrates  in the gas condense before the gas exits to the gathering system.
Processes to keep the wellhead operational include the removal of water and condensates that accumulate in limited onsite storage breakdown. Automated shut off systems stop production if these tanks fill to capacity. (During the February 2011 event this factor stopped production at many wells that could not be reached by maintenance crews because of icy roads).
Failure to replenish consumable chemicals such as hydrate and corrosion inhibitors that prevent condensation in gathering pipelines causes freezing because these are not added to the gas stream into the gathering system.