With a natural viscosity ranging from molasses to tar, the world’s vast resources of heavy crude (SN: 7/21/79, p. 42) require heating–generally with steam–before they can be coaxes dout of underground reservoirs. Now engineers at Sandia National Laboratories in Albuquerque have identified an energy-robbing phenomenon that’s costing heavy oil producers dearly: It’s called wellbore refluxing.
the outer casing of a steam-injection well is a large pipe into which the narrower steam-conveying pipe is inserted. The space inside the casing and outside the steam-injection pipe is the wellbore. Though the long segments of inner steam pipe are heavily insulated to limit energy loss, small metal couplings between segments are not. And tests last year at Sandia showed that heat lost at couplings is unexpectedly high owing to the presence of some mositure in most wellbores.
When the injected steam’s heat gets conveyed through a coupling’s wall, explains Billy Marshall, geo-energy manager at Sandia, moisture in th e wellbore flashes to steam and begins rising up the bore. As it hits the cooler surfaces of the casing or insulated pipe some distance up, the steam cools, condensing to water, and begins trickling back toward the hot coupling again (see diagram). The phase change from water to steam at these couplings allows the wellbore to steal six times more energy than would have been intuitively expected for a dry wellbore, Sandia research shows. But there’s a simple solution: line couplings with doughnutshaped plastic insulators.
Calcultions by Sandia’s Dan Aeschliman indicate that if only the injection wells already in use were insulated at these couplings, U.S. heavy oil producers could save more than $1 billion over the next 10 years (just based on current production rates and no inflation). Moreover, the insulation should make possible steam delivery to even greater depths.
