Couterflow

This Letter addresses a practical problem that is encountered in, for example, a recent oil gushing in Mexico Gulf. To 'top kill' the oil well, one may pump "mud" from above to suppress the welling from bottom. However, in this "top kill", one needs consider an issue as described in the mentioned Letter, the so-called "Helmholtz instability", namely, "Dense fluids, i.e., mineral suspensions called ‘‘mud’’ [1,2], are introduced into oil wells to provide hydrostatic pressure to offset hydrocarbon (oil and gas) fluid pressure in deep formations, stopping upward flow and reducing the fluid pressure at the surface to near ambient. If hydrocarbon is flowing upward in the well, there is a counterflow between the upwelling hydrocarbon and the descending mud. Successful top kill requires that the mud descend despite this counterflow. However, upwelling at speeds > 1m/s, as in the uncontrolled Macondo well ‘‘blowout’’ in the Gulf of Mexico in 2010, may lead to a Kelvin- Helmholtz instability [3]."[PRL 106, 058301 (2011)] How to avoid the instability ? Suggested to use viscoelastic materials.