What is gravity override?

Answer 1

This is a problem encountered with extraction of Petroleum from the ground. Petroleum engineers use steam flooding processes to extract the oil from a reservoir. The Phase relationships of steam, water and hydrocarbons are critical to this process and can result in either Steam override or Gravity override.

A physical characteristic of steam injection processes that must be considered is steam override due to gravitational forces. Gravity override is caused by the fact that steam is much less dense than liquids and therefore will tend to rise in the porous medium. At the same time, hot water produced by the condensation of steam is more dense than some contaminants, so may tend to underride the zone to be treated. When there is a difference in density between the injected fluid and the fluid initially present, the slope of the interface between the fluids is equal to the difference between the vapor and liquid water phase viscous forces divided by the gravitational forces. The degree of gravity override increases as the difference in density between the liquid and vapor phases increases, as the permeability of the medium decreases, and as the viscosity of the liquid phase being displaced increases. One operational parameter that affects override is the steam injection rate. Source: Steam Injection for Soil and Aquifer Remediation by Eva L. Davis