When drilling through depleted and/or fractured reservoirs, total losses are a common occurrence. In the worst case, if the well penetrates the top of the formation (fracture) above the gas-oil contact there is a high risk of a blowout if insufficient fluid is not pumped down the annulus at all times between encountering total losses and cementing/landing the production casing.
If gas bubbles enter the well bore above the loss zone when drilling, tripping, logging or running casing and insufficient fluid is not pumped down the annulus or open hole they will rise through the liquid, joining together as they do so. Due to joining together and expansion resulting from the decrease of hydrostatic head as the bubbles rise through the liquid, a point will be reached when a large bubble is formed which almost fills the available flow cross section. This slug-flow bubble will then push the liquid ahead of it up the hole and rise with ever increasing velocity i.e. the well begins to kick.
If there is a downward flow of water such that the velocity is higher than the bubble rise velocity, the bubbles entering the well bore will be carried downwards to the loss zone. The smaller the bubble below a certain size then the smaller is the rise velocity. However if the gas is entering the well bore from a fracture it will be entering as a continuous jet and a slug-flow bubble could be formed at the point of entry into the wellbore.
To be safe at all times therefore the flowrate down the annulus should be sufficient to prevent the rise of a slug flow bubble. Small bubbles which are carried down the hole to a major loss zone can accumulate at the top of the loss zone or fractures into which losses are occurring and as the accumulation grows in volume suddenly re-enter the wellbore in the form of a jet, this forming a slug-flow bubble at this level in the wellbore.
Note: It is not only gas that is a problem with losses. Fill up speed and type of fluid is another factor while drilling through reservoirs or losses with high or virgin pressures while the objective reservoir below is depleted. Not filling up and having losses in the depleted reservoir will allow the overlying reservoir to kick.