Dual Gradient Drilling is a combination of rotary and casing drilling techniques that removes the Effective Circulating Density (ECD) from the open well bore to reduce surges that lead to lost circulation.
As shown to the right and below, the casing or liner is run into the previous casing and then a drill string is run through the new casing or liner. The new casing or liner is secured to the drill string at top and bottom and the combined string is run to bottom.
Drilling occurs while the casing or liner is carried with the drilling assembly, wherein the drill string may rotate with the new casing or liner and returns are taken between the drill string and new casing or liner.
Hydrostatic pressure is maintained on the outside of the new casing or liner with hydrostatic the the fluid friction ECD occurring within the new liner or casing to provide a dual gradient when drilling.
The method would be especially useful in weak formations onshore or offshore. For example, within the top hole section of an offshore the conductor could be carried with the drill string to set is significantly deeper than would otherwise be possible.
Additionally, as shown below, reverse cementing can be used with gravity assisting placement of the cement around the new casing or liner to ensure zonal isolation.
Oilfield Innovations' Dual Gradient Circulating and Reverse Cementing Tool provides normal well control by crossing over the combined drilling and casing or liner string back to drill pipe.
Conductor, casing or liner is first run into open water, the riser or the previous casing. A drill string is then run into the conductor casing or liner suspended in the rotary table and a drill string is run on a false rotary until the circulating tool can be attached.
The entire cross-over string is run to bottom and drilling occurs. If the entire string cannot be run into the available water depth or cased hole, when sufficient hole has been drilled, the string is pulled back to the crossover and additional conductor, casing or liner is run.
Once the intended conductor, casing or liner depth is reached conventional cementing or reverse cementing can take place.
The method is very useful in offshore environments or onshore environments where extremely weak formations cause significant time and cost.
Where hole troubles are expected the conductor, casing or liner can be drilled through weak formations and cemented in place once the trouble area is behind casing to avoid costly hole instability and/or well control issues.
Conventional Casing does not have rotary connections and can fail during conventional casing drilling. Oilfield Innovations Dual Gradient Drilling combines the best of rotary drilling and casing drilling for circumstances where weak formations prevent effective drilling. Various configurations and cementing options are available.
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