Fluid Retention Coring
Fluid retention coring or as it is more commonly know - sponge coring - is a specialized technique of coring which utilizes an oil wet sponge lined aluminum inner core barrel.
During the trip out of the hole the pressure differential forces some of the oil out of the core sample. The sponge liner then absorbs this bleed off which can be extraced at the lab for further analysis. Gas and water are allowed throught the sponge and escape into the drill mud contained in the annulus of the outer core barrel through vent holes drilled into the aluminum inner barrel.
Whie the sponge coring technique was used in wells where oil water saturation was critical, the technology is limited to lengths of 9 Meters (30 Feet) in a single run and is being replaced by enhanced accuarcy in the algorithms of the labs. Utilizing the fluid retention system further limits the maximum core diameter to 3.5".
Larger diamter conventional or containerized cores can be cut so it is possible to spot a sponge core section and resume coring without having to open the well bore.