Imbibition
- Also known as
- spontaneous imbibition, forced imbibition
Imbibition is the process by which a fluid that preferentially wets a rock displaces a fluid that does not, such as water displacing oil in water-wet rock. When capillary forces alone draw the wetting fluid in, it is called spontaneous imbibition.
What is imbibition?
In a porous rock containing two fluids, one of them usually wets the rock surfaces more strongly. Imbibition is the increase in saturation of that wetting fluid as it moves into the pores and pushes the non-wetting fluid out. In water-wet rock, water imbibes and displaces oil; in oil-wet rock, the reverse. The opposite process, the non-wetting fluid displacing the wetting one, is called drainage.
Spontaneous and forced imbibition
- Spontaneous imbibition happens without any applied pressure: capillary pressure draws the wetting fluid into the small pores, the way a paper towel soaks up water.
- Forced imbibition continues the process with an applied pressure difference, displacing more of the non-wetting fluid than capillary forces alone would.
Imbibition can be co-current, with both fluids moving in the same direction, or counter-current, with the wetting fluid entering and the displaced fluid leaving through the same face of the rock.
Why imbibition matters
- Naturally fractured reservoirs and shale. Injected or fracturing water flows mainly through the fractures. Oil stored in the rock matrix between them is recovered largely by water imbibing into the matrix and expelling oil into the fractures. The rock’s wettability therefore strongly affects recovery; oil-wet matrix resists imbibition.
- Fracturing fluid retention. Imbibition is one reason much of the water pumped during hydraulic fracturing is not recovered as flowback.
- Wettability testing. The Amott test measures how much fluid a core takes up by spontaneous and forced imbibition to classify wettability.
Imbibition and surfactants
Surfactants that shift oil-wet or mixed-wet rock toward water-wet can promote spontaneous imbibition, so that water enters the matrix and releases oil. This is a key mechanism in surfactant treatments for tight and fractured reservoirs. Surfactants also lower interfacial tension, which reduces the capillary force that drives imbibition, so formulations balance the two effects.
Frequently asked questions
In imbibition, the wetting fluid displaces the non-wetting fluid. In drainage, the non-wetting fluid displaces the wetting fluid, as when oil first migrates into water-filled rock.
Because fluids move mainly through fractures, and the oil in the tight rock between them is released largely when water imbibes into the rock. How readily that happens depends on wettability.
Sources
- imbibition, SLB Energy Glossary
- Spontaneous imbibition, SPE PetroWiki