Water contained in the pores of a reservoir rock is called what?

• A. Capillary water
• B. Groundwater
• C. Interstitial water
• D. Formation water

Answer: (C)

Water occupying the spaces between rock grains is interstitial water. The term “interstitial” means between solid particles, so this water fills the pore network that makes up the rock’s skeleton. It describes the pore-scale fluid inside the reservoir rock, differentiating it from water held in tiny capillaries (capillary water) and from groundwater in broader subsurface systems. Formation water is another way people refer to pore water in the formation, but the word interstitial specifically highlights its location between the grains.