Imbibition is the process of absorbing a wetting phase into a porous rock. Imbibition is important in a waterdrive reservoir because it can advance or hinder water movement, affecting areal sweep. Spontaneous imbibition refers to the process of absorption with no pressure driving the phase into the rock. It is possible for the same rock to imbibe both water and oil, with water imbibing at low in situ water saturation, displacing excess oil from the surface of the rock grains, and oil imbibing at low in-situ oil saturation, displacing excess water. An imbibition test is a comparison of the imbibition potential of water and oil into a rock. The wettability of the rock is determined by which phase imbibes more.