The movement of water through formations having capillary openings is generally laminar and obeys Darcy's law, at least down to very low gradients. About 1,000 samples tested in the Geological Survey laboratory have coefficients of permeability ranging from .001 to 90,000, indicating probable velocities ranging from a fraction of a foot to a few miles in a year. The Thiem field method gives promising results for determining permeability. Movement through subcapillary openings is important but poorly understood. Molecular attraction of the water offers great resistance to movement and compression.

Water in rocks tends to move from a position of high pressure to a position of low pressure, both along and across the strata. Pressure changes are transmitted with much lag under water-table conditions and less lag under artesian conditions, according to the transfer of water involved. With respect to development of cones of depression, oil wells somewhat resemble water-table wells. Essentially static conditions of water or oil may be found in underground reservoirs that are completely sealed or sealed at one end or that contain fluids that differ in specific gravity, as water and oil, or fresh and salt water.