A single, impenetrable barrier to fluid flow in a reservoir that causes a change of a factor of two in the slope of buildup or drawdown curves. These are often observed in a normal test if the barrier is close (a few hundred feet or less) to the tested well. Most tests are not long enough to detect the presence of distant barriers. Two perpendicular barriers cause a change in the slope of a factor of four, and so forth, and models exist for a variety of geometries, typically for up to four barriers.