Oil well bore hole filter cake breaker fluid test apparatus and method

An apparatus for testing the effectiveness of a breaker fluid in breaking a filter cake on the wall of a borehole penetrating a subterranean formation, said apparatus comprising:

• (a) a filter medium cell having a generally vertically disposed cylindrical wall with an inner surface and an outer surface such that at least a portion of said cylindrical wall is formed of a filter medium;
• (b) a first chamber for said fluid in open communication with the inner surface of said cylindrical wall;
• (c) a second chamber in open communication with the outer surface of said cylindrical wall;
• (d) a generally vertically disposed, removable, rotatable shaft, at least a portion of which is received in said filter medium cell, generally surrounded by said cylindrical wall;
• (e) a rotator for said rotatable shaft for imparting shear to fluid;
• (f) means for forming a repeatable, uniform filter cake on said filter medium under controlled shear and differential pressure;
• (g) a piston and associated means for instantaneously monitoring the amount of fluid that passes through the filter medium as a function of time;
• (h) means for changing fluids in said first chamber;
• and
• (i) means for removing said shaft without significantly disturbing filter cake on said filter medium.