2.4 The volume of water expelled from the mud, during the creation of the mudcake, will displace the formation fluid, creating the so-called invasion zone. The thickness of this zone, in which the formation fluid has been displaced by mud filtrate, will depend on the formation porosity. Show that the radius of invasion ri is given by: dv = +πrbh Π Ton ²). (2.1) where dV is the volume of mud filtrate/unit length displaced into the formation, is the porosity, and rbh is the borehole radius.