which contains petroleum gas and mixtures thereof a string of casing 14 extends from the earth 39 s surface 11 to the bottom of the wellbore 10 the space between the casing 14 and the wall of the wellbore is filled with cement 15 the cement 15 extends as illustrated from the bottom of the wellbore 10 to earth 39 s surface 11 the casing 14 is capped by a suitable wellhead 16 to which is coupled a suitable flowline the casing and the surrounding cement sheath are provided with a plurality of perforations 17 penetrating the formation 13 the well may be provided with a suitable packer 18 which isolates the production from formation 13 from the remainder of the well using a tubing string 20 which extends from the wellhead 16 through packer 18 the tubing string is provided with a suitable flowline not shown for the introduction and withdrawal of fluids to and from the well if the well does not have the desired productivity it is common practice to treat the well to improve the well 39 s production characteristics this may be accomplished by acidizing hydraulic fracturing or other methods which comprise forcing a treating material down the casing and into the producing formation through the perforations 17 in the casing as mentioned above it is sometimes desirable to selectively close those perforations through which fluid is flowing during the treating operation so that treating fluid is forced into the formation adjacent to other perforations in the casing prior to illustrating any specific embodiments of this invention it is appropriate that the following definitions be established to clarify the relative terminology used to describe ball sealer and fluid density characteristics namely light or low density fluids refer to fluids having density less than.