underneath upper slip ring 30 as shown in fig1 b examples of suitable slip rings are described in u s pat no 5 540 279 typically upper slip wedge 32 will be designed as a partial cone so as to provide a ramp or wedge for splitting and radially expanding upper slip ring 30 when frac plug 10 is moved into its set position upper slip wedge 32 abuts expandable sealing element 34 located below upper slip wedge 32 located below upper slip wedge 32 is expandable sealing element 34 the frac plug 10 includes at least one such expandable sealing element but can include two three or more such elements expandable sealing element 34 has unset and set positions corresponding to the unset and set positions of frac plug 10 respectively expandable sealing element 34 is radially expandable from the unset position to the set position in response to the application of axial force on expandable sealing element 34 in the set position the expandable sealing element 34 engages an inner wall of a casing in the wellbore to create a seal to prevent flow through annulus between frac plug 10 and the casing limiter rings can be positioned at the upper and lower ends of expandable sealing element 34 so as to limit longitudinal or axial expansion of expandable sealing element 34 when frac plug 10 is moved into its set position upper slip wedge 32 is disposed at the upper end of expandable sealing element 34 there is a lower slip wedge 36 disposed at the lower end of expandable sealing element 34 lower slip wedge 36 is similar to upper slip wedge 32 but oriented opposite to upper slip wedge 32 as shown the upper end of expandable sealing element 34 resides directly against the abutting.