How do well operators prevent filling up all the porous area in the injection zone so the liquid doesn't back up the pipe and have nowhere to go?

The pore space (void space inside the rock, picture a hard sponge) in the formations at several thousand feet of depth in the vicinity of disposal wells are already 100% saturated with natural brines.  In many injection zones, these brines have specific gravities of between 1.05 and 1.2 gm/ml.  This salinity can be several times the salinity of seawater.  When operated, Class I wells have fluid inside the tubing from the surface all the way down to the injection zone.  By adding a carefully controlled amount of pressure to the fluid by pumping during disposal operations, wells can be used to add the injectate to the brine already present in the injection formation. This is accomplished by pushing natural fluids out of the rock pore spaces near the well and into neighboring pore spaces by compressing these moving fluids and the fluid already contained inside those rock spaces to a higher pressure. This effect continues radially away from the well.