Though the fracturing itself uses tremendous pressure, it's also allowed to come back up the hole and "vent", for lack of a better term. Likewise, what starts out around 3000psi eventually dwindles down to almost nothing, effectively depressurizing the subsurface.
Disposal wells inject under pressure but it's not allowed to come back up. Some drillers I've spoken with say that it's dissolving salt domes and other fabric on a large and widespread basis.
Over 200 earthquakes have recently occurred in Arkansas but when four disposal wells were ordered to be shut down, the quakes subsided. One may draw his own conclusions as to why.
Recycling flowback could effectively end the need for disposal wells. Fountain Quail just set up shop in Kenedy and also in Arkansas in response to the disposal wells being shut down there.
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Will someone explain to me how the disposal wells can cause an earthquake. Could it be the extra pressure put on the formation. But they say that fractruing the formation, which uses 10,000+p.s.i., is not the cause.