Being a highly technical engineer-type, I’ve always wondered what the actual pressures are in fracking for gas?
We had a dry well fracked successfully, but that only took a small 3,000 lbs/psi.
Don’t know, but a column of merely static liquid over a mile high would express one helluva pressure at the bottom.
In the absence of tectonic forces, Pressure to maintain an open vertical fracture the rock, (PR/(1-PR))X(Sig V - Pres) + Pres
where PR = poissons ratio of material = lateral strain/longitudinal strain under longitudinal applied stress. about 0.15 to 0.25 for hard sedimentary sandstone reservoir rock, higher for limestone and shale.
Sig V = vertical stress, aka overburden. basically pressure exerted by overlying rock. 1.1 to 1.15 psi/ft
Pres = Pressure in the reservoir pore space, normal is 0.43 to 0.45 psi/ft, water gradient
it gets more complicated for laminated rocks, grains laid out more like plywood than bound spheres in a sandstone or silt matrix. one must apply variability of horizontal properties (parallel to bedding) and vertical properties (perpendicular to bedding). you can get close on the low side with general isotropic equation above.
induced fractures are almost always vertical, as the horizontal stresses in the earth are almost always less than overburden. Vertical growth stops when layers with stress higher than is within the fracture is encountered. you can build pressure in the fractures exceeding frac pressure which induces greater width. generally extending fracture length is the easier path than creating large widths. widths are on the order of 0.05 to 0.25 inch.
breakdown pressure, necessary to get a fracture started is higher than fracture pressure. usually by a factor of 1.1 to 1.4
so without spending a million or ten acquiring data, usually around 0.55 to 0.80 psi/ft (vertical feet depth)plus all the friction getting the fluid down there. 5000 psi in shallow basins is common (8000’ or less) 10,000 in deeper basins and 12,000+ psi in deep basis with high reservoir pressure. Fracs have been done over 20,000 psi at surface, but rare. the 3000 psi job mentioned was probably on what’s nicknamed a post hole, under 3000’ deep.