ABSTRACT

ABSTRACTThe process of fracture initiation and propagation,from a pressurized borehole into a fluid-saturatedstratified formation, is examined indepth toward better understanding of the implicationsfrom pressure records (e.g. toward boundingof in-situ stresses under minifrac conditions) andto assist in better designs of pumping sequences,especially for containment in pay zones. The paperaddresses two major groups of problems: firstly,some simple estimates of rupture pressures aremade, allowing for frac fluid penetration, materialvariability and pre-existing flaws (or strengthanisotropy) in borehole walls, which exhibit a widerange of deducible tectonic conditions for any particularrate and pressure required to cause breakdown(unstable crack propagation) from the wellbore.Secondly, dominant mechanisms of crack growthstabilization (due to pore-fluid effects) are rationalized:required driving stresses show a monotonicdependence on growth speed which reveals onepossible source for inhibition of spreading intoadjacent shale strata (as against lateral propagationinto more permeable sandstones). Other potentialbarriers against vertical migration of fractureshave been characterized, particularly as dueto contrast of moduli, toughness and coherence or toblunting at interfaces. Our rationalization herenow lends credence to additional highly-desirablein-situ stress barriers which may be generated simplyby drawing down the reservoir pressure: thisconclusion runs counter to that based on manyother models. However, it is strongly emphasizedthat conventional hydraulic fracturing designs,adopting constant flow rate, tend to overpowerthese possibly favorable formation characteristicsby achieving excessively high pressures inthe region near the wellbore: simple calculationsfor the frac fluid flow (coupled to the openingdisplacement of the fracture) show that sufficientlateral penetration cannot occur to maintain out- ward spreading and resulting driving energy nearthe borehole is adequate to break through upwards,even against high resistance.

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