Fluid-driven Cyclic Propagation of a Joint in the Ithaca Siltstone, Appalachian Basin, New York

Crack-seal veins, millifractures, and joints with rhythmic c-type plume patterns are common examples of cyclic crack propagation in rocks. Although in some cases cyclic propagation could result from periodic external forcing by far-field stress changes or fluid-pressure pulsation, the regularity and rhythmic nature of several types of fractures suggest that cyclic propagation also arises from dynamic instability of the fracture-fluid-rock system. A 40-90-m-long cross-fold joint that propagated within a single bed of the Devonian Ithaca Siltstone near Watkins Glen, New York has a plumose surface morphology with multiple arrest lines indicating that cracking occurred in increments rather than in one smooth rupture. The crack increments increase in overall length in the propagation direction over the final 28-m portion of the exposed end of the study joint with the largest increments increasing in length from 0.6 m to 1.0 m. At least three conceptual models based on linear elastic fracture mechanics and fluid flow along joints can be imagined to explain incremental crack growth under conditions of constant stress and pore pressure: the compressibility-limited propagation model; the flow-limited propagation model; and the infiltration-limited propagation model. This surface morphology of the study joint provides constraints on the propagation process so that the growth of the joint may be analyzed in terms of these three models. Based on quantitative evaluation of the cracking process, compressibility-limited propagation is favored and the driving fluid is identified as a gas rather than a brine. The gas is identified as a natural gas on the basis of geological constraints.