Mechanisms controlling rupture shape during subcritical growth of joints in layered rocks

Abstract

During joint propagation, the orientation of plume lines on joint faces allows for mapping the path taken by the crack tip line during rupture growth. Rupture shapes in layered, clastic sediments of the Devonian Catskill Delta (Finger Lakes district, New York) indicate joint growth through three stages, where velocity of the tip line, v tl , varies as a function of the crack-tip stress intensity, K I . The initial stage of growth is characterized by a rupture of approximately circular or elliptical shape that expands from an initiation fl aw with a velocity, v tl . In some cases, primary growth involves self-correction, where an elliptical rupture redistributes the crack-tip stress so that the rupture returns to a more stable circular shape. The crack tip line eventually propagates to a bedding interface where properties of either a plastic or a noncohesive bedding interface cause v tl → 0 along a portion of the tip line. The onset of secondary growth occurs when the rupture tip line intersects both bedding interfaces and splits into two discontinuous segments that propagate synchronously as a single, coherent rupture. Nonuniform, nonsystematic v tl eventually interferes with the rupture's ability to grow coherently and thus leads to the transition to tertiary growth, characterized by the detachment of the coherent rupture into one or more independently propagating, noncoherent tip lines. Throughout all three stages of rupture growth, the K I -dependent v tl points to subcritical propagation. Variation from a smooth to rough plume morphology is consistent with propagation through region I of the subcritical regime with a transition at v tl ≈10 −5 m/sec. Rupture velocity may enter region II of the subcritical regime, but such a broad range of K I must be crossed to reach region III and quasistatic behavior at the critical stress intensity, K Ic , that region II acts as a 'barrier' through which few joints pass, thus greatly limiting the number of postcritical joints in the Catskill Delta if not in the crust of the Earth.

Gas-driven subcritical crack propagation during the conversion of oil to gas

“…A penny-shaped crack is considered since it has typically been observed in sedimentary rocks (Lacazette & Engelder 1992;Savalli & Engelder 2005;Olson 1993). We assume that the crack is initially filled with oil in a linearly elastic source rock of essentially zero permeability and the final state is a gas-filled crack when all the oil has been converted to gas.…”

Microseismic shearing generated by fringe cracks and bedding-plane slip

“…In some cases tensile joints are interpreted to be natural hydraulic fractures formed in geologic environments comparable to unconventional hydraulic fracture stimulations (e.g., Savalli and Engelder, 2005). Tensile joint surfaces are characterized by plume-like structures that record the fracture growth direction and sense of propagation.…”

Joint development and tectonic stress field evolution in the southeastern Mesozoic Ordos Basin, west part of North China

“…Furthermore, the discontinuity of the plumose structure is a characteristic of natural hydraulic fractures (Fig. 7a, Lacazette and Engelder, 1992;Savalli and Engelder, 2005).…”