Hydrofracture Propagation Research Articles

An analysis of shear waves generated by the Sterling explosion

We show that the near‐field shear waves from the decoupled explosion Sterling were likely caused by the cavity shape, which was approximately spherical except for a flat floor due to melted and recrystallized salt. We model the impact of the explosion shock wave on the cavity walls using a two‐dimensional (2‐D) Eulerian finite difference code that simulates the evolution of the air shock in the cavity coupled with 2‐D Lagrangian finite difference code that simulates the nonlinear region outside the cavity. Calculated shear waves generated by the asymmetric impact of the shock wave on the cavity walls match the observed initial shear wave amplitudes and radiation pattern. The observations also have substantial P and S coda, which are not reproduced by the calculations. Similarity of coda waveforms with distance indicates that their source is at or very near the cavity. Longer time modeling of the air shock evolution appears to produce a more realistic source function and provide a better match to the coda and indicates that the coda are caused by air reverberations between the top and bottom of the cavity. We assess the likelihood that fractures created by the tamped explosion that formed the cavity Salmon were reopened by Sterling. Modeling of hydrofracture propagation driven by the Sterling explosion, by coupling stress wave dynamics in rock with fluid mechanics in the fractures, shows that the cavity pressure is insufficient to overcome the overburden to propagate fractures into rock, except for the area immediately below the explosion on the cavity floor.

How hydrofractures become arrested

Fluids in the earth's crust are commonly transported by hydrofractures, such as dykes and mineral veins, many of which become arrested at various crustal depths. Hydrofractures are commonly arrested – some showing blunt tips – at contacts between soft (low Young's modulus) and stiff (high Young's modulus) layers. For example, many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows, and vein tips at contacts between soft marl and stiff limestone. Theoretical models indicate that overpressured, buoyant hydrofractures in homogeneous, isotropic host rocks should normally reach the surface. In layered host rocks, however, abrupt changes in Young's moduli, horizontal discontinuities, and layers with unusually high fracture‐perpendicular stresses encourage hydrofracture arrest. It is proposed that for layer‐parallel loading, stiff layers favour hydrofracture arrest during active compression but soft layers during extension. It is concluded that for hydrofracture propagation to occur, the stress field along its potential pathway must be essentially homogenous.

New experiment to model self-organized critical transport and accumulation of melt and hydrocarbons from their source rocks

A new, simple, and easily reproducible experiment was designed to simulate the production, accumulation, and transport of melt within rock. The transport was found to be of the self-organized critical type. The emergence of self-organized criticality is explained by the availability of hydrofracture propagation as a rapid or ballistic transport mechanism. This mechanism also serves as a mechanism for stepwise accumulation. These findings are confirmed by a numerical model, which shows the emergence of self-organized critical behavior when Darcian transport cannot accommodate transport and the dormant transport mechanism of hydrofracture propagation is activated. Ballistic and self-organized critical transport may play a significant role in the transport and accumulation of geological fluids, such as melt and hydrocarbons. This conclusion has a profound impact on the modeling of many transport processes in geology (e.g., accumulation of melt, oil, and gas).

流理性および成層性岩石における水圧破砕き裂進展のＡＥによる標定

In this study hydrofracture propagation in discontinuous rocks were investigated by Acoustic Emission (AE) locations. Two kinds of rocks, Inada granite and Tako sandstone, were used in the experiments. Inada granite is an anisotropic rock with rift planes and Tako sandstone has discontinuous planes. AE measurements were carried out by 12 piezoelectric transducers.Hydrofracture propagation in Inada granite were controlled by rift planes. Under unconfined condition hydrofractures were oriented to rift plane. Under confined condition, however, the initiation fracture were caused by the stress condition. In the stage of fracture extension, the fracture changed its direction depending on the rock anisotropy.In the case of Tako sandstone discontinuous plane had a great influence on the propagation of hydrofracture. Discontinuous planes prevented fractures from propagating through these planes. And, the majority of the AE events were located below the discontinuous plane and predominant fracture growth was downwards.

Laboratory observations of the effect of geologic discontinuities on hydrofracture propagation : Blair, S C; Thorpe, R K; Heuze, F E; Shaffer, R J Rock mechanics as a guide for efficient utilization of natural resources: Proc 30th U.S. symposium, Morgantown, 19–22 June 1989P443–450. Publ Rotterdam: A A Balkema, 1989

We have performed a series of laboratory tests to study the propagation of a hydrofracture into and through an interface between two rock-like materials. Test specimens were prepared by embedding sandstone tablets (lenses) in blocks of gypsum cement. These blocks were hydrofractured under true triaxial loading conditions, at a constant fluid injection rate. The injection path and applied state of stress were designed so that we obtained a single-wing fracture, propagating in a place perpendicular to the interface. Fracture growth was tracked via extension failure of fine tungsten wires embedded in the gypsum. After testing, the blocks were dissected and the extent of fracturing and fluids leakoff were recorded. In each test, the hydrofracture propagated into and through the discontinuity. Pressure-time and fracture tracking data were consistent for all tests. Distinct step increases on the pressure-time record were also noted in all tests, and are related to the interaction of the hydrofracture with the sandstone lens. All the fracture showed step-crack behavior upon entering or exiting the sandstone tablet. In addition, a finite element model was used to simulate the experiments. Pressure-time records from the model and the experiments compare favorably. 8 refs., 9 figs.