In this work, we argue that resistance (apparent toughness) to fracture propagation is an inherent characteristic of cohesionless particulate materials. We developed experimental techniques to quantify the initiation and propagation of fluid-driven fractures in saturated particulate materials. The fracturing liquid is injected into particulate materials, where the fluid flow is localized in thin, self-propagating, crack-like conduits. By analogy, we call these conduits ‘cracks’ or ‘hydraulic fractures.’ The experiments were performed on three particulate materials – (1) fine sand, (2) silica flour, and (3) their mixtures. Based on the laboratory observations and scale (i.e., dimensional) analysis, this work offers physical concepts to explain the observed phenomena. The goal of this study is to determine the controlling parameters of fracture behavior and to quantify their effects. When a fracture propagates in a solid, new surfaces are created by breaking material bonds. Consequently, the material is in tension at the fracture tip. In contrast, all parts of the cohesionless particulate material (including the tip zone of the hydraulic fracture) are likely to be in compression. In solid materials, the fluid front lags behind the front of the propagating fracture. However, for fluid-driven fractures in cohesionless materials the lag zone is absent. The compressive stress state and the absence of the fluid lag are important characteristics of hydraulic fracturing in particulate materials with low, or negligible, cohesion. At present, two kinematic mechanisms of fracture initiation and propagation, consistent with both the compressive stress regime and the absence of the fluid lag, can be offered. The first mechanism is based on shear bands propagating ahead of the tip of an open fracture. The second is based on the reduction of the effective stresses and material fluidization within the leakoff zone at the fracture tip. Our experimental results show that the primary factor affecting peak (initiation) pressure and fracture aperture is the magnitude of the confining stresses. The morphology of the fracture and fluid leakoff zone, however, changes significantly not only with stresses, but also with other parameters such as flow rate, fluid rheology, and permeability. Typical features of the observed fractures are multiple offshoots (i.e., small branches, often seen only on one side of the fracture) and the bluntness of the fracture tip. This suggests the importance of inelastic deformation in the process of fracture propagation in cohesionless materials. Similar to solid materials, fractures propagate perpendicular to the least compressive stress. Scaling indicates that in the experiments performed in the regime of limited leakoff (i.e., the thickness of the leakoff zone is much smaller than the fracture length); there is a high-pressure gradient in the leakoff zone in the direction normal to the fracture. Fluid pressure does not decrease considerably along the fracture, however, due to the relatively wide fracture aperture. This suggests that hydraulic fractures in unconsolidated materials propagate within the toughness-dominated regime. This is the main conclusion of our work. In addition, the theoretical model of toughness-dominated hydraulic fracturing can be matched to the experimental pressure-time dependences with only one fitting parameter. Scale analysis shows that large apertures at the fracture tip correspond to relatively large 'effective' fracture (surface) energy, which can be orders of magnitude greater than typical for hard rocks. In this work, we present a comprehensive experimental development focusing on four main parameters: (1) confining stresses, (2) fluid rheology, (3) injection rate, and (4) permeability. Another important conclusion is that the primary parameter in determining the peak injection pressure is that of confining stresses.

We review techniques for measuring fluid flow and advective heat output from seafloor hydrothermal systems and describe new anemometer and turbine flowmeter devices we have designed, built, calibrated, and tested. These devices allow measuring fluid velocity at high‐ and low‐temperature focused and diffuse discharge sites at oceanic spreading centers. The devices perform at ocean floor depths and black smoker temperatures and can be used to measure flow rates ranging over 2 orders of magnitude. Flow velocity is determined from the rotation rate of the rotor blades or paddle assembly. These devices have an open bearing design that eliminates clogging by particles or chemical precipitates as the fluid passes by the rotors. The devices are compact and lightweight enough for deployment from either an occupied or remotely operated submersible. The measured flow rates can be used in conjunction with vent temperature or geochemical measurements to obtain heat outputs or geochemical fluxes from both vent chimneys and diffuse flow regions. The devices have been tested on 30 Alvin dives on the Juan de Fuca Ridge and 3 Jason dives on the East Pacific Rise (EPR). We measured an anomalously low entrainment coefficient (0.064) and report 104 new measurements over a wide range of discharge temperatures (5°–363°C), velocities (2–199 cm/s), and depths (1517–2511 m). These include the first advective heat output measurements at the High Rise vent field and the first direct fluid flow measurement at Middle Valley. Our data suggest that black smoker heat output at the Main Endeavour vent field may have declined since 1994 and that after the 2005–2006 eruption, the high‐temperature advective flow at the EPR 9°50′N field may have become more channelized, predominately discharging through the Bio 9 structure. We also report 16 measurements on 10 Alvin dives and 2 Jason dives with flow meters that predate devices described in this work and were used in the process of their development. This includes the first advective measurements in the Lau Basin and at the EPR 9°39.5′N. We discuss potential error sources and how they may affect the accuracy of measurements by our devices and other devices. In particular, we use the turbulent plume theory to evaluate the effect of entrainment of ambient seawater.

Hydraulic fracturing of unconsolidated materials has been a long-recognized technique to control sand production. This work describes the results of an experimental investigation on the dominant parameters of hydraulic fracturing in unconsolidated sands. The variables in this study, each varied by several orders of magnitude, are sample permeability, fluid rheology, remote stresses, and fluid injection rate. We have quantified the results of these experiments in terms of injection pressuretime history, fracture aperture, and leakoff thickness. Directly measuring the magnitude of fracture aperture and leakoff thickness is significant in hydraulic fracturing of unconsolidated materials to constrain modeling this process. Laboratory observations presented here suggest that in our experiments, the leakoff process is primarily confined to the tip of the fracture and that leakoff precedes fracture propagation. This may be the first direct experimental observation of the dominance of the leakoff process in the tip region of a propagating fracture in cohesionless materials. Our experimental results also suggest that the primary factor affecting peak (initiation) pressure and fracture aperture is the magnitude of the confining stresses. However, the morphology of the fracture (and fluid leakoff zone) changes significantly not only with stresses, but also with other parameters such as flow rate, fluid rheology, and sample permeability. Typical features of the observed fractures are multiple offshoots, a blunt fracture tip, leakoff primarily confined to the fracture tip region, and material shear banding. Dimensional analysis of the recorded experimental data results in the power-law dependence of the peak injection pressures on the dominant experimental parameters. The results presented here quantify the effect of the experimental parameters on injection pressures, magnitude of leakoff, and fracture aperture. This is a significant development and our data set is consistent with available field observations, also discussed in this work.

The application of high-resolution fiber optic distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) during multistage hydraulic fracturing has increased in recent years. However, fiber optic has been used for communication networks, surveillance, and monitoring and diagnosing wellbore integrity for quite some time. The technology has also been used to diagnose reservoir and well performance, e.g., production and injection profiling. Optical fibers were placed in a small stainless steel tube, which was clamped to the casing while running in the hole in a horizontal unconventional gas well. The casing was then cemented in place, and fracturing treatments were completed by limited-entry techniques using the "plug & perf" process, resulting in multicluster (each stage), multistage fractures. Perforations were placed on the topside of the casing, oriented to avoid damaging the optical fiber (zero degree phasing, at 5 shots per foot, one foot per cluster). This paper shares images and results from DAS and DTS data that allow for interpretation of fracturing treatments in multicluster, multistage horizontal wells. The images illustrate the dynamic nature of the spatial and temporal fluid distribution pertaining to initiation, propagation and arrest of hydraulic fractures during simultaneous stimulation of multiple clusters. First, fracture initiation appears not only during early time, as expected, but also late time initiation events may develop in previously dormant clusters. Second, propagation of multiple fractures can occur randomly within a cluster array, and these multiple fractures are sometimes in close proximity to one another. Third, dominant clusters are often observed during stimulation. Further, during the treatment cycle, the position of dominant cluster(s), within a given stage, may actually change. We leverage these observations with various other data sources in a modeling exercise to demonstrate the implications of unequal fracturing fluid distributions on propped fracture performance.