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  1. Abstract

    The feedback between dyke and sill intrusions and the evolution of stresses within volcanic systems is poorly understood, despite its importance for magma transport and volcano instability. Long-lived ocean island volcanoes are crosscut by thousands of dykes, which must be accommodated through a combination of flank slip and visco-elastic deformation. Flank slip is dominant in some volcanoes (e.g., Kilauea), but how intrusions are accommodated in other volcanic systems remains unknown. Here we apply digital mapping techniques to collect > 400,000 orientation and aperture measurements from 519 sheet intrusions within Volcán Taburiente (La Palma, Canary Islands, Spain) and investigate their emplacement and accommodation. We show that vertically ascending dykes were deflected to propagate laterally as they approached the surface of the volcano, forming a radial dyke swarm, and propose a visco-elastic model for their accommodation. Our model reproduces the measured dyke-aperture distribution and predicts that stress accumulates within densely intruded regions of the volcano, blocking subsequent dykes and causing eruptive activity to migrate. These results have significant implications for the organisation of magma transport within volcanic edifices, and the evolution and stability of long-lived volcanic systems.

     
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  2. Abstract

    Hydraulic fractures that grow in close proximity to one an other interact and compete for fluid that is injected to the wellbore, leading to dominance of some fractures and suppression of others. This phenomenon is ubiquitously encountered in stimulation of horizontal wells in the petroleum industry and it also bears possible relevance to emplacement of multiple laterally propagating swarms of magma‐driven dykes. Motivated by a need to validate mechanical models, this paper focuses on laboratory experiments and their comparison to simulation results for the behavior of multiple, simultaneously growing hydraulic fractures. The experiments entail the propagation of both uniformly and nonuniformly spaced hydraulic fractures by injection of glucose or glycerin‐based solutions into transparent (polymethyl methacrylate) blocks. Observed fracture growth is then compared to predictions of a fully coupled, parallel‐planar 3D hydraulic fracturing simulator. Results from experiments and simulations confirm the suppression of inner fractures when the spacing between the fractures is uniform. For certain non‐uniform spacing, both experiments and simulations show mitigated suppression of the central fractures. Specifically, the middle fracture in a 5‐fracture array grows nearly equally to the outer fractures from the beginning of injection. Furthermore, with some delay, the other two fractures that are suppressed with uniformly spaced configurations grow, and eventually achieve a velocity exceeding the other three fractures in the array. Hence, these experiments give the first laboratory evidence of a model‐predicted behavior wherein certain nonuniform fracture spacings result in drastic increases in the growth of all fractures within the array.

     
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  3. Abstract

    Hydraulic fracturing enables oil and gas extraction from low‐permeability reservoirs, but there remains a need to reduce the environmental footprint. Resource use, contaminant‐bearing flowback water, and potential for induced seismicity are all scaled by the volume of injected fluid. Furthermore, the greenhouse gas emissions associated with each extracted unit of energy can be decreased by improving resource recovery. To minimize fluid use while maximizing recovery, a rapidly computing model is developed and validated to enable the thousands of simulations needed to identify opportunities for optimization. Lower pumping pressure approaches that minimize pressure loss through the wellbore perforations combined with nonuniform spacing are shown to be capable of substantially reducing fluid consumption and/or increasing created fracture surface area when the stress variation is mainly from fracture interaction instead of in situ stress. When in situ stress variation is dominant, “limited entry” methods promote more uniform growth but with higher pumping pressures and energy consumption.

     
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  4. Abstract

    Optimal hydraulic fracturing stimulations of horizontal oil and gas wells maximize created fracture surface area and/or maximize the uniformity of stimulation. Here, we use a new, rapidly‐computing hydraulic fracture model to investigate how surface area and uniformity are impacted by interplay among multiple growing hydraulic fractures driven through permeable rocks by fluids of various viscosities. The results show the existence of a surface‐area‐optimizing viscosity that is large enough to control leak‐off but not so large that leads to unnecessarily large fracture aperture.

     
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