Showing papers by "Herbert H. Einstein published in 2022"
DOI•
TL;DR: It is shown that the wormholes created in core flood tests have lengths following a power‐law distribution, with more short ones than long ones, which allows one to experimentally study the wormhole competition in 3‐D: Many wormholes develop initially near the inlet, but only a few develop further.
Abstract: Understanding how wormholes develop and how they change the permeability of the porous medium is important for many natural and industrial processes in subsurface solid‐fluid systems. We conduct core flood tests to study the wormhole formation in a porous medium under different flow rates and at different moments before breakthrough. We show that the wormholes created in core flood tests have lengths following a power‐law distribution, with more short ones than long ones. This statistical nature of the wormhole lengths allow one to experimentally study the wormhole competition in 3‐D: Many wormholes develop initially near the inlet, but only a few develop further. Our experimental results also show that the existing wormhole‐matrix models underestimate wormhole lengths because of the neglected additional pressure drop caused by the radial flow near the wormhole tip. We improve upon the existing models by approximating the radial flow near the wormhole tip with a Rankine ovoid model.
1 citations
TL;DR: In this article , the authors present experiments on Barre granite with precut flaws where the material is loaded to high far-field stresses close to shear failure, and then the fluid pressure in the flaws is increased to move the Mohr's circle to the left and observe the initiation and propagation of fractures using high-speed imaging and acoustic emissions (AEs).
Abstract: Hydraulic fracturing is frequently used to increase the permeability of rock formations in energy extraction scenarios such as unconventional oil and gas extraction and enhanced geothermal systems (EGSs). The present study addresses uncertainties in the hydraulic fracturing process pertaining to EGSs in crystalline rock such as granite. Specifically, there is debate in the literature on the mechanisms (i.e. tensile and/or shear) by which these fractures initiate, propagate, and coalesce. We present experiments on Barre granite with pre-cut flaws where the material is loaded to high far-field stresses close to shear failure, and then the fluid pressure in the flaws is increased to move the Mohr's circle to the left and observe the initiation and propagation of fractures using high-speed imaging and acoustic emissions (AEs). We find that the hydraulic fractures initiate as tensile microcracks at the flaw tips, and then propagate as a combination of tensile and shear microcracks. AE focal mechanisms also show elevated levels of tensional microfracturing near the flaw tips during pressurization and final failure. We then consider a numerical model of the experimental setup, where we find that fractures are indeed likely to initiate at flaw tips in tension even at relatively high far-field stresses of 40 MPa where shear failure is generally expected.
1 citations