53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: A series of laboratory in-situ visualization experiments was conducted on shale samples with different ductility (or brittleness) for improved understanding of the process of hydraulic fracture closure. In these experiments, a circular shale disc (diameter 44 mm, height 19 mm) was pressed against either a transparent glass disc with a roughened surface or a sapphire disc mediated by a single layer of proppant (grain size ∼1 mm). The compaction experiments were conducted under room temperature (∼25°C), with the maximum effective stress of ∼27 MPa. The test durations were 2 weeks and 1 month. The fracture closure and proppant crushing and embedment were visualized optically, using UV-induced fluorescence of dye mixed with the pore fluid (5% NaCl aq.). The closure of the fracture and the permeability reductions were monitored throughout the experiment. Two Marcellus shale samples with very different mineralogical compositions exhibited very different fracture closure behavior. A brittle, high-calcite-content sample caused progressive crushing of quartz proppant. In contrast, the other, high-clay-content sample resulted in strong proppant embedment and matrix disintegration. Interestingly, in spite of the very different properties, the creep closure of the fractures in both samples exhibited near-perfect semi-logarithmic time dependency.
High-performing hydrocarbon reservoirs usually consist of brittle, clay-poor, high TOC (total organic carbon) shales (e.g. Bourg, 2015. See Fig.1). However, clay-rich, ductile reservoirs with a good production potential do exist (e.g. Pair, 2017). Additionally, as the high-quality, “low-hanging-fruits” of the hydrocarbon-bearing shales get depleted, there will be increasing needs to produce from more difficult, ductile shales. Hydraulic fracturing has become an indispensable tool for enhancing permeability of otherwise very impermeable shales containing oil and gas. However, clay-rich, ductile shales are difficult to fracture, and the hydraulic fractures created in the rock tend to be short and have a smaller surface area. Proppant placed in these fractures tends to be embedded in the soft fracture walls, and the open space created by the fracture can be filled by mobilized clay minerals and by the expanded fracture walls if clay swelling happens. This is particularly a problem for the far-ends of the created fracture network, where proppants are distributed as a sparse monolayer, or no proppant present. Although these poorly propped fractures provide a significant drainage footprint for hydrocarbon production, they tend to close prematurely and lead to rapid permeability loss.
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