Laboratory In-Situ Visualization of Long-Term Fracture Closure and Proppant Embedment in Brittle and Ductile Shale Samples

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.