Economic hydrocarbon production from organic rich shale has been made feasible by advances in horizontal drilling and hydraulic fracturing. Proppants are pumped to keep the fractures open and provide a high conductive path from the reservoir to the wellbore. Effects of proppant size, proppant crushing, fines migration, rock mineralogy and fluid chemistry on the long-term fracture conductivity have been studied experimentally in detail by Mittal (2017, 2018). This study further investigates the impact of proppant concentration, size and presence of different volcanic ashes on fracture conductivity along with different conductivity impairment mechanisms including proppant crushing, embedment and diagenesis under simulated reservoir conditions.
Experiments have been conducted by varying the proppant concentration of 60/100 mesh Ottawa sand from 2 lb/ft2 to 4 lb/ft2. The proppant pack was placed between metal platens and subjected to axial load of 5000 psi and temperature of 250 °F. Proppant pack conductivity was then measured by flowing 3% NaCl brine for periods of 7-15 days. We observed a sharp decline in permeability, with almost 98% decline within 3 days with low concentration compared to only 60% decline in permeability with higher concentration of proppant. Particle size analysis reveals overall 5% higher percentage crushing at lower proppant concentration, suggesting major crushing occurs at the platen interfaces which reduces with increased proppant pack concentration.
Presence of volcanics in the major shale plays like Eagle Ford and Vaca Muerta has been reported in literature. To simulate similar environment and study the impact of diagenesis on fracture conductivity, experiments have been conducted by flowing high pH (~10) brine through the proppant pack mixed with volcanics like obsidian and basalt and placing the proppant between Eagle Ford shale platens. Experiments were conducted with 20/40 Ottawa sand mixed with obsidian and 60/100 mesh Ottawa sand mixed with basalt. We observed a sharper decline in permeability with 60/100 sand as compared to 20/40 sand in the first two days. However, the permeability for both the proppant sizes continues to decline with a difference of an order of magnitude even after 30 days. SEM images shows significant particle crushing, embedment and diagenetic growth on the shale surface and verify that these factors are responsible for permeability decline. To further understand the impact of proppant size on permeability, dry crush tests have been conducted on 20/40 and 60/100 Ottawa sand by varying compaction pressure from 1500 psi to 3000 psi and 5000 psi. We observed that 60/100 mesh sand undergo overall higher compaction and crushing compared to 20/40 mesh sand at each compaction pressure.