Hydraulic fractures act as conduits connecting a wellbore to nanodarcy-permeability unconventional reservoirs. Proppants are responsible for enhancing the fracture conductivity, and they help in maintaining high production rates. This study is focused on the measurement of long-term conductivity of proppant packs at simulated reservoir-temperature and pressure conditions. Various conductivity-impairment mechanisms such as proppant crushing, fines migration, embedment, and diagenesis are investigated.
Testing was performed with a conductivity cell that allows simultaneous measurement of fracture compaction and permeability. The proppant-pack performance during compression between metal and shale platens was compared. The proppant-filled fracture (concentration of 0.75–3 lbm/ft2) is subjected to axial load (5,000 psi) to simulate closure stress. Brine (3% NaCl + 0.5% KCl) is flowed through the pack at a constant rate (3 cm3/min) at 250°F during an extended duration of time (10–60 days). In this study, Ottawa sand proppant was used between platen facies fabricated from Vaca Muerta and Eagle Ford shales.
Testing between metal platens indicated that the reduction in permeability with 20/40-mesh Ottawa sand (≈30% during 12 days) was less than that of 60/100-mesh Ottawa sand, which suffered a 99% reduction in only 4 days.
Measurements with 20/40-mesh Ottawa sand between shale platens were conducted at 1.5 lbm/ft2. During a duration of 10 days, the Eagle Ford platens proppant pack exhibits a greater reduction in permeability, in comparison with Vaca Muerta platens. The normalized compaction for Eagle Ford shale platens is 20% more than Vaca Muerta platens because of greater proppant embedment. Particle-size analysis and scanning-electron-microscopy (SEM) images verify proppant crushing, fines migration, and embedment as dominant damage mechanisms. These factors are observed to depend on the testing of shales. The results suggest a substantial degradation of permeability during the initial 5 days of testing, after which the permeability appears to stabilize. Crushed proppant and dislodged shale-surface particles contribute to the fines generated; a greater concentration of fines is observed downstream.
In a separate study between Vaca Muerta platens, under similar closure stress and temperature conditions at 2-lbm/ft2 proppant concentration, the permeability reduced by almost three orders of magnitude during a duration of 60 days. It was also observed that growth of diagenetic smectite is accelerated by making the fluid more basic (pH of 10).