Hydraulic fractures act as conduits connecting a well-bore to nanodarcy permeability unconventional reservoirs. Fracture conductivity is the key to maintaining high production rates after stimulation. Proppants are responsible for enhancing the conductivity of hydraulic fractures and comprise a considerable proportion of completion expense. This study focuses on the long-term conductivity of proppant-packs at elevated temperature and pressure under simulated reservoir conditions in laboratory. Various conductivity impairment mechanisms such as proppant crushing, fines migration, embedment, and diagenesis are investigated.
Testing was done using a Hastelloy conductivity cell which allows simultaneous measurement of fracture compaction and permeability. We compare proppant-pack performance during compression between metal and rock platens. The proppant filled fracture (concentration: 0.75–3 lb/ft2) is subjected to axial load (5000 psi). Brine (3% NaCl + 0.5% KCl) is flowed through the pack at a constant rate (3 ml/min) at elevated temperature (250°F) over extended periods of time ranging from 10–60 days. Ottawa sand (20/40 mesh and 60/100 mesh) proppants were used in this study. Shale platens fabricated from Vaca Muerta were used as platens.
We measured a 30% reduction in propped fracture permeability in 12 days for 20/40 Ottawa sand and a 99% reduction in 4 days for 60/100 mesh Ottawa sand; both were tested between metal platens at concentrations of 2 lb/ft2. Particle size analysis indicated that proppant crushing and fines migration are the major causes of permeability reduction.
For 20/40 mesh Ottawa sand, over a duration of 12 days, greater reduction in permeability is observed with Vaca Muerta shale platens. 90% reduction in permeability is observed with Vaca Muerta shale platens as compared to 30% with metal platens. Scanning Electron Microscope imaging is used to investigate proppant crushing, fines migration and embedment damage mechanisms. The normalized compaction for shale platens is 5% more than metal platens, owing to proppant embedment. The results suggest a substantial degradation of permeability during the initial 5 days of testing, after which the permeability stabilizes. Crushed proppant and dislodged surface particles contribute to the fines generated; a greater concentration of fines is observed, as expected, downstream.