Managed Saffman-Taylor instability during overflush in hydraulic fracturingстатья
Статья опубликована в высокорейтинговом журнале
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Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 27 марта 2018 г.
Аннотация:Overflushing is a common practice in multistage completions of horizontal wells in unconventional formations. It consists in displacing the suspension by a thin fluid, away from the wellbore and into the fracture. It can, however, overly displace proppant and leave a significant portion of the fracture unsupported near the wellbore, or trigger gravity slumping, again leaving unpropped area near the wellbore. That unpropped area can then close during production, damaging the performance of the fracture. A novel modeling approach was investigated to gain insight into the overflush process and determine secure bounds for maintaining the fracture performance.
From a fluid mechanics viewpoint, overflushing is the displacement of a Hershel-Bulkley fluid by a power law fluid in a Hele-Shaw cell, leading to Saffman-Taylor instability at the fluids interface. Whereas most hydraulic fracturing simulators use power-law rheology model, we used a novel numerical approach accounting for the yield-stress behavior of the slurry. Using the lubrication approximation, we derived a model, which includes transport equations for fluid volume fractions and a nonlinear elliptic equation for pressure with mixed-type boundary conditions. Validation was previously performed against three sets of experiments in Hele-Shaw cells, comprising gravitational slumping and displacement of fluids with fingering.
Based on fracture mechanics, we analyzed how large a portion of the fracture may be left unsupported before it is severely pinched during drawdown. A parametric study was performed on the displacement of the yield-stress slurry by the overflushing fluid. Qualitatively, when fingers of the overflushing fluid can be created at the overflush/slurry interface, large slurry pillars are preserved in the near-wellbore area, which may keep the fracture open. Three main scenarios of fluids distribution are identified and classified in terms of the oveflush fluid-to-slurry viscosity ratio ξ: (i) the slumping-dominated regime, where light clean overflush fluid goes on top of sedimenting heavy proppant-laden suspension, (ii) an intermediate scenario of slumping combined with fingering, and, finally, (iii) a pure fingering-dominated scenario, when the slurry viscosity is high. The third scenario minimizes the geomechanical risks of overflushing by providing tiny fingers that are unlikely to be pinched out during fracture closure.