High-resolution modeling of overflush and its implications for fracture productivity

Abstract

Hydraulic fracturing operations typically aim to create a highly conductive fracture that will intensify well production or injectivity. At the end of pumping, a fluid without proppant, called flush, is injected to clean the wellbore and tubing of the remaining slurry. Overflush is the situation when flush volume exceeds the volume between the wellhead and perforations so that flush fluid enters the propped fracture and washes out the near-wellbore region of propped fracture. In this work, we examine overflush impact on fracture closure near perforations in detail by considering several overflush scenarios and running simulations using a suite of high-resolution models of fracturing itself, fracture closing on proppant, and productivity. We show that at certain conditions, overflush has no impact on well productivity or may even enhance it. Additionally, we develop a simplified analytical model of well production for an axisymmetric overflushed region and compare the model results to precise numerical simulations. We consider several overflush scenarios, both beneficial and unfavorable. It was found that the dominant role in the formation of overflushed region geometry is that of Saffman–Taylor instability, the manifestation of which leads to the formation of long channels that with high possibility do not cause fracture closure with pressure decrease during production and can remain open even at large pressure drawdown. The work has both research and practical interest and demonstrates an integral approach to numerical overflush modeling. In addition, it gives basis for the revision of the existing “harmful overflush” paradigm.