Flow Behavior of Water Through Nanopores of Unconventional Reservoirs: Confinement and Slip Effects

Abstract

Hydraulic fracturing and horizontal well technologies are central to the success of unconventional reservoirs development. However, hydraulic fracturing remains poorly understood, including the poor recovery and an incomplete understanding of the potential trapping mechanisms for hydraulic fracture water, which in turn has raised serious technical and environmental concerns. Flow behavior of water through nanopores is critical for the fate of hydraulic-fracture water. In this work, water flow behavior through nanopores, including organic and inorganic nanopores, with different size and wettability is investigated. The interaction forces of fluid-fluid and fluid-wall are equally important in the nano-scale pores, which resulting in that the viscosity of nanoconfined water is much larger than that of bulk water, whereas it greatly decreases and approaches to the bulk water viscosity when the nanopore size increases. Slippage processes always exit in water transport through nanopores, which depend on the wettability of the nanopores wall. These two unique phenomenon are coupling and significantly influence the water flow behavior through the nanopores. A modified form of no-slip Darcy flow, combining the viscosity variation and slip effect, is successfully proposed to describe the flow behavior of water through nanopores. The viscosity in nanoconfined water at different nanopores size is obtained with the experiment data. The slip length of water in nanopores with different wettability is calculated by fitting the experiment data. Results show that the viscosity dependence on the nanopores size decreases the water transport capacity while the nanopores size decreases. The water viscosity in nanopores with the diameter 1.4 nm is 2.7 times that of bulk water, and increases to 3.3 times while the diameter decreases to 0.6 nm. However, the slip effect increases the water transport capacity, which becomes more obvious while the nanopores wettability are increasingly hydrophobic. The slip length increases from 0.24 ± 0.38 nm to 12.0 ± 3.3 nm while the static contact angles of water for the nanopores walls changes from 4 ± 3° to 85 ± 4°. This new understanding of the interplay between viscosity, wettability and boundary slip captures the water flow behavior more accurately in tight shale formations, which can be used to control water flow through nanopores during unconventional oil and gas reservoirs development.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016