An experimental study on stress sensitivity of tight sandstone gas reservoirs during nitrogen drilling

Abstract

The “Fizz Phenomenon,” observed by real-time monitoring and reservoir evaluation technology while nitrogen drilling, is ubiquitous in tight sandstone gas reservoirs. Experiments are used to simulate stress sensitivity when encountering tight gas reservoirs in the Xujiahe formation of the central and western Sichuan Basin. The original reservoir effective stress is assumed to be the effective confining pressure reflecting the real severity of damage to the formation permeability. Results show that 1) the permeability of samples reduces with confining pressure increasing and pore pressure reducing. 2) The denser the rock, the stronger stress sensitivity is, which can explain the mechanism of low permeability reservoir sensitivity differences by using X-ray diffraction (XRD), scanning electron microscopy (SEM), casting thin sections, and constant-speed mercury injection experiment. What’s more, throat size and shape and plastic mineral content and type contribute to the differences of permeability sensitivity. 3) Mineral content and type of plastic are the major factors affecting the sensitivity of permeability differences. The stress sensitivity of tight reservoir permeability rock becomes strong with the increase of plastic mineral content.