Understanding stress-sensitive behavior of pore structure in tight sandstone reservoirs under cyclic compression using mineral, morphology, and stress analyses

Tight sandstone reservoirs have excellent potential as oil and gas exploration fields. During exploitation, drilling upsets the stress balance of the reservoir. The top priority is studying the stress state in subsequent hydraulic fracturing and production operations. This study selected the samples of the Lower Shihezi Formation in Ordos Basin as research target, and investigated the stress sensitivity of tight sandstones based on casting thin sections (CTS), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and X-ray diffraction analysis (XRD). The results show that the fluctuation of permeability is larger than that of the porosity under the cyclic effective stress. Pore types can be divided into fracture-like, irregular oval, spherical arches, and triangle pores. Non-homogeneous deformation of microscopic structures and minerals eventually leads to permeability evolution. Overall, this study proposes novel pore models and throat types and provides insight into the stress sensitivity of tight sandstone reservoirs. A comprehensive analysis of stress sensitivity control factors for tight sandstones was then performed considering the combined effect of mineral components, microscopic morphology, and mechanical stability. However, we acknowledge that laboratory stress sensitivity evaluation generally does not completely represent the field-scale stress sensitivity. Therefore, the sample preparation and experimental conditions and process need to simulate in situ formation conditions as comprehensively as possible in future studies.