Experimental study on the in-situ stress field of tight sandstone in the He1 member of Fuxian Block, Ordos Basin

Abstract

Accurate characterization of the in-situ stress field is a critical prerequisite for efficient hydraulic fracturing development in tight sandstone gas reservoirs. This study focuses on the tight sandstone of the He 1 Member in the Fuxian Block of the Ordos Basin. A comprehensive investigation was conducted by integrating ultrasonic velocity anisotropy measurements, acoustic emission (AE) Kaiser effect experiments, and X-ray diffraction (XRD) analysis of clay minerals to systematically determine the orientation and magnitude of in-situ stresses and elucidate their petrological basis and engineering implications. The results indicate that the reservoir exhibits pronounced stress anisotropy, with the maximum horizontal principal stress (σH) oriented between 150° and 160° (relative azimuth). The estimated stress magnitudes suggest a transitional strike-slip-to-reverse faulting stress regime, characterized by σV ≈ σH > σh, with a σH gradient of approximately 2.20 – 2.44 MPa / 100 m. XRD results show that the clay mineral assemblage is dominated by illite (42 – 62 %) and chlorite (21 – 41 %), accompanied by a certain proportion of illite/smectite mixed-layer minerals (16 – 19 %), with a smectite layer content of about 40 %. This mineralogical composition implies a moderate-to-strong potential risk of water sensitivity while maintaining relatively high matrix brittleness, which is favorable for the formation of complex fracture networks. This study establishes an integrated understanding of the “stress field–mineral composition—engineering response” relationship in the Fuxian Block, providing a scientific basis for optimizing hydraulic fracturing design and mitigating reservoir damage.