Investigation of brittle characteristics and hydraulic fracture propagation in deep sandstone reservoirs in the central Junggar Basin

In recent years, the exploration of oil and gas resources has been continuously extended to deep reservoirs, such as deep sandstone reservoirs in the central Junggar Basin, Western China. Hydraulic fracturing for increasing production is significantly affected by rock brittleness and in-situ stress level. In this study, a geometric mean-based comprehensive index (GMCI) was proposed from experimental analyses. Subsequently, the effect of brittleness on failure modes and acoustic emission (AE) characteristics was analyzed. Finally, a three-dimensional numerical model was established considering pore pressure, in-situ stress, and fractures at the field scale to analyze the hydraulic fracturing in deep sandstone oil reservoirs, which was validated by microseismic monitoring (MS) results. The research results indicate that the brittleness of deep sandstones can be effectively evaluated with the GMCI method. With the increase of confining pressure, the overall trend of the brittleness index decreases; this is consistent with the failure mode in the experiments. With increasing brittle mineral content, the failure modes change from single shear failure to composite failure with multiple fractures, resulting in different AE vibration modes. With increasing burial depth of the reservoir by 400 m, the fracture length and width were reduced by 35.5% and 36.5%, respectively, and the stimulated reservoir volume (SRV) was reduced by 33.6%. The brittle response of a sandstone reservoir is limited, resulting in a significant decrease in fracture complexity and fracturing effect. The research results can provide some references for the mechanical response, brittle evaluation and field fracturing design of deep sandstone.