Experimental Study on Rock Failure Patterns and Mechanisms of Hydraulic Frac-Packing in Weakly Consolidated Sandstone Reservoirs

Abstract

Frac-packing is an integrated stimulation and sand-control technology widely used in weakly consolidated sandstone (WCS) reservoirs. However, frac-packing treatments in the Sebei Gas Field exhibit substantially higher proppant intake than conventionally expected. Typical frac-packing intensity in WCS reservoirs is approximately 0.2–2 m3/m, whereas values of 2–15 m3/m have been reported in Sebei. Proppant volumes are mainly clustered around 78 and 97 m3, with some treatments reaching approximately 150 m3, and show only weak correlations with effective interval thickness, designed fracture dimensions, and injection rate. These observations indicate that proppant accommodation in the Sebei reservoir cannot be explained solely by the geometric volume of a conventional fracture. To investigate the underlying mechanism, true-triaxial frac-packing experiments were conducted using 200 mm × 200 mm × 200 mm cubic WCS samples, followed by post-fracturing 3D scanning and fracture-surface reconstruction. Three representative failure–packing modes were identified: fracture-dominated mode (FDM), plastic deformation-dominated mode (PDM), and fracture–plastic coupled mode (FPCM). In FDM, proppant mainly entered the main and branch fractures, with equivalent packing thickness ranging from several millimeters to 20–30 mm and locally reaching 30–40 mm. In PDM, no distinct throughgoing fracture formed, and proppant was primarily retained within near-wellbore plastic failure and local dilation zones; the representative PDM-P1 experiment showed an average equivalent packing thickness of 22.32 mm. FPCM exhibited both main-fracture packing and near-wellbore plastic-failure-zone packing. The corresponding laboratory pressure curves showed a distinct breakdown peak, no identifiable peak, and a weak peak for FDM, PDM, and FPCM, respectively, and representative field curves exhibited similar qualitative characteristics. The results suggest that the unusually high proppant intake in the Sebei Gas Field may arise from the combined contributions of locally large equivalent packing thickness, branch-fracture accommodation, and near-wellbore plastic dilation zones. The proposed failure–packing modes provide an experimental basis for interpreting high proppant intake in WCS reservoirs and offer guidance for frac-packing design and proppant-demand evaluation.