MATERIAL STRENGTH REDUCTION IN UNCONSOLIDATED SANDSTONE DURING WATER BREAKTHROUGH AND SILICA DISSOLUTION UNDER MULTIPHASE FLOW: A SCREENING FRAMEWORK FOR SANDING RISK

This study quantifies material strength loss in weakly consolidated sandstone during multiphase water invasion and links the degradation to sanding risk. The work combines triaxial testing on offshore Azerbaijan type core plugs (N = 18) with a coupled screening model that updates cohesion during water breakthrough using capillary-bond collapse and silica dissolution. Tests were performed at Sw = 30 percent, 60 percent, and 100 percent and at 30°C, 60°C, and 90°C under 10 MPa confinement. Dissolved silica increased with temperature and reached 32 mg/L at 90°C. UCS decreased from 9.56 MPa at the low-saturation baseline to 5.46 MPa at full saturation, a 42.9 percent loss. The screening model propagates this weakening into drawdown limits and shows a strong narrowing of the safe operating window, with critical drawdown pressure collapsing from 70.1 MPa to 0.7 MPa under the combined post-breakthrough scenario. The output is a practical screening framework that ties water-cut progression and temperature to formation strength reduction, drawdown margin, and expected sanding regime for late-life wells. Keywords: unconsolidated sandstone, water breakthrough, silica dissolution, strength degradation, triaxial testing, critical drawdown pressure, sanding risk.