Integrated stimulation technology for ultra deep tight sandstone gas reservoirs with ultra low permeability in Sichuan Xujiahe formation

The development of tight sandstone gas reservoirs in the XUJIAHE Formation of western Sichuan faces critical challenges due to extreme reservoir conditions, including burial depths exceeding 4500 m, abnormal pore pressures (pressure gradient > 1.8 MPa/100 m), high breakdown pressures (> 90 MPa), elevated temperatures (> 120 °C), and ultralow permeability (< 0.1 mD). Conventional fracturing technologies exhibit low success rates (< 40%) and inadequate proppant placement efficiency (< 50%) under these conditions, severely limiting commercial gas production. This study presents an integrated fracturing strategy combining three novel approaches to address these operational and geological constraints. First, a high-density potassium formate brine system (1.5-1.6 g/cm³) reduced the bottomhole fracture pressure gradient to 0.0326 MPa/m through hydrostatic compensation, achieving a 21.83 MPa reduction in surface pump pressure compared to conventional fluids. Second, a hybrid fracturing design combining low-viscosity slickwater (3-5 mPa·s) and high-concentration cross-linked gel (80-120 mPa·s) simultaneously enhanced fracture complexity (Fracture Complexity Index > 2.5) and conductivity (> 100 Darcy), achieving a fivefold improvement in proppant transport capacity (600 kg/m³). Third, gelled acid pre-treatment (15% HCl system) dissolved calcareous interlayers (CaCO₃ >70%), reducing injection pressure by 14.83 MPa and increasing proppant migration efficiency by 30% in challenging Xu 4 intervals. Field implementation demonstrated breakthrough performance: fracturing success rates improved to 82%, with average post-fracture gas production reaching 2.7 × 10⁴ m³/d-a 210% increase over conventional methods. These innovations tight gas stimulation by synergistically addressing high-stress environments, fracture network complexity, and lithological heterogeneity, providing a replicable framework for deep unconventional reservoirs.