Massive Acid Fracturing for Limy Dolostone in Ultra-Deep Well: A Case Study

Well X was drilled into dolomitic limestone in Ordovician stratum to explore the hydrocarbon bearing properties. We faced with the following difficulties in acid fracturing: (1) High well depth (5331 m–5446 m), high in-situ stress (95–103 MPa) and high temperature (168–173 °C); (2) Poor reservoir properties, average gas permeability is 0.0009 mD, average porosity is 2.5–6.8%; (3) Large longitudinal span (113 m), strong heterogeneity and high stress difference; (4) High compressive strength (550 MPa) and high Young’s modulus (63–67 GPa); To obtain a deep penetration and large stimulate volume, the following integrated technologies were applied: (1) Laboratory acid-etching experiment to obtain basic parameters for acid fracturing design. (2) Use 5 different functional fluids including low-friction slickwater, high and low viscosity acid, diverting agent and gel, slickwater to lower temperature and initial cracks, high-viscosity acid to increase acid-rock reaction rate and penetrate deeper, low-viscosity acid to corrode and connect natural fractures, diverting agent to increase fracture complexity, gel to carry sands. (3) Mini-fracture test to analysis the stress situation, adjust the pumping procedure according to the on-site analysis. (4) A hybrid pumping mode was applied. In the fields test of this well, a total of 3600 m3 treating fluids were injected into the ground. After acid fracturing, this well has daily gas production of 11 × 104 m3 and oil production of 31 t/d. Integrated of this acid fracturing design method can be reference for similar rock formations.