Reservoir blockage removal/pressure boosting in production wells at Yanchang oil fields

The principles of reservoir blockage removal/pressure boosting by CO₂ (BR/CO₂PB), the composition and the performance properties of the working fluids used, and the application of this combinational technology for oilwell stimulation in low permeable (1.3 × 10^-3 sq μm in average) and low porosity (9.7% in average) reservoirs in Yanchang, North Shaanxi, are presented. The blockage removing fluid (BRF) is a polycomponent compounded mixed acidizing fluid containing hydrochloric, hydrofluoric, glacial acetic, and phosphoric acids up to 12% and enhanced by gaseous CO₂; the pressure boosting fluid (PBF) is a chemical solution with gaseous CO₂ dissolved under pressure and can evolve a large amount of heat and gases, mainly CO₂, in contact with the mixed acids. The viscosity of a heavy crude oil is reduced by > 90% at 25°-55°C after the crude oil having reacted with an experimental mixed BR/CO₂PB fluid at 10 MPa initial pressure over 50 min; 91-99.4% dissolving rate at 60°C is observed for CaSO₄, CaCO₃, and oily fractions of the precipitates in some production wells; at 10 MPa initial pressure, the PBF develops the highest pressure of 18 MPa in 58-65 min, which gradually drops down owing to dissolution of CO₂ in the crude oil. The technology, well selection and results of field BR/CO₂PB operations are presented.