Oil production in fractured vuggy carbonate reservoir has unpredicted behaviors due to complex vug connectivity, variation of oil-water interface, and oil-water multiphase flow. Early water breakthrough and rapid increased of water cut as well as decreased of productivity are often encountered in this kind of reservoir. Experiments were conducted in vug-pore artificial cores with the size of 4.5 × 4.5 × 30 cm to study the influence of viscosity and different inclination angle on the oil recovery (OR). The influence of N2 and CO2 on the relative location of the water-oil/gas-water interface were compared. These results indicate that OR increases with the decrease of oil viscosity but not always with increase of inclination angle. Under CO2 conditions, however, influence of inclination angle on OR becomes more sensitive and gravity separation becomes more important factor at higher pressures.
Stainless steel tube with inner diameter (ID) of 1 mm was used to simulate the fracture. Time for CO2 and N2 gas cap formation are different as soon as the fracture width decreases to 1mm. For N2, the time increases with increase of pressure, and takes more than 50 hours as the pressure approaches to 4 MPa. While for CO2, however, a reverse behavior is observed and the time decreases to several seconds when pressure reaches to 8 MPa.
For vug carbonated reservoir with vertical connection of pore or fracture, CO2 can efficiently change the location of oil-water interface especially at smaller connecting space and produce much more oil. The N2 gas cap can maintain the high pressure to production layer due to its lower solubility both in oil and water.
Hence, great performance can be expected in the filed performance by injection of N2/CO2 gas mixture because of gas cap formation which attributes to pressure build-up and alteration of water-oil location by N2/CO2 injection.