The Applicable Limits of the High Strength Gel System to the Tight Sandstone Fractured Reservoir in CO2 Flooding

The matrix permeability of the tight sandstone reservoir is very low, which causes difficulty to inject water, the CO₂ flooding is applied widely to these reservoirs for the better oil displacement and low injection pressure, but natural fractures develop as well as artificial fractures exist in the tight reservoir, makes gas-channeling along the fractures severe when proceeding a CO₂ flooding process, blocking the fractures is necessary for displacing the residual oil trapped in the matrix. The self-developed tight sandstone fractured cores which can simulate different conditions of fracture width and matrix permeability had been used to evaluate the blocking effect of the high strength starch gel system, the mechanism that the high strength gel system blocked the fractures and activated the residual oil in matrix was analyzed by comparing the pressure when CO₂ broke through and the incremental recovery after blocking in CO₂ flooding experiments, then the applicable limits that the high strength starch gel system improved the oil displacement of CO₂ flooding in the tight sandstone fractured reservoir had also been discussed. The results showed that the high strength starch gel system could be used to block the fractures, and the blocking effect and the incremental recovery were the best when the fracture width was 0.42mm, however both effects became worse when the fracture width decreased to 0.08mm or increased to 0.65mm because of the injection limitation or the reduction of blocking effect; furthermore, the residual oil in the matrix could be activated effectively after the high strength starch gel blocked the fractures even if the matrix permeability reduced to 0.1mD, the research results can provide reference to the design and optimization of project when proceeding a CO₂ flooding process in the tight sandstone reservoir.