An experimental research on gas-injection huff and puff recovery after the cold heavy-oil production of foamy-oil reservoirs

Taking degassed oil and active oil as a reference, an unconventional PVT experiment was carried to reveal features of gas dissolution in foamy oils and to define the dissolution capacity of natural gas in foamy oils and influential laws of formation pressure on gas dissolution. In addition, the feasibility of gas huff and puff development in enhancing oil recovery was evaluating by means of a core displacement experiment, and effects of gas-injection occasions and cycles on gas huff and puff development of foamy oils were also defined. The experimental results showed that the process of natural gas dissolution can be divided into three stages, i.e. rapid dissolution, fluctuation decline and stable stages. At the early stage of gas injection, the gas dissolution rate is relatively great and the volume of cumulative dissolved gas increases rapidly. The dissolution capacity of natural gas decreases gradually with depth increasing, which can form a certain vertical miscible area. An increase in formation pressure can increase the gas dissolution rate and volume of cumulative dissolved gas. The dissolution capacity of natural gas in foamy oils is smaller than that in active oil and degassed oil under different pressure, but the total gas volume in foamy oils is the highest and the viscosity of foamy oils is lowest at the same depth. Compared with cold production, gas huff and puff experiment demonstrated that gas huff and puff development for three cycles can increase the oil recovery by 7.8%. The gas injection occasion should be between bubble point pressure and quasi-bubble point pressure. Soaking time should not be too long, and the formation pressure be increased maximumly.