Application of high temperature clean CO2 foam fracturing fluid in shale gas reservoirs

In order to efficiently develop special fractured oil/gas reservoirs with low pressure, low permeability, strong water sensitivity and easy water locking, the multipolymer, viscosity synergist, modifiers, etc. are hence mixed with CO2 to form a high temperature clean CO2 foam fracturing fluid (BCF fracturing fluid). Comprehensive performance test was conducted to the fracturing fluid system and the test result shows that BCF foam fracturing fluid is capable of resisting a temperature of up to 140 ℃, under 90 ℃, the foam has good sand-carrying performance. Its half-life period is over 5 h. The filtration coefficient of foam fluid is equivalent to that of guar gel. In addition, the gel breaking in fracturing fluid is complete, and gel breaking fluid has as low as 1 mg/L of residues and its surface tension is below 24 mN/m. BCF fracturing fluid system was successfully applied to YY2 shale gas well in Yanchang Oilfield. The sand adding successful rate was 100%, and the open gas production of single well after well flow was 1.0×105 m3/d, which was three times that of adjacent well. Tests indicate that this fracturing fluid has some advantages over conventional CO2 foam fracturing fluid and is of great significance to efficient development and protection of special oil/gas reservoirs.