While high working pressure and complex procedure restrict application of conventional foam fracturing, in-situ foam can overcome the limitations because it is liquid while pumping, reducing flow friction and dosage of special equipment. It gradually foams in the formation with large amount of heat released and pressure increased, improving flowback performance. Thus, this study developed an in-situ foam fracturing fluid stabilized by a novel microbial polysaccharide called diutan gum, evaluated its performance, and investigated its proppant suspension mechanism at high temperature.
First, based on the foam comprehensive value, the polysaccharide stabilizer and foaming agent systems of N2 foam and CO2 foam were selected separately. Second, the self-generated N2 systems and self-generated CO2 systems were screened in terms of gas production efficiency and rate. Third, on the premise of meeting compatibility, the selected foam systems and self-generated gas systems were combined, and necessary additives were introduced to prepare in-situ N2 and in-situ CO2 foam fracturing fluid systems, respectively. The stability and foaming ability of in-situ foams were evaluated at high temperature, and the optimal ones were selected. Then, the proppant suspension performance, heat and shear resistance, and viscoelasticity of the optimal ones were evaluated at high temperature, and this study tailored a method for evaluating proppant suspension performance of the in-situ foam fracturing fluid due to its difference from the conventional ones. Finally, based on experimental data and rules, the proppant suspension mechanism of in-situ foam fracturing fluid at high temperature was revealed.
The combination of diutan gum and AOS exhibited outstanding ability in enhancing the foam comprehensive value of both N2 and CO2 foam, and two kinds of CO2 foam and N2 foam systems with higher comprehensive values were selected respectively. The self-generated nitrogen and carbon dioxide systems with the highest gas production rate and efficiency were respectively selected, with the highest gas production efficiency reaching 95.9%. Thanks to these two excellent components, the in-situ N2foam volume reached 518mL which was 26 times of the base fluid of 20mL and remained 480mL within 90 minutes even at 70°C, demonstrating excellent foaming ability and foam stability. However, the stability of the in-situ CO2 foam was poor, as the foam volume dropped from 515mL to 250mL in just about 13 minutes. The in-situ N2 foam fracturing fluid obtained remarkable proppant suspension performance that with only 20mL of base fluid, it fully suspended 25mL of 70/140 mesh ceramic proppant for up to 120min, achieving proppant volume fraction as high as 55.6%. The in-situ CO2 foam could not even suspend 5mL of proppant, so it was eliminated and the in-situ N2 foam fracturing fluid was determined as the optimal system whose rheological properties was also extraordinary. After continuous shear for 2h at 70° and 170s−1, it maintained a viscosity of 59.4mPa·s, and it exhibited brilliant elasticity that its storage modulus was always greater than the loss modulus, ensuring its excellent proppant suspension performance. Ultimately, its proppant suspension mechanism was revealed in four stages.
The results suggest that the in-situ foam fracturing fluid stabilized by diutan gum obtains promising applications and is supposed to be further studied.
