A foot-scale high-temperature true-triaxial fracturing apparatus was used to conduct hydraulic fracture tests on rock samples. The fracturing tests used different fluids including water, CO2, CO2 with water, and CO2 with a 1% polyallylamine (PAA)/water solution. The tests were conducted with both constant pressure injection and constant flow rate injection modes. CO2-based fracturing fluids were found to produce higher breakdown pressures, high transient flow rates, and produce higher-conductivity fractures as compared to water-based fracturing fluids. Additionally, faster pressurization rates with CO2-based fracturing fluids (obtained when fracturing at constant flow rate mode) are found to be associated with higher fracture conductivities. When fracturing with CO2-based fluids in the presence of a PAA aqueous solution, for example if the rock is saturated with 1wt% PAA in water, the volume expansion caused by CO2-induced cross-linking of PAA leads to a faster pressure increase due to the associated volume expansion and increase in viscosity. It was also found that CO2 as a fracturing fluid injected in hot dry rock (HDR) attain the highest fracture conductivity only when injected at very high flow rates, followed very closely by the CO2/PAA fracturing fluid system that generates fractures with, on average, similarly high conductivity values though independently of injection flow rate and using 1/6 of the volume of CO2 as compared to CO2 in HDR. Breakdown pressures were also similar for CO2 stimulation in HDR and CO2/PAA fluid system under identical injection flow rates. Finally, the well-known low viscosity of CO2 phase prevents the efficient transport of proppants while the reacted CO2/PAA fluid system is known to form a high viscosity binary fracturing fluid system with the potential to carry proppant in addition of being a fine fracturing fluid.

Press the Back button in your browser, or search again.

Copyright 2020, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.