Fabrication and Characterization of Oxalic Acid-Paraffin Microcapsules

To mitigate incomplete gel breaking of fracturing fluids and the “cold injury” caused by cold fluids during hydraulic fracturing in low-temperature, shallow reservoirs, we propose and validate a slow-release catalyst within an in-situ thermogenic system—oxalic-acid/paraffin microcapsules. Using a modified pan-coating method, oxalic acid was encapsulated with paraffin to produce microcapsules with varied core-shell ratios; their morphology, oxalic-acid loading, release behavior, and low-temperature gel-breaking performance were systematically characterized. The microcapsules exhibited a mean diameter of 200 μm, with the oxalic-acid cores effectively embedded in paraffin and pore channels enabling sustained release. Increasing the core fraction significantly raised oxalic-acid loading, whereas increasing paraffin during coating decreased it. All formulations showed a “rapid-then-slow” release profile, with higher loading yielding faster release. In an in-situ thermogenic fracturing-fluid system, neither the absence of a catalyst nor direct addition of free oxalic acid met gel-breaking requirements at low temperature. In contrast, introducing the oxalic-acid/paraffin microcapsules afforded well-regulated kinetics: the fluid maintained high viscosity during the initial stage (viscosity > 50 mPa·s over the first 60 min), followed by accelerated exothermic reaction that generated substantial heat and gas, rapidly elevated the temperature, and achieved complete gel breaking within 90 min (viscosity reduced to 3 mPa·s). These results demonstrate that the slow-release acid catalyst effectively overcomes gel-breaking limitations under low-temperature conditions, enhances post-fracturing flowback efficiency, and holds strong promise for field application.