Production decline in acid-fractured reservoirs result from the elastic, plastic, and creeping response of highly confined vertical stresses that can destroy the conductive channels created by the acid. This paper introduces a novel, highly viscous acid system having the potential to suspend proppant and perform simultaneous acid and proppant fracturing treatments.
During production, impure carbonates tend to decrease productivity as quartz and other minerals migrate, leading to channel closing and/or collapse. Conventionally, acid fracturing treatments are performed by fracturing without proppant followed by different acid stages, including gelled acid. The idea of proppant-fracturing-acidizing (PFA) system entails fracturing in a single stage operation, although to avoid post-treatment plugging of the propped fractures the application of the process would be limited to very clean carbonates. This paper describes PFA fluid prepared by emulsifying hydrochloric acid (HCl) in the internal phase using diesel as the external phase. PFA offers distinct advantages compared to conventional acid systems, namely high viscosity to carry proppant, simultaneously acid-etching the rock, while attaining deeper live acid penetration. Another potential advantage is lowering operational time because mixing can be performed on-the-fly.
Preliminary experimental studies have been carried out to investigate the performance of PFA fluid and validate its applicability to perform simultaneous acid and proppant fracturing. PFA was prepared at various HCl concentrations and tested up to 28%. PFA fluid viscosity under different shear rates was measured at high-pressure/high-temperature (HP/PT). It exhibited viscosity as high as 2424 cp at 100°F and 300 cp at 300°F at 50 1/s shear rate. The oil external nature and the stability of the PFA fluid were examined using HP/HT autoclaves at 300°F. The effectiveness of corrosion inhibitors in PFA fluid was determined at 300°F. Static proppant settling tests showed that PFA fluid has good proppant carrying capacity at different particle loadings. After spending PFA fluid in carbonate, the viscosity was reduced significantly, exhibiting breaker-free cleanup.
Carbonate reservoirs having closure stress of more than 5,000 psi could be propped to enhance conductivity using PFA system. Enhancing the stimulated reservoir volume could be plausible using PFA system as a single-fluid treatment option, particularly if acid leakoff control and diversion, deeper penetration, and acid and proppant fracturing can be simultaneously implemented.