Millimeter-sized (10 μm~mm) particle gels have been used widely to control water flow through super-high-permeability zones and fracture zones in mature oil fields. During particle gel extrusion into target zones, the gel can form a cake on the surface of low-permeability, unswept formations. This cake reduces the effectiveness of conformance control as well as the amount of oil that can be recovered from unswept oil formations. Thus, we evaluated the effectiveness of using hydrochloric acid (HCL) to remove gel cakes induced during conformance-control treatments.
The interactions between HCL and particle gels were evaluated to understand the swelling, deswelling, and the gel strength after adding acid. A Hassler core holder was then used to determine the core permeability after gel and acid treatments. Gels swollen in brine concentrations of 0.05%, 1%, and 10% were injected into a sandstone core having a variety of permeabilities. Brine was then injected in cycles through the gel into the core. The core permeability was measured after gel particle injection and after the core surface with the gel cake was soaked in the acid solution for 12 hr. The results indicate that particles swollen in brine concentrations of 0.05% caused more damage than those swollen in higher concentrations of brine. The damage increased as the core permeability increased for all the swollen gels. The results also show that HCL removed the gel cake effectively, and varying HCL concentrations did not exhibit a significant difference in the gel cake-removal efficiency. The gel was found to swell much less in HCL solutions than in brine. After it was deswollen in acid, the gel strengths were measured and found to be higher than those swollen in brine. This work concludes that HCL can be used effectively to mitigate the damage induced by particle gels.