When polymer-based acids can be used? A core flood study

In-situ gelled acids that are based on polymers have been used in the field for several years, and were the subject of many lab studies. An extensive literature survey reveals that there are conflicting opinions about using these acids. On one hand, these acids were used in the field, with mixed results. Recent lab work indicated that these acids can cause damage under certain conditions. There is no agreement on when this system can be successfully applied in the field. A core flood study was conducted using Indiana limestone cores (1.5 in. diameter & 20 in. long) at high temperatures and pressures. Propagation of the acid, polymer, and cross-linker inside the long cores was examined for the first time in detail. Samples of the core effluent were collected and the concentrations of calcium, cross-linker, and acid were measured. Permeability enhancement and location of any precipitation was detected using CT scanner to the core after, and before the acid injection. Different sections were cut from the core to investigate the propagation, and precipitation of polymer and cross-linker. Material balance was conducted to determine the amounts of cross-liker that retained in the core. Experimental results show that this acid system caused damage in some cases. At low shear rates, in-situ gelled acid formed a gel inside the core, as indicated by the cyclic behavior noted in the pressure drop across the core, and the acid changed direction inside the core several times. However, this gel significantly reduced the core permeability. At high shear rates, a smaller amount of gel was formed inside the core and the acid did change its direction inside the core. A significant permeability enhancement was achieved. Wormhole length increased as the shear rate was increased, while the diameter of wormhole increased as the cumulative injected volume of acid was increased. CT scan indicated the presence of gel residue inside the and around the worm hole. Gel residue increased at low shear rates. Material balance on the cross-linker indicated that a significant amount of the cross-linker was retained in the core. This amount increased at low shear rates.