Effect of Shear Rate on the Propagation of Polymer-Based In-Situ-Gelled Acids Inside Carbonate Cores

Abstract

Summary In-situ-gelled acids that are based on polymers have been used in the field for several years and were the subject of many laboratory studies. An extensive literature survey reveals that there are conflicting opinions about using these acids. These acids were used in the field with mixed results. Recent laboratory work indicated that these acids can cause damage under certain conditions. There is no agreement on when this system can be applied successfully in the field. A coreflood study was conducted using Indiana limestone cores (1.5-in. diameter and 20-in. length) at high temperatures and pressures. Propagation of the acid, polymer, and crosslinker inside the long cores was examined in detail. Samples of the core effluent were collected, and the concentrations of calcium, crosslinker, and acid were measured. Permeability enhancement and location of any precipitation was detected using a computed-tomagraphy (CT) scanner on the core before and after the acid injection. Different sections were cut from the core to investigate the propagation and precipitation of polymer and crosslinker. Material balance was conducted to determine the amount of crosslinker 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 reduced the core permeability significantly. At high shear rates, a smaller amount of gel was formed inside the core, and the acid's ability to change its direction inside the core was less than that noted at low shear rate. A significant permeability enhancement was achieved. The wormhole length increased as the shear rate was increased, while the diameter of the wormhole increased as the cumulative injected volume of acid increased. CT scan indicated the presence of gel residue inside and around the wormhole. Gel residue increased at low shear rates. Material balance performed on the crosslinker indicated that a significant amount of the crosslinker was retained in the core. This amount increased at low shear rates.