A study of diversion using polymer-based in-situ-gelled acids systems

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. These acids used in stages to block the treated zone and forcing the next regular acid stages to the untreated zones. 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. On the other hand, recent lab work indicated that these acids can cause damage under certain conditions. The ability of polymer-based in-situ gelled acids to divert regular acids was studied using parallel core flood setup. The sequence of the injection involved In-situ gelled acid at 5 wt% HCl stage followed by regular acid at 15 wt% HCl until acid breakthrough. Experimental results show that even for nearly the same permeability formation (1:1.1), regular acid preferred to flow only through the high permeability zones. Therefore, diversion was needed for all acid treatments. At an injection rate of 1 cm³/min, in-situ gelled acid plugged the two cores. For low permeability contrast (1:2), polymer was able to divert the acid with permeability enhancement in both cores. For high permeability contrast (1: 20 up to 25), in-situ gelled acid was able to divert the acid only at injection rate less than 10 cm³/min. However, at higher injection rates, in-situ gelled acid was not able to build enough pressure that could force the regular acid into the low permeability core. Increasing the injection rate reduced the viscosity of the in-situ gelled acid. Therefore, acid injection rate should be determined based on the expected fluid viscosity in the formation. The results obtained can be used to better design acid treatments in carbonate reservoirs.