It is well known that the efficiency of regular acid treatments goes through a maximum at an optimum acid injection rate where it gives the maximum wormhole propagation rate (ratio between wormhole length to pore volume of acid required to breakthrough). This rate is a strong function of the acid-rock reaction: diffusion of the acid to the rock, surface reaction, and mass-transport of the reaction products. Adding a polymer and other additives to the acid may affect the wormhole propagation rate. However, literature survey didn’t highlighted in details how the wormhole propagation rate will change when polymer and crosslinker will be existing with the regular acid. Therefore, the objective of this study is to determine the wormhole propagation rate of gelled and in-situ gelled acids as a function of the injection rate. The effects of temperature, lithology, and initial permeability on the wormhole propagation rate were considered in this work.
A coreflood study was conducted using calcite cores at temperature range 100 to 250 °F. Injection rate was varied from 1 to 20 cm3/min using 5 wt% HCl regular, polymer gelled, and in-situ gelled acids. Calcium, cross-linker, and acid concentrations were measured for the core effluent samples.
The wormhole propagation rate of gelled acid required was higher than regular acid where gelled acid required less acid volume to breakthrough. However, the wormhole propagation rate of in-situ gelled acid required was lower than regular acid where in-situ gelled acid required a higher pore volume to breakthrough. Three regions were obtained for the wormhole propagation rate of in-situ gelled acid as a function of the injection rate: Region I, where acid injection rate is low, and in-situ gelled acid wasn’t able to breakthrough from the cores at injection rate less than 5 cm3/min. Region II, where the acid volume required to breakthrough decreased as injection rate increased in the range between 5 and 10 cm3/min. Region III, where the acid volume injected increased with the injection rate at injection rate higher than 10 cm3/min. The optimum injection rate wasn’t affected by temperature, or initial core permeability. The volume required to breakthrough decreased as the temperature was increased, and as the initial core permeability increased.