Propagation and retention of viscoelastic surfactants following matrix acidizing treatments in carbonate cores

Viscoelastic surfactants have been applied extensively in the field, because of their unique characteristics of forming rod-like micelles with the increase of pH and calcium concentration. There is continuous debate in the industry on whether the gel generated by these surfactants causes formation damage or not. This is especially true in dry gas wells. The objective of the present study is to conduct core flood experiments using surfactant-based acids and measure the concentration of the surfactant concentration in the core effluent. Material balance on the surfactant will shed lights on the amount of surfactant recovered, which will help in assessing damage due to surfactant gel retention in the cores. Core flood tests were performed using calcium carbonate cores 1.5 in. diameter and 20 in. length. The cores were injected with a surfactant-based acid of 15 wt% HCl that contained 7 vol% surfactant and other acid additives. Core flood test were conducted at constant injection flow rate that was varied from 3 to 40 cm³/min. Carbonate cores of various initial permeability were used. In all core tests, surfactant concentration was measured in the injected acid and core effluent using the two-phase titration method. Mutual solvent was used to break surfactant gels. Considerable amounts of viscoelastic surfactant were retained in the carbonate cores after the acid treatments, and the retained surfactant gel is a source of formation damage if not effectively removed. The amount of surfactant-based acid required to create wormhole in carbonate matrix is high at both low shear rate (< 900 s ^-1) and high shear rate (> 3,000 s^-1), and is low at intermediate shear rates with the minimum value at shear rate of 1,500 to 2,000 s^-1. The surfactant retention in the carbonate matrix is lowest at shear rates near 2,000 s^-1, and is high at both high and low shear rates. Surfactant was retained in gelled form. Mutual solvent was able to remove as much as 21% of the total amount of surfactant injected into the core.