Development of a Novel Cationic Polymer-Based In-Situ Gelled Acid System That Improves Acid Diversion in a Heterogenous Carbonate Reservoir

Abstract

Viscosified acid systems have been used for acid diversion in heterogeneous carbonate reservoirs for several decades. However, the anionic polymers used in these systems are difficult to clean post treatment, leading to formation damage. This work evaluates a new cationic-polymer acid system with self-breaking ability for application as an acid diverter in carbonate reservoirs. The apparent viscosities of the live and the partially neutralized acids at pH from 0 to 4.5 were measured as a function of the shear rate (1 to 1,000 s^-1). The effects of salinity and temperature (80 to 250℉) on the rheological properties of the acid system were also studied. The viscoelastic properties (G’ and G”) of the gelled acid system were evaluated using an oscillatory rheometer. Single coreflood experiments were conducted on Indiana limestone cores to study the nature of diversion caused by the polymer-acid system. The impact of permeability contrast on the process of diversion was investigated by conducting dual coreflood experiments on Indiana limestone cores which had a permeability contrast of 1.5-20. CT scans were conducted to study the propagation of wormhole post acid injection for both single and dual corefloods. The live acid system (5 wt% HCl and 2.5 vol% polymer concentration) displayed a non-Newtonian shear thinning behavior with the viscosity declining from 117 to 19.4 cp when the shear rate increased from 10 to 500 s-^1 at 250oF. Spent acid tests showed an increase in viscosity once the pH reached 2. The viscosity of the system increased from 117 cp at 10 s^-1 at pH 0 to 185.8 cp at pH 2. The gel exhibited a strong elastic behavior (G’- 5.18 Pa at 1 s^-1). The viscosity of the system continued to increase until it broke down without a breaker to 69 cp (at 250oF and 10 s^-1) at a pH of 4.8, providing a self-breaking system which prevents formation damage due to polymer precipitation. Coreflood studies indicated that the wormhole propagation and the diversion processes are dependent on the temperature and the flow rate. There was no indication of any damage caused by the system to the cores. The strong elastic nature of the gel generated by the partially neutralized acid system proves its suitability as a candidate for use as a diverting agent. The system offers an improvement over existing polymer-based acid systems because of its self-breaking ability and lack of precipitation, thus preventing formation damage.