Surface Modified Nanoparticles Based Novel Gelled Acid System - A Unique Formation Damage Free Well Stimulation Technology

Abstract

Gelled acid systems based upon gelation of hydrochloric acid (HCl) are extensively used in both matrix acidizing and fracture acidizing treatments to prevent acidizing fluid leak-off. The gelled-up fluid system helps retard the acid reaction to allow deeper wormhole propagation. Conventional in-situ crosslinked gelled acid systems consist of a polyacrylamide polymer, a crosslinker (such as iron-based crosslinker), a chemical breaker, other additives, along with acid. However, these systems can lead to damaging the formation due to several reasons including unbroken polymer residue or scaling, resulting in lowering of hydrocarbon productivity. To mitigate these drawbacks, we have developed a self-breaking, formation damage-free, novel nanoparticles based gelled acid system to replace the polymer based gelled acid system. The new gelled acid system is based on, surface modified nanoparticles to make them compatible in acidic environment, a gelation activator, acidizing treatment additives along with HCl to overcome the challenges the conventional systems face. The new system can work with up to 28& of HCl up to 300°F with low viscosity at surface, making it easy to be pump. As the acid spends due to reaction with the formation the pH of the fluid transitions from acidic to basic pH. The gelation phenomenon of the new system is controlled by the increasing pH. As the pH increases beyond pH 1 gelation of the nanoparticles occurs thus gelling up the acidic fluid. As the pH further continues to rise beyond pH 4 the nanoparticles lose their capability to gel up and the fluid viscosity decreases to pre-gelation level, facilitating easy post treatment flow back. A systematic experimental protocol was followed to develop the new system that is documented in this paper. It is shown that the gelation properties are pH dependent phenomenon providing the critical control over the gelation time and avoiding any premature gelation during pumping the treatment. The effectiveness of the system by not damaging the formation was investigated using regain permeability studies. The new system showed excellent regain permeability. The obtained data confirmed several advantages of the new system over conventional polymer based gelled acid systems. Gelation experiments were performed to gather a better understanding of the gelation mechanism and also to get effective control on the gelation and break properties. The uniqueness about the new system is that, it is formation damage free without the need to use polymers or iron based cross-linkers that may lead to potential damage mechanisms.