Engineering a Synthetic Friction Reducer to Combat Undesirable Formation of FR-Metal Complex/Precipitation in Slickwater Fracturing

Abstract

Abstract During stimulation and production, a highly viscous and rubbery precipitation can form due to incompatibility of friction reducer polymers (cationic, anionic or amphoteric) with ferric ions, particularly in formations with high iron content. This material plugs up proppant packs, even production strings, and is extremely detrimental to well productivity. A straightforward sequestration approach with chelants does not work because of poor outcome and prohibitive economics. Compatible biopolymer FRs, as an alternative approach, have limited applications due to their moderate FR performance compared to synthetic PAM based polymers. This work shows the development of a novel synthetic friction reducer to address this challenge. The polymer was designed by systematically optimizing monomer compositions, molecular weight and surfactant packages. Friction reduction performance of the newly developed FR was evaluated in friction loops under various water conditions. Iron tolerance tests were performed by mixing ferric iron with prehydrated FRs under different pHs, at high concentrations, and salinities. The mixture solutions were then placed in a water bath for heat treatment to simulate downhole conditions and to accelerate the formation of the ferric/FR complexes. Comparative experiments were performed using conventional FRs. In order to probe the interaction between polymers and the iron species, zeta potential analyzer was applied to measure charge changes of the polymer strands. The newly developed FR showed superior FR performance with fast hydration and high overall friction reduction, in both fresh water and synthetic brines. In iron tolerance tests, rubbery precipitations formed in solutions for all three types of conventional FRs, while no such precipitations were observed with the newly developed FR, even in the presence of 500 ppm ferric ion. This test was repeated in a wide range of pH and salinity conditions and no significant viscosity change of the FR polymer solution was observed before and after the test. Zeta potential measurements confirmed the validity of the polymer design to minimize the interaction between the new FR polymer and iron ions. This paper demonstrates that the newly developed friction reducer successfully solves the incompatibility issue of FRs with iron spices, i.e., without flocculation on the surface or formation of gummy precipitations downhole. Its superior friction reduction performance with no concerns of potential damages make it a strong candidate for iron-rich fields. Mechanism of the interaction between iron and synthetic polymers is proposed and confirmed by zeta potential results. The manuscript discusses in depth the strategy of the design of the newly developed copolymer, including selection of monomers, molecular weight control, and inverting surfactants.