In certain wells where relatively high levels of iron are present, the use of polyacrylamide-based friction reducers (FR) for hydraulic fracturing completions can lead to poor performance and negative chemical interactions including the formation of unusual semi-solid accumulations. The accumulations, often referred to as "gummy bears" due to their rubbery texture, can form in surface and downhole equipment and can inhibit well production. This paper summarizes work performed to evaluate the performance of FRs in the presence of iron, identifies the specific causal factors for the formation of the accumulations, and provides practical solutions to mitigate the problems associated with the negative iron impact in order to improve overall well performance.
Iron can present itself during fracturing operations in different forms and from different possible sources including source water, tubulars, and within the rock formations themselves. To study the interactions between iron sources and anionic friction reducers, synthetic and field water sources were used to identify and quantify the negative effects that iron has on performance parameters for FRs and viscosifying friction reducers (VFR) such as friction reduction, viscosity development and the development of polymer accumulations. The second portion of this paper is given to identify methods to improve overall FR performance and to mitigate the risk of developing the accumulations in iron-rich environments. Field case histories are presented to support the results of this work.
Over the years, the oil and gas industry has documented many of the detrimental effects that iron can have on well completion operations. Iron sulfide scale, for example, is the result of hydrogen sulfide and iron interacting with each other and can lead to problematic issues including loss of injectivity in water injection and disposal wells, plugging of artificial lift mandrels and perforations, reduced reservoir permeability, and other mechanisms that can limit overall well production (Nasr-El-Din et al, 2001). In hydraulic fracturing operations, the presence of iron can also have negative effects on the performance of fracturing fluids. Many, if not most, of the polyacrylamide polymers used in fracturing operations are negatively charged (anionic) in nature. When positively charged ions such as calcium (Ca2+), magnesium (Mg2+), ferrous iron (Fe2+), or ferric iron (Fe3+) come into contact with the negatively charged polymer, the result is usually a reduction in the overall performance of the polymer.