Viscosification of Oilfield Brines: Guidelines for the Prevention of Unexpected Formation Damage

Abstract Polymer viscosified oilfield brines have been accepted by the industry and routinely used to transport sand in gravel pack applications, to control fluid loss to the formation, and in other oilfield applications. However, recent observations of precipitate formation from heat aged viscosified brines indicated that the thermal stability of viscosified oilfield brines can vary dramatically. Variations were observed not only for the same polymer in different oilfield brines, but also for the same polymer from a different source in the same brine, and even for the same polymer in different concentrations of the same brine. This paper details the systematic study conducted to identify and map the safe operating environments for selected oilfield polymers used to viscosify standard oilfield brines. Polymer viscosified brines were prepared by standard procedures, and statically heat aged at prescribed temperatures. The clarity of aged samples, including precipitation and separation tendencies, retained viscosities, and return permeability measurements were used to determine thermal stability. Systematic polymer-brine composition-temperature relationships were observed, and certain combinations were found to be a significant source of unexpected polymer precipitation and formation damage. Results from this study provide the basic guidelines for the selection and field application of polymer viscosifiers for oilfield brines presented in this paper. Details concerning the viscosification process, its impact on TCT (True Crystallization Temperature), and the viscosification of zinc based brines are outlined. A two step viscosification process for brines containing low zinc concentrations is presented.