Nanocellulose as a New Degradable Suspension Additive for High-Density Calcium Brines

Abstract

Abstract Calcium-based brines, principally calcium chloride and calcium bromide, find widespread use as completion fluids and base fluids for reservoir drill-in fluids (RDF). The high ionic strength of these brines, combined with the chemical compatibility issues inherent with divalent ions, such as calcium, makes identifying additives for developing functional fluids challenging. While additives, such as hydroxyethyl cellulose, can provide viscosity to calcium brines, imparting the suspension properties needed for the transport of solid bridging agents or drill cuttings can be particularly problematic in divalent brines. Even the widely used and potent suspension agent, xanthan gum, in addition to being difficult to degrade, suffers from gelation and precipitation under certain conditions in high-calcium environments. In this study, nanocellulose, which has previously shown promise as a suspension additive in freshwater and low-salinity brines, was evaluated in high-density brine fluids. Extensive laboratory evaluation of the commercially sourced nanocellulose was conducted in sodium chloride, calcium chloride, and calcium bromide brines. Rheological determinations were made of simple systems containing only brine plus the novel additive, as well as fully formulated RDF containing the additive with sized calcium carbonate bridging solids, alkalinity, and filtration control additives. This data was compared before and after aging at various temperatures and was further compared to other biopolymers that are used today in divalent brines. Stability of the fluids and the capability of the formulated fluids to suspend solids were also examined. Fluid-loss control of full formulations was compared under API conditions. Single-phase return permeability studies were conducted. While results in sodium brines were mixed, nanocellulose was shown to outperform existing biomaterials in calcium brines. Nanocellulose was shown to be readily hydrated in the challenging high-density divalent environment and provided superior suspension to currently used fluid components at similar concentrations. Rheological performance of nanocellulose was found to be synergistic with bridging solids. In one particularly interesting example, it was also shown to complement a synthetic polymer in a high-temperature calcium bromide application. Nanocellulose provides superior rheological performance and reduced formation damage potential compared to conventional materials in difficult high-density calcium chloride and bromide brines. The improved rheological profile and enhanced thermal stability of fluids formulated with nanocellulose, combined with a greater diversity of removal options, can facilitate expansion of the utility of calcium-based brines in well construction and completion applications.