RIASSUNTO
Summary
High molecular weight polymers are routinely used to viscosify brines for drilling, workover and completion operations. Of the commercially available polymers, hydroxyethyl cellulose (HEC) is among the most versatile and widely used product. HEC has been used in NaCl, NaBr, KCl, NH4Cl, CaCl2, and CaBr2 brines over a wide range of brine density, pH environment, and temperature. Typically, the performance issues addressed in HEC studies focus on dispersibility, hydration rate and viscosity development, rheology at elevated temperature, and formation damage. Under typical oil field conditions, the published data indicates HEC produces minimum formation damage and provides predictable rheological behavior when properly prepared.
Recently, an evaluation of HEC stability at elevated temperature was performed in high density brines using commercial sources of HEC and common brine compositions. Results of this study indicated HEC solubility was drastically reduced at elevated temperature in specific brine compositions. The HEC-viscosified fluids were prepared using controlled, laboratory mixing procedures to simulate field mixing conditions and yielded fluids with the expected properties at room temperature. However, upon heating, the HEC precipitated from solution producing an unviscosified aqueous phase and a large mass of insoluble polymer. To further define the conditions causing HEC precipitation, testing with a variety of brines was performed over a wide temperature range. From these data, empirically derived HEC-solubility diagrams were developed to illustrate single-salt brine compositions and temperatures that cause HEC precipitation. To quantify the extent of damage that could be caused by HEC precipitation, core flow testing was performed with HEC-viscosified brines simulating conditions encountered during various HEC applications. Background
HEC is a commonly used gelling agent for brines in many completion and workover operations. HEC is often recommended to viscosify carrying fluids for gravel packs, circulating fluids for washing particulate from wells, and kill pills to limit fluid loss. HEC is prepared from cellulosic fibers that have been reacted with ethylene oxide to impart the hydrophilic properties necessary for use in aqueous fluids. Although HEC is nonionic, the hydroxyethyl functional groups are sufficiently polar to promote water solubility. This nonionic polymer is reported to be relatively insensitive to salt concentration and is often used in concentrated brines containing NaCl, KCl, and CaCl2, as well as many other salts.
HEC is provided as either a dry powder or a liquid concentrate (powder slurried in a non-hydrating liquid, such as diesel or glycol). HEC gels are prepared by dispersing the polymer in an aqueous fluid and allowing the polymer to hydrate or yield. The rate of hydration (viscosity development) is controlled by a number of factors including brine composition, temperature, and brine pH.
To properly prepare HEC-viscosified fluids, the polymer must be completely dispersed in the brine prior to hydrating. If hydration occurs before dispersion, large clumps of dry powder are encapsulated by a very viscous, highly concentrated layer of polymer. These lumps, commonly called ""fish-eyes"" are difficult to break up and can cause severe formation damage. In addition to ""fisheyes"", microgels can be formed in HEC viscosified brine. ""Microgels"" are microscopic aggregates of HEC polymer in the size range of 10 to 100 microns.