Abstract

Polymers used in completion and workover operations can consist of cellulose, cellulose derivatives, guar and guar derivatives. Normally these polymers are used in completion and workover fluids consisting of potassium, sodium and ammonium chloride. As more wells are being completed in deeper and hotter environments, fluids composed of calcium chloride, calcium bromide, zinc bromide and combinations of all three are being used to control well pressures.

Studies were conducted on hydration rates in various brines for these polymers and are presented in graphical form. A correlation was also established to predict the hydration rates and the ultimate viscosity for hydroxyethylcellulose at temperature based on brine viscosity. These correlations are particularly useful for field operations during cold weather months.

Introduction

Water-soluble polymers are extensively used in oil and gas operations. In high ionic completion fluids, many of these polymers do not dissolve or require long mixing times to dissolve and thicken the fluid. Darlington, et al, showed that dry hydroxyethylcellulose (HEC) added to a 13.0 ppg calcium chloride/calcium bromide solution had no increase in viscosity at 511 sec–1 from initial to four hours.1 Scheuerman determined hydration rates for HEC in completion fluids at various temperatures and densities and showed a relationship with time to maximum viscosity with brine density. Basically, the greater the brine density and the lower the temperature the greater the hydration time. Since completion fluids can be prepared with various crystallization temperatures and brine viscosities having the same density, Scheuerman's relationship can only be used as a rough guide.

Polymer systems are used in a variety of applications in the oil industry including loss circulation material, push pills, well bore clean out pills, gravel pack operations and components of drill-in fluids. Unfortunately, damage may be introduced to the formation due to improperly hydrated polymers. The occurrence of "fish-eyes" has been well documented.2 Additionally, failure to fully hydrate the polymer is an all too common occurrence when working with weighted brines.

Both Darlington and Scheuerman along with others developed prehydrated HEC slurries to decrease the mixing time for thickening brines.1–4 Vollmer, et al, showed a similar situation with xanthan gum by using the prehydrated form to viscosify a 13.1 ppg potassium formate solution.5

Due to the ease of mixing, liquid polymer systems have enjoyed wide spread acceptance and are operationally preferred to their dry polymer counterparts. These systems vary widely in base carrier fluids and polymer activity level. These liquid polymer systems must be able to allow the polymer to effectively disperse in the base fluid before hydration begins avoiding the possibility of "fish eyes". Additionally, environmental concerns regarding oil and grease content in the systems must be considered. With the wide variety of liquid polymer systems and diversity of base mixing fluids, the possibility for introducing formation damaging mechanisms increases if the polymer is not allowed to completely disperse and hydrate.

After reviewing the work of Darlington and Scheuerman, a relationship was observed with brine viscosity, i.e., the greater the brine viscosity the longer the hydration rate. Therefore, work was conducted to determine the limits for various water-soluble polymers to viscosify conventional completion fluids by relating the brine's viscosity to the polymer's hydration rate. Since the brine's viscosity changes with temperature this would elucidate why heating typically increases hydration rates. This relationship is important when deciding on slurry types for field use.

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