Guidelines for Using HEC Polymers for Viscosifying Solids-Free Completion and Workover Brines

Abstract

Summary Solids-free brines are used increasingly in well completion and workover operations. One technique to minimize downhole losses of expensive, high-density brine is to spot a pill of viscosified brine across the thief zone. Hydroxyethylcellulose (HEC) is the polymer used most frequently for this purpose. This report discusses the properties of HEC-thickened brines and presents guidelines for their use for completion and workover fluid-loss control. Introduction When using solids-free brines, well control is achieved by using a slight overbalance. With calcium chloride and lower-density brines, the fluid loss resulting from this overbalance often is ignored because the brine costs are relatively low. For example, saturated CaCl2 brine [11.6 lbm/gal) costs about $15/bbl. However, costs rise sharply with increasing brine density. For example, a 19-lbm/gal CaBr2/ZnBr2 brine currently costs about $820/bbl. There are two basic methods for minimizing fluid loss--adding solids or increasing brine viscosity. The traditional approach has been to add filter-cake-forming particles, preferably of an acid-soluble material such as particles, preferably of an acid-soluble material such as calcium or iron carbonate. There is increasing concern regarding our ability to remove these filter cakes adequately. The problem is that the zone where the acid first breaks through may be the only section that gets cleaned. Other sections of the wellbore then may not be exposed to sufficient acid and/or contact time for good cleanup. This problem is expected to be more severe when using iron carbonate (siderite, FeCO3), since its dissolution rate in acid is at least 30 times slower than that of calcium carbonate. More recently, the method of using solids-free viscosity pills is receiving increased attention. With this method, a clean brine pill is viscosified with a soluble polymer and spotted across the thief zone to develop a polymer and spotted across the thief zone to develop a bank of high-viscosity fluid in the formation, which will retard further fluid losses. As might be expected, there are a variety of factors related to both the polymer and brine that influence the effectiveness of viscosity pills in controlling fluid loss. This paper discusses some of these factors and presents data and guidelines that will be useful when employing viscosity pills to control fluid loss. Discussion The chemistry and rheology of polymer solutions are complex. Hence, within the limited scope of this study, we were able only to define trends and general-use guidelines.Polymer solution apparent viscosity is very shearsensitive.Increasing shear rate decreases viscosity and increases fluid loss.Increasing brine and HEC concentration increases viscosity and decreases fluid loss.Increasing temperature decreases viscosity and increases fluid loss.Increasing brine density above about 12 lbm/gal decreases the hydration rate of HEC polymer sufficiently so that heat and/or extended mixing times are required to develop full solution viscosity. Further discussion of these effects follows. JPT P. 306