Chemical Modification of Biopolymers to Design Cement Slurries with Temperature-Activated Viscosification

Polymeric viscosifiers are added to cement slurries for a variety of reasons, including prevention of particle settling and control of fluid loss, gas migration, and free-water. Many of these functions are critically important after the cement slurry has been placed behind the casing but before the setting of the cement. Some functions, such as particle-settling prevention, are also important during the pumping phase. Unfortunately, most of the viscosifying polymers suffer from thermal thinning at bottomhole temperatures, especially under shear. The amount of polymer required to maintain the required level of viscosity at elevated bottomhole temperatures causes excessive surface-slurry viscosification at ambient temperature. Pumping such slurries can require higher pump pressures, or in cases where formation breakdown pressure might be exceeded. This becomes a serious challenge when the window between the fracture pressure and the pore pressure of the formation is narrow. It would be a significant improvement to oilfield cementing technology to develop polymers that do not cause excessive slurry viscosification on the surface but gradually increase the slurry viscosity as it reaches downhole temperatures, with the maximum viscosity reached at the time the slurry becomes static behind the casing. This paper describes an economical chemical method, not based on encapsulation, for modifying biopolymers and their derivatives—for example, hydroxyethylcellulose, xanthan, and guar—that renders them insoluble in cement slurries at room temperature (RT). When the cement slurries are heated, the slurries develop viscosity, as reflected by rheological measurements. The method also provides for increased viscosification efficiency of the modified polymers because of the increased molecular weights of the modified biopolymer products. Synthesis details, slurry rheologies at different temperatures, and job-placement simulation details are presented. A possible reaction mechanism that is operative in the chemical-modification step is also discussed. Introduction Designing cement slurries to meet all the requirements for effective primary cementing requires the addition of many additives, some of which are functional in the slurry placement and some in the set cement. The additives that are functional in the slurry phase are added primarily to retard/accelerate the cement setting, optimize the rheological properties (by particle dispersion), prevent settling, provide fluid-loss control, and, in some cases, provide gas migration control. The latter three functions are accomplished by water-soluble, synthetic or biopolymer-based polymeric additives. Such polymeric additives, either because of their molecular weights or adsorption on cement particles, tend to excessively increase the surface-slurry viscosities if added in quantities sufficient to be effective at bottomhole circulating or static temperatures, referred to as BHCTs or BHSTs. This is often the case because most of the polymers undergo severe thermal thinning at BHCT or BHST. As a result of high initial slurry viscosities, either slow pumping rates or higher pump pressures are required for slurry placement, which could be problematic in cases where the fracture gradients of the formation are low or where the window between the fracture pressure and the pore pressure of the formation is narrow. In such cases, a compromise between surface and bottomhole viscosities is attempted by the addition of a dispersant and adjusting the viscosifier-to-dispersant ratio. This approach, at best, is poorly effective and requires extensive slurry testing because of the unintended influence of such additives on other properties, such as set retardation/acceleration, fluid loss, or strength development. Additionally, it is well established that, with additives requiring adsorption of cement particles to function, adding more of the additive will establish a competition between the additives for adsorption on the cement-particle surface. Depending on the relative adsorption capability of the additives, a strongly adsorbing additive might block the adsorption of a polymeric additive; thus, preventing manifestation of a property at desired timing. As an example, if a dispersant is strongly adsorbed on cement, it might prevent the adsorption or displacement by another polymer added for providing fluid-loss control (Plank et al. 2006). Similar