Xanthan gum is the staple viscosifier and fluid loss control agent used in reservoir drilling and completion fluids. In over 40 years of use in this application xanthan has built up a deserved reputation for reliable performance in drilling and completion fluids, and for generally minimizing damage to reservoir formations. Its only peculiarity is its low transition or melting temperature (Tm) in low salinity fluids and in bromide brines, meaning that its viscosity can collapse very suddenly at 70-120°C. This can be a nuisance in applications where viscosity maintenance at high temperatures is important, but a benefit if the application requires a self-breaking polymer.
Previous research has shown that formate brines are capable of increasing the thermal stability of xanthan gum by increasing its Tm and by providing anti-oxidant protection. The extent to which the thermal stability of xanthan may be increased depends primarily on the type and concentration of the alkali metal formate salt in solution. The most effective brine from this perspective is a very concentrated potassium formate brine, which can increase the Tm of xanthan to over 200°C (396°F) and raise the 16-hour thermal stability to almost 180°C (356°F). The objective of the study described in this paper was to investigate if it was possible to use combinations of commercial polymer stabilizing additives to further increase the temperature ceiling of xanthan gum dissolved in formate brines. The opportunity was also taken to look at the high-temperature behavior of xanthan in rubidium formate brine and cesium acetate brine.
Before starting on the additive screening program the thermal stability of xanthan in formate brine was compared against other well-known natural biopolymers used in drilling and completion fluids. The tests confirmed that xanthan really was the best performing viscosifier in buffered and alkaline formate brines, although welan showed some promise in unbuffered formate brines at neutral pH. Tests also showed that there was no significant difference in thermal stability between six different commercial xanthan products in formate brines.
The transition temperature of xanthan in cesium formate brine was found to reach a maximum of almost 180°C (356°F) at a brine density of 2.20 g/cm3 (18.4 lb/gal) compared with almost 200°C (396°F) in 1.57 g/cm3 (13.1 lb/gal) potassium formate brine, and blends of the two brines produced Tm values in the range 180-200°C (356-396°F). High-density rubidium formate brines raised the xanthan Tm to around 185°C (365°F) at a density of 2.11 g/cm3 (17.6 lb/gal). Hot-rolling tests of cesium acetate brines viscosified with xanthan have shown that cesium acetate is able to stabilize xanthan to similar temperatures as cesium formate.
The best additive package for increasing the thermal stability of xanthan in potassium formate brine was found to be a blend of magnesium oxide and 5% v/v of a polyethylene glycol. Potassium formate brine containing xanthan and this additive package was found to retain some viscosity after hot rolling for 16 hours at at least 194°C (381°F). It seems likely that the glycol acted as a sacrificial scavenger, mopping up free radicals before they could attack the xanthan. This finding validates a suggestion from work carried out in 1980’s that the best thermal stabilizer package for xanthan would include a concentrated salt brine containing a glycol and an oxygen scavenger
Deployment of the stabilizer package identified in this study should raise the thermal stability ceiling of xanthan in buffered formate-based non-damaging drilling and completion fluids by more than 20°C (36°F).