Understanding and Managing the Natural Self-Breaking Behaviour of Xanthan in Reservoir Drill-In, Completion and Workover Fluids

Abstract

Xanthan gum is a non-damaging viscosifier and fluid loss control agent commonly included in reservoir drill-in, completion and workover fluid formulations. One of the key benefits offered by xanthan in these applications is that it is not an especially robust polysaccharide and under downhole conditions it will eventually self-break through molecular collapse and/or depolymerisation reactions. With only a temporary presence in the filter cake and formation invaded by filtrate, xanthan can never pose a serious or permanent threat to oil and gas production but clearly it would be good to be able to control and manage the rate at which it degrades. The rate at which xanthan degrades downhole is a function of the temperature, the presence of oxidants and the ionic environment. The purpose of the experimental program described in this paper was to measure the rate of natural self-breaking of xanthan under different temperature conditions when dissolved in various water and brine systems containing formate and halide ions. A simulated North Sea formation water and a low-sulfate seawater were included in the test program. The samples were dynamically and statically aged for periods up to 12 months, and their degree of natural self-breaking was tracked by viscosity measurements. Several other classes of polysaccharide were tested in the program, either alone or in combination with xanthan. The tests confirmed that at temperatures in the range 124–170°C (255–338°F) xanthan and the other polysaccharides all degraded over time and the resulting "broken" fluids had low viscosities. It was found that the self-breaking rate varied hugely with brine type and concentration. Concentrated formate brines, rich in antioxidants and high molar concentrations of water-structure making ions, allowed steady rates of polymer breaking over weeks and months while the same polysaccharides dissolved in brines containing significant amounts of sodium bromide degraded very quickly. These results suggested that clear brine systems of any density within the compatibility limits of the blended components could be engineered to self-break within a set period of time by blending formate and sodium bromide brines in appropriate ratios. Degradation tests at 124°C (255°F) of xanthan in a typical North Sea formation water and a low-sulphate seawater, showed very rapid self-degradation, resulting in hardly any remaining viscosity after only 4 days of ageing. It seems likely that xanthan gum and other polysaccharides that are stabilized in formate brines, will lose their viscosity rapidly if contacted by formation water or well injection water. In fact, an overflush of the filter cake and near wellbore formation with any low salinity fluid would make an effective breaker system for xanthan in the applicable temperature range. The learning points from this study offer a solution to the problem of filtrate retention as an artifact in laboratory coreflood tests of viscosified brine-based fluids. Not exposing the fluid to the reservoir temperature for a realistic time period between invasion and drawdown may leave some viscosified brine in the pore space and in the filtercake, that is hard to remove during the standard drawdown time. Such retained filtrate has an adverse effect on the core's water saturation and thereby on the effective permeability to hydrocarbons.