Water-based drilling fluids: the contribution of xanthan gum and carboxymethylcellulose on filtration control

This study aimed mainly to evaluate the influence of xanthan gum (XG) and carboxymethylcellulose (CMC) in the filtration process of water-based drilling fluids, considering the conformational changes suffered by the polyelectrolyte with the addition of salt (NaCl) in different concentrations (0.17, 0.34 and 0.51 M). It was also evaluated the behavior of the fluid by the addition of calcite (calcium carbonate, CaCO₃). Sixteen drilling fluids were prepared with the same xanthan gum concentration (major application as thickener to transport the cuttings), but different salinities and having or not CMC and calcite at constant concentrations. This strategy was adopted to evaluate the real contribution of each additive in the control of filtration performance, frequently unclear in the papers. In general, the fluids prepared in all brines showed lower viscosities and higher filtrate loss compared to fluids of sweet water. This effect was more strong in fluids containing only XG. The addition of CMC enhanced the viscosity and reduced the filtrate loss. However, the best results were obtained when calcite was added. The results indicated that XG contribution as thickener is not enough to control efficiently the fluid filtration even in the presence of calcite. However, the addition of CMC and calcite to XG salt solution increased the viscosity and decreased significantly the filtrate loss. This result was attributed to synergistic interactions between XG, CMC and calcite.