RESUMO Testes de cisalhamento utilizando um viscosímetro de cilindros coaxiais foram realizados no intuito de se determinar as propriedades reológicas de dispersões de goma xantana com bentonita sódica em presença de diferentes concentrações de cloreto de sódio (NaCl) e um valor fixo de concentração de cloreto de cálcio (CaCl2) à temperatura ambiente. A reologia das dispersões com diferentes concentrações de NaCl e bentonita sódica foram preparadas fixando as concentrações de goma xantana e CaCl2 em 0,47% e 0,455% respectivamente (ambas as porcentagens em massa) e utilizando água deionizada como meio dispersivo. As dispersões mostraram um bom ajuste ao modelo reológico de potência modificado (Herschel-Bulkley) através de regressões não lineares. Foi feito um Planejamento Composto Central (PCC) em dois níveis no qual as variáveis independentes foram as concentrações de bentonita sódica e cloreto de sódio e as variáveis dependentes foram o limite de escoamento (τ0), o índice de consistência (k) e o índice de fluxo (n). As amostras foram preparadas sob agitação com 24 horas de antecedência e as medições dos pontos da curva reológica foram obtidas através do viscosímetro Haake. Os resultados da análise mostraram que a tensão limite de escoamento é fortemente influenciada pela concentração de bentonita enquanto que os índices de consistência e fluxo são influenciados tanto pela concentração de bentonita como pela concentração do cloreto de sódio. Esses resultados mostram que é possível melhorar o desempenho de fluidos de perfuração argilosos através da manipulação do teor de sais e argila no mesmo.

ABSTRACT Shear tests using a coaxial cylinders viscometer were performed in order to determine the rheological properties of xanthan gum dispersions with sodium bentonite in the presence of different concentrations of sodium chloride (NaCl) and a fixed concentration of calcium chloride (CaCl2) at room temperature. The dispersions rheology with different concentrations of NaCl and sodium bentonite were prepared by fixing the xanthan gum and CaCl2 concentrations of 0.47% and 0.455% respectively (both mass percentages) using deionized water. The dispersions showed a good fit to modified power rheological model (Herschel-Bulkley) by non-linear regressions. A Central Composite Design was made for two levels. The independent variables were sodium bentonite and sodium chloride concentrations and the dependent variables were the yield point (τ0), the consistency index (k) and the flow index (n). Samples were prepared with stirring 24 hours in advance and the measurement of the rheological curve points were obtained by Haake viscometer. The results of the analysis showed that the yield stress is strongly influenced by the concentration of bentonite while the consistency and flow indexes are influenced by both sodium chloride and sodium bentonite concentrations. These results show that it is possible to improve the performance of clay drilling fluids through manipulation of salts and clay content.

Shear tests using a coaxial cylinders viscometer were performed in order to determine the rheological and thixotropic properties of xanthan gum dispersions with sodium bentonite in the presence of high salt concentrations of sodium and calcium chlorides. The dispersions rheology with different temperatures and concentrations of NaCl, CaCl2 and xanthan gum were prepared by fixing the bentonite concentration of 1%. The dispersions showed a good fit to power law model through linear regressions with the lowest coefficient of determination value of 0,98. A screening was made initially to decide which independent variables potentially affect the rheological and thixotropic parameters. The independent variables were temperature, the xanthan gum and sodium and calcium chlorides concentrations. Then a full factorial design in duplicate with the remaining variables on the same tracks was performed. The results eliminated calcium chloride as an independent variable and revealed a strong dependence of the rheological and thixotropic parameters with temperature and xanthan gum and NaCl concentrations.