A NEW METHOD FOR EVALUATING STABILITY OF HIGH DENSITY OIL-BASED DRILLING FLUID

摘要

High density oil-based drilling fluid can reduce the emulsion dispersion capacity due to high solid phase content, thus, resulting in the complex situation of drilling fluid stratification and settlement instability presenting and suspension instability, which can make the rheological property of drilling fluid hard to control, causing sinking card drilling and other underground accidents. Therefore, the steady- state dispersion capacity is vital to high density oil- based drilling fluid. In this study, a multi-light scattering instrument (AGS) was used to continuously test the change of scattered light at different in high density oil-based drilling fluid, and the comprehensive stability of oil-based drilling fluid in different time was obtained. The test method was simple and the data repetition rate was high. The influence of different density, different saturated NaCl, CaCl_2 solution and CaSO_4 pollution on the stability of oil- based drilling fluid were tested. The experiment results show that with the same density, the micro instable phenomena of high density oil-based drilling fluid were that sinking solid phase particles of weighting agent caused clarification on the top of drilling fluid, and the solid phase agglomerated at the bottom of cuvette, especially, the closer to the cuvette bottom is, the agglomeration more obvious is. With the gradual increase of density (2.0 g/cm~3-2.6 g/cm~3), the comprehensive stability of drilling fluid got better, which indicated that the increase of solid phase was not the main reason for causing the instability of drilling fluid, but it had a relationship with emulsified wetting system in drilling fluid. Increasing the concentration of saturated NaCl had little influence on drilling fluid, but when increasing the concentration of CaCl_2 (5%-25%), the stability of system reduced. In terms of the insoluble CaSO_4 solid phase particles, when its concentration below 12%, increasing content made the stability declined. However, when its concentration up to 15%, its stability was improved. When CaSO_4 increased to a certain extent, the viscosity of the whole oil-based drilling fluid system improved, the settlement ability decreased, and the stability improved.