Experimental investigation on size degradation of bridging material in drilling fluids

Solid particle size distribution (PSD) of drilling fluids is one of the key factors for the formation damage and lost circulation control. However, particle size degradation occurs during the long-term process of drilling fluid circulation in the wellbore, consequently resulting in severe formation damage. In this study, size degradation experiments of calcium carbonate solids in the period of drilling fluid circulation are conducted to understand the effect of initial particle size, rotation speed, shear time, fluid viscosity, temperature, pH, salinity, and solid concentration on size degradation of bridging material (BM). The size degradation rate of D-90 is taken as the characterization of particle size degradation. The results show that (1) the degree of size degradation sharply increases with increasing solid initial size, rotation speed, and shear time. Under the experimental conditions, size degradation rate of sample with D-90 = 44.153 mu m is up to 30%-40% at a rotation speed of 1000 rpm over 30 min; (2) there is a critical particle size in the range 15-20 mu m for size degradation over 90 min. When the particle size drops to this critical size, no size degradation occurs obviously; (3) size degradation decreases with increasing fluid viscosity; (4) fluid temperature, pH, salinity, and solid concentration have little effect on size degradation. The evaluation criterion of size degradation is established, and an empirical model is developed to calculate the size degradation. Two mechanisms of the formation damage induced by size degradation are revealed. Based on the particle size degradation, the strategy for the optimization of PSD and solid supplement is presented. We believe that this strategy will be of great significance to designing the particle size distribution of BM for drilling fluids in the field. (C) 2018 Elsevier B.V. All rights reserved.