Abstract
Two important aspects often overlooked in the design of bridging solids packages for drilling fluids are: a) obtaining accurate particle size measurements, and b) allowing for the influence of particle attrition. This paper explores both aspects and illustrates, using field data, the challenges and positive impact on productivity (or injectivity) of getting the distribution right.
Commonly-used bridging solids were sized in the laboratory by sieve analysis and laser diffraction techniques and the results compared. The two methods gave similar results for particles in the range ~ 50–500 μm, but there was a significant discrepancy for larger particles. Since sieving best reflects the functional purpose of the particles (to block pores or fractures) it is suggested that sieve analysis should be the preferred method for particles with a d90 above about 500 μm.
Studies of particle grind down (attrition) showed the critical attrition parameters to be the particle type (material), the initial particle size, and the shear rate. Large particles ground down faster and to a greater extent than smaller ones. In the laboratory tests, large 1400 μm particles were found to reduce in size by up to 80% compared with only a 5% reduction in size for 300 μm sized particles over a 30 minute test period. Fluid viscosity also had an influence on attrition but particle concentration appeared to have only a marginal effect.
Two field case studies are presented. These used small bridging particles for pore sealing to minimize formation damage in two drill-in fluid applications. A grind-down effect was observed in case 1 (water based mud) which used 50 μm marble particles, and so steps were taken to restore the size distribution. There was no apparent attrition effect in case 2 (oil mud) which used a mixture of smaller 20 μm marble particles and barite. Careful management of the fluids led to a successful outcome in terms of well performance in both cases.
