Dynamic Filtration Loss Control Through Optimization of Drilling Fluid Rheological Properties: A Comparative Study of the Fluid Viscoelasticity Versus Shear Viscosity Effects
Authors: Chen Hongbo , Kuru Ergun 
Published: September 2023

Shear viscosity and elasticity have been identified as two of the most influential factors controlling the volume of drilling fluid invasion into reservoir and the resultant formation damage. Past studies were inconclusive regarding individual effects of fluid shear viscosity vs elasticity, as it was challenging to disintegrate and measure their impacts independently. Therefore, we investigated the relative contributions of the fluid shear viscosity and the elasticity on the fluid invasion and the resultant productivity impairment.

24 water-based drilling fluids were prepared using various blends of three different molecular weight PHPA polymers. Detailed rheological characterizations of these fluids were carried out by conducting amplitude sweep and controlled shear rate tests. Viscoelastic properties of the fluids were quantified in terms of energy dissipation, which physically signifies the amount of energy required per unit volume to cause an irreversible deformation in the fluid’s internal structure. Static filtration tests and core flooding experiments were conducted to determine the static fluid loss, pressure drop across the cores at different flow rates, and the resultant formation damage induced by each fluid.

Using a unique technique we developed in our previous work, we have formulated two groups of fluids; one group with the same shear viscosity and variable elasticity and the other group with the same elasticity and variable shear viscosities. Hence, we could investigate the individual effects of shear viscosity and elasticity on the static and dynamic filtration loss and the resultant formation damage. By investigating the independent effects of viscoelasticity and shear viscosity on the fluid filtration loss characteristics, we have observed that: 1-) The static filtration rate can be more effectively reduced by altering fluid viscoelasticity as compared to the fluid shear viscosity. 2-) Both shear viscosity and viscoelasticity have a direct relationship to the pressure drop associated with the core flow. However, the effect of viscoelasticity on the pressure drop is more pronounced. 3-) Increasing fluid viscoelasticity inhibits fluid invasion into the formation better than that of the fluid shear viscosity. 4-) The results have suggested that viscoelasticity can be effectively used for developing non-invasive fluids, which would reduce static filtration rate, increase pressure drop (i.e. building internal cake), and minimize formation damage by effectively reducing fluids invasion.

The study introduces an innovative approach to investigate the fluid loss and formation damage characteristics of viscoelastic, solids-free drilling fluids. The sole effects of shear viscosity and viscoelasticity on filtration loss characteristics were investigated and compared. Understanding the mechanisms of internal cake formation and their quantitative relation to fluid viscous and elastic properties will help the design of optimum drilling and completion fluids and, hence, minimize the productivity reduction associated with applications of these fluids.