Pore-Scale Mud Invasion in Different Rock Samples and Wellbore Conditions: Implications for Lithology Dependent Wellbore Strengthening

In this study, pore-scale dynamic mud invasion in a radial system was investigated for different rock samples. The rocks were selected to generate the mud invasion patterns, filter cake permeability profiles, and change in stress profiles of the lithologies they represent. Each rock was cut into a thick-walled cylindrical shape to simulate drill pipe rotation inside the inner diameter. In the experimental design, lost circulation material (LCM) concentration and rotary speed were varied based on the results of preliminary experiments with ceramic filter tubes. Water based mud (WBM) was formulated with calcium carbonate, and the viscosity profile of the fluid with change in temperature was also determined. The results from the experiments revealed different dynamic mud invasion and filter cake permeability patterns for different rock samples. These patterns were controlled by the rock porosity, permeability, temperature, and rotary speed. Increasing the concentration of calcium carbonate beyond a certain threshold may not always reduce pore-scale dynamic mud invasion at elevated conditions. Analytical results showed that rock permeability and filter cake permeability profile largely control the changes in wellbore stress profile. This approach can be used in the planning and well design process for drilling similar lithologies.