Formation damage from mud invasion is a complicated process that is influenced by mud type, downhole fluid particle size distribution (PSD), mud rheology, rock type, lithology, permeability, drill pipe rotation, geochemical factors, pressure, and temperature. Many of these variables are often ignored, simplified, or underestimated. Thus, their individual and combined effects on formation damage is not well understood. Among these factors, the porous media complexities play a key role in determining the extent of drilling mud damage to the formation.
This paper is focused on characterizing water based mud's (WBM) invasion and damage to different rock cores representing various lithologies, under simulated dynamic wellbore condition. The experiments performed in this study were selected from a statistical design of experiment method. Based on formation damage relevance, the following factors were considered and varied: pore throat diameter of homogenous porous media (ceramic filter tubes), lithology type, and temperature. The levels of each factor were determined from a mud invasion data base considering high and low limits. Other constant factors were rotary speed, type of lost circulation material (LCM), concentration of LCM, and pressure. An LCM was carefully added to the WBM to reduce the fluid's invasion. Dynamic wellbore condition (pipe rotation only) was simulated with a drilling simulator, capable of rotating a shaft that is centered between the inner diameter of the damaged porous media. Using this approach, the formation damage profile of the WBM on four different lithologies was investigated. The results revealed that rock permeability and porosity are critical factors that control mud invasion, damage propensity, and filter cake permeability profile. Statistical analysis showed that increase in temperature can significantly increase the degree of mud solids cross flow and fines mobilization through the porous media. Thus, leading to the reduction of formation permeability. In a radial system, ceramic filter tubes representing homogenous porous media are often used to quantify mud invasion. However, further investigation revealed no significant change in the damage profile exhibited by the two (5μm and 20μm) pore throat diameter filter tubes used in this study. These filter tubes are typically calibrated to match permeability values of actual rocks, but are not true representation of lithology complexities. The novelty of this study lies in integrating different constant and varying mud invasion factors in relation to the damage of four lithology types.