The impact of weighting materials on carbonate pore system and rock characteristics

The infiltration of the drilling fluids into the drilled formation causes significant alterations in the rock properties due to the interaction between the drilling fluid and the rock pore system. The objective of this study is to evaluate the impact of the most common weighting materials used in water-based mud (WBM) on carbonate pore system and rock characteristics. Rock-mud interaction was imposed by using a customized high-pressure high-temperature (HPHT) filtration test cell under 2068 kPa differential pressure and 93°C temperature to simulate downhole conditions. For filtration properties, ilmenite WBM showed the maximum values (10.9 cm³ filtrate volume and 8.7 mm thickness), while baryte recorded the lowest filtrate volume (6.2 cm³) and thickness (4.2 mm). Nuclear magnetic resonance (NMR) profiles illustrated the changes in the rock pore system due to two aspects: precipitation and dissolution. A general porosity reduction was recorded with all formulations, namely 7.5% and 10.1% for hematite and ilmenite, respectively. The rock permeability showed severe damage after mud exposure that caused the rock average pore size to decrease from macro to meso-porous. After the mud invasion, the rock electrical resistivity showed alterations with all drilling fluids. Compressional wave velocities (Vp) showed an increasing trend that ranged from 2.52% increase for baryte-WBM to 6.35% increase by Micromax-WBM. A general reduction was found for shear wave velocities (Vs) after mud exposure, Micromax-WBM showed no changes in Vs, while hematite and ilmenite showed the largest decreases of 6.71% and 5.56%, respectively.