Investigating the Alteration of Sandstone Pore System and Rock Features by Role of Weighting Materials.

Drilled formations are commonly invaded by drilling fluids during the drilling operations, and as a result, the rock pore system will have alterations that consequently alter the rock properties. The objective of this study is to investigate the impact of the most commonly used weighting materials in water-based mud (WBM) on the Berea Buff sandstone pore system and rock characteristics. Rock-mud interaction was imposed by using a customized high-pressure high-temperature filtration test cell under 300 psi differential pressure and 200 °F temperature to simulate downhole conditions during drilling that affect the rock-mud interaction. Extensive lab analysis was accomplished to investigate the rock characteristic alterations in terms of rock porosity, permeability, pore size distribution, flow characteristics, resistivity, and acoustic properties. Ilmenite-WBM showed the maximum values (8.3 cm3 filtrate volume and 7.6 mm cake thickness), while barite recorded the lowest filtrate volume (5.3 cm3) and thickness (3 mm). Nuclear magnetic resonance profiles illustrated the changes in the rock pore system due to the dominant precipitation or dissolution effects. A general porosity reduction was recorded with all mud types that ranged from 4.2 to 9.9% for ilmenite and Micromax, respectively. The rock permeability showed severe damage after mud exposure and a reduction in the pore throat radius. After mud invasion, the rock electrical resistivity showed alterations based on the mineralogical composition of the weighting materials that replaced the saturated brine from the rock pores. Compressional wave velocities (V p) showed an increasing trend as V p of Micromax-WBM increased by 4.5%, while hematite- and ilmenite-WBMs recorded the minimum increase of 1.8%. A general reduction was found for shear wave velocities (V s); Micromax-WBM showed the highest V s reduction by 6.6%, while ilmenite-WBM recorded the minimum reduction of 1.8%. The pore system alterations are the main reason behind V p increase, where the rock lithology alterations controlled the V s changes. The study findings will add more for the rock logging interpretation and rock properties alterations after the mud exposure.