Abstract
One of the most crucial decisions in drilling operations is the choice between water-based mud (WBM) and oil-based mud (OBM). Although WBM is often preferred, mainly due to its low cost, OBM usage is on the rise in light of drilling deeper and increasingly more complex wells. A key disadvantage of using OBM is the effect of mud-filtrate invasion on formation properties in the near-wellbore region, which can complicate interpretation of petrophysical measurements. Rock surfaces for this study were prepared by grinding with silicon carbide powder and polishing with diamond paste. Surface roughness was quantified and OBM surfactant adsorption was visualized, both by atomic force microscopy. Wettability changes on the rock surfaces upon conditioning with surfactant, contained in OBM filtrate, were characterized by contact angle measurements. It was found that, regardless of surface roughness and lithology, conditioning of rock surfaces with low concentrations of surfactant, contained in OBM, resulted in surface alteration to more oil-wet, as shown in the current study for calcite, dolomite, quartz, and mica surfaces. When applied to porous media, this suggests that part of the capillary bound water in the invaded zone becomes more likely to be displaced by the hydrocarbon phase, a factor that needs to be considered when interpreting shallow reading petrophysical measurements.