Alteration effect on petrophysical properties of reservoirs rocks by drilling fluid emulsifiers

Surfactants formulated into oil-based mud can adsorb to pore surfaces causing changes in rock wettability, which are widely believed to impair formation permeability when oil-based mud invades the near-wellbore region. In the present study, the effects of oil-based mud with Versawet and Versacoat surfactants, on the petrophysical properties and wettability of drilling cutting samples from sandstone reservoirs were investigated through relative permeability, wettability index, and adsorption measurements at standard conditions in a micromodel device. The X-ray analysis and flame photometer were used to characterize the drilling cuttings samples, and the functional groups of surfactants were identified by the infrared spectroscopy. Relative permeability was studied using the unsteady state method, and the wettability index was measured by the Amott method after the aging with drilling formulation for 24 to 720 h and then the adsorption behavior of surfactants using Langmuir isotherm. The results show that drilling cutting is mainly composed of quartz 71.9%. Surfactant in oil-based mud reduces permeability and causes a change in wettability to a more oil-wet state, and these effects become more pronounced as the contact period with the surfactant increases. The wettability of a reservoir rock was altered due to surfactant adsorption and the formation of a hydrophobic layer on the surface of drilling cutting samples. The adsorption and retention of polar part of the surfactants -NH2 for Versawet and -COOH for Versacoat on the silanol groups and the calcium atoms included in the structure of the reservoir rocks. They are the key interactions in the wettability alteration process.