Effect of barite and gas oil drilling fluid additives on the reservoir rock characteristics

Abstract

This research presents a study of the effect of drilling fluid on the reservoir rock properties. The interactions between the clay minerals existing in the formations and the drilling fluids have been studied. Two types of drilling fluids, which are water-based ferrochrome lignosulfonate base mud and lime mud, were prepared for the study. About 100 core plugs were prepared from seven oil wells of Zubair Formation from depths of over (3000) m. The core plugs were cut, cleaned, dried, and then subjected to petrophysical tests, which are permeability, saturation, X-ray diffraction, and the petrographical analyses. Clay minerals like kaolinite, illite, and montmorillonite were found in the core samples. The static immersion test shows that the clay minerals reacted with the drilling fluids and created swelling and spalling in the core samples and changed the original petrophysical rock properties. Different concentrations of gas oil and barite were added to improve the properties of the used drilling fluids and to decrease their negative impact on the petrophysical rock properties. A comparison is made between the permeability and water saturation before and after exposure to the drilling fluids. The percentage change in permeability and water saturation is calculated. The results showed that the additives to the drilling fluids reduced the formations damage. Finally, the study indicates that the major formation damage in southern Iraqi oil fields is induced by mechanical and chemical mechanizes.