A CT scan aided core-flood study of the leak-off process in oil-based drilling fluids

Abstract

An experimental study on the leak-off of oil based drilling fluid sandstone cores is reported. First we revised the theoretical models for the rheology of the drilling fluid, the flow behavior of drilling fluids in the drill pipe and annulus, and filtration mechanisms. Then systematic static leak-off experiments were carried out using an innovative method where CT scans taken at time intervals were used to visualize and accurately quantify infiltration of fluids in a sandstone core. Different compositions of oil based drilling fluids were investigated, to examine the influence of various particles on the external filter cake and internal filtration. Scanning electron microscopy was used to characterize the external filter cake and internal filtration. The results give accurate measurements of the filtration volume of the drilling fluids. Depending on the composition of the drilling fluid, the formation of external filter cake could be visualized on CT images. The core flow experiments are matched to the theory for linear static filtration. The results lead to new insights concerning the build of external filter cake and internal filtration. The experiments use real sandstone cores giving more realistic data than using an API press test and filter paper. This work creates a basis for future improvement of oil based drilling fluid, by providing a better understanding of mechanisms involved in leak-off control.