Effect of water-based drilling fluid components on filter cake structure

• Filter cake is heterogeneous not only in structure but also in composition.
• Bentonite coalescence reduces the compactness of filter cake.
• Higher polymer content in the filter cake contributes to its compactness.
• Hematite is easier to deposit compared with barite during the fluid loss test.

Filter cake plays an important role in the filtration performance of drilling fluid. Drilling fluid components greatly influence the characteristics of the filter cake. The homogeneity of the filter cake, in turn, influences the properties of the filtration process. This study is devoted to understand the structure of drilling fluid filter cake in the non-reservoir sections and provide a new insight into cake layer structure. The spatial distribution of physical, chemical structure of cake layers was characterized by various analytical techniques, including high-temperature high-pressure (HTHP) fluid loss test, scanning electron microscopy (SEM), energy dispersive spectrum (EDS), particle size distribution (PSD), X-ray diffraction (XRD), and methylene blue test (MBT). The results showed that the filter cake was heterogeneous not only in structure but also in composition. The structures and composition of filter cake change significantly from the top layer to the bottom layer. Drilling fluid components have a non-negligible effect on filter cake characteristics. High clay content would result in reduced compactness of filter cake because of the clay coalescence under high temperature, the critical coalescence temperature decreases with the increase of clay content. SEM images and elemental compositions illustrated that electrolyte contamination results in larger pore size of the filter cake as compared to the cake formed by uncontaminated freshwater based drilling fluid, the “honeycomb” structure collapsed because of its inhibitory action on the hydration and swelling capabilities of the pore wall of filter cake, and the filter cake of high density drilling fluid had a loose structure when compared with that of low density drilling fluid. EDS and XRD studies showed that there was an increase in the mass-ratio of hematite to barite of filter cake from the top layer to the bottom layer. Through the PSD results, the filter cake was found to have a larger particle size in the bottom layer than in the top layer.

Scanning electron microscopy images for filter cake show that the compactness of low density one (cake #7) is better than high density one (cake #9). Deformable colloidal particles in cake #7 can be observed obviously other than in cake #9. Particle size distribution results obtained in cake #7 show that filter cake is heterogeneous.Figure optionsDownload as PowerPoint slide