A study of Iraqi shale samples when subjected to different drilling fluids

The interaction between aqueous drilling fluids and clay minerals has been established as a significant factor in the instability of wellbore and shale formation. Previous models of wellbore stability take into consideration the interactions between aqueous drilling fluids and pore fluid but neglect the interactions with shale matrix. This paper provides an understanding of the behavior of shale sample when subjected to water based drilling fluid through laboratory experiments. The gravimetric test method used in the laboratory to acquire a better understanding of water and ion flow into or out of the shale. To avoid shale instability problem, the drilling fluid should be designed in such a way that less water and more ions can diffuse to the shale. In this study, three different salts were added to water base mud to understand which one causes less shale swelling. Shale samples were chosen from two Iraqi oilfields Al-Khabaz and East Baghdad (E.B.) field. These samples were subjected to several steps of drying, immersion, direct and indirect exposure to three chemical solutions namely KCl, NaCl and CaCl2. When E.B. shale was immersed in KCl solution; the shale sample weight increased by just 0.19 % through water diffusion into the shale. KCl solution also increased the sample weight by 0.75 % through the diffusion of ions. As a result, KCl was shown to be more effective in maintaining shale stability compared to NaCl and CaCl2. Al-Khabaz shale sample responded to NaCl drilling fluid better where water increase was approximately zero and sodium cations absorption raised the shale sample weight by 0.62 %.