Abstract
Chemical wellbore instability is mostly attributed to the “unfavorable interaction” between the formation pore fluid, its shale constituents and the drilling fluid, this is mostly experienced in terms of shale swelling that tends to under gauge the drilled well. This study was conducted to examine the impact of brine concentration and mineralogy on the swellability of shale. Sixteen shale samples were obtained from selected wells across two selected fields (OR and OG) in the Niger Delta. X-ray diffraction analysis was carried on the shale samples to determine its mineralogy distribution. A Nacl brine solution was prepared alongside a water based mud formulated using sodium chloride brine of varied concentration. The shale samples were tested for its swellability in both brine solution formulated water based mud for 24 hours and 48 hours. Results obtained showed that there were more clay minerals (56%) across both fields and that Palygorskite was the most abundant clay minerals for both fields. Also, for the swelling test (shale-in-brine and shale-in-mud) that the shale samples swelled in both medium as expected, but to different degrees, mainly as a function of the mineralogy content of the shale and the brine concentration of the fluid in which they are immersed. The import of this goes to buttress the necessity for understanding the place of mineralogy and concentration in the study of wellbore instability caused by chemical interaction.

Key words: Wellbore instability, Shale-fluid interaction, X-ray diffraction, Chemical interaction