Influence of Mineralogy on the Index Properties and Durability of Crushed Shales

Weak shale formations in oil shale production are the primary cause of instability in drill holes due to sloughing and swelling. During and after the drilling, shales continuously absorb water and consequently deteriorate with time. The effect of the deterioration results in the transformation of a hard rock with high cohesion and frictional resistance to a soft, fine-grained mass without cohesion, lower shear strength, and high swelling susceptibility. As a result, the shales swell and slough into the hole. Shales are primarily composed of varying amounts of clay minerals (i.e. chlorite, mixed layer illite-smectite, illite, and kaolinte), quartz, and other minerals such as carbonates, pyrite or iron oxides. In particular, understanding the influence of clay minerals on the shale is crucial to understanding the mechanical behavior of shales. This is because clays are highly reactive minerals in the presence of water. Due to this reactivity, shales tend to be highly susceptible to deterioration. This paper investigates the association between shale mineralogy and mechanical characteristics. For this study, shale samples were collected from a variety of locations in Kentucky. The specimens were assumed to be representative of hard and soft shales with high, medium, and low durability. These samples were crushed and laboratory testing was performed. The lab tests included index tests, mineralogical tests, and durability tests. It was observed from the results of this study that the mineralogy explained the activity and variations in several index properties, as well as the durability of the samples.