Experimental study of the main controlling factors of desiccation crack formation from mud to shale

Abstract

Numerous studies have been done on the formation mechanism and distribution law of desiccation cracks. However, the formation mechanism of micro-cracks in shale is still unclear. In this study, the most common clay minerals of shale (montmorillonite, illite, kaolinite) were experimentally studied to identify the main controlling factors of desiccation crack formation, and show the influence of these factors on crack geometry. To imitate the process of desiccation crack formation, a diagenetic crack imitator machine and a laboratory air drying were used. The results show that clay content, temperature, and thickness of the clay layer are the main factors that control the crack formation. Our samples suggest that crack width increases with increased montmorillonite content, and decreases with increased kaolinite and illite content. Temperatures of 100 °C and 230 °C induced further desiccation cracks in pure mineral solutions, while the mixed samples did not show any additional crack propagation. Crack width was shown to increase with layer thickness. In addition, 92% of all samples exhibited the three-armed pattern, where the angle distribution was between 90-100 and 170–180°. We further propose that the water content in the dehydration process plays an important role in the formation of cracks, and the crack formation can be divided into four stages. The majority number of cracks occur at the initial and development stages, where the main mono-crack forms and starts to divide. The cracks intersect each other at the complex third stage. Ultimately, at the weak stage the last few random thin cracks might form. The formation of the first crack also depends on clay type and varies in time within an order from early to late: montmorillonite, kaolinite, illite. Based on the visual similarity between diagenetic micro-cracks in shale and mud desiccation cracks, as well as their mechanism of cracks formation, this study may provide a new perspective on the formation of the diagenetic micro-cracks in shale reservoirs and complements previous studies through virality of pure homogeneous and mixed clay samples.