Visualization of Complex Shale Swelling at Lamina-Scale

摘要

Shale-fluid interaction is a source of wellbore instability and inadequate stimulation performance in clay-rich shale formations such as Tuscaloosa Marine Shale which is the subject of this study.Moreover,shale formations usually exhibit significant fine-scale heterogeneity while the current laboratory techniques such as LVDT or strain gauges are not capable of distinguishing such fine-scale variation in the deformation of shale formations with complex mineralogy and texture.This study aims to measure the full-field deformation of shale during exposure to deionized water using digital image correlation.To overcome the current limitation in laboratory techniques for the evaluation of shale swelling-induced deformation,we are proposing the application of the digital image correlation(DIC)technique as a non-contact method for full-field measurement of microscopic shale deformation in interaction with water.A speckle pattern is applied to the specimen and then the deformation of the specimen during imbibition is captured using a digital camera.The images are analyzed using a 2D-DIC image processing software to obtain displacements and strains.Tuscaloosa Marine Shale samples with 35% to 52% clay are tested in interaction with water.This study shows the implementation of DIC technique to capture deformation during the imbibition process and the impact of clay content on fine-scale deformation and consequent fracturing of clay-rich shales.DIC enables the visualization and quantification of full-field deformation.Full-field monitoring helps measure the strain distribution and localization,as well as the evolution of strain field with time.The strain localization is influenced by the distribution of the minerals in the specimen.The results provide a better understanding of strain development during imbibition in comparison to traditional LVDT based measurements that give only an average strain value.The results from the study show large strains get localized along a few select laminations.These laminations showed a large tensile strain in the direction perpendicular to the bedding plane.A new technique is proposed for the evaluation of shale swelling at a fine-scale.This technique overcomes the limitation of current laboratory practices on the evaluation of shale formation with a significant amount of fine-scale heterogeneity.Similar to other microscopic techniques such as SEM,we are proposing a microscopic optical technique that can provide deformation at the fine-scale.Therefore,the role of each lamina on deformation and subsequent fracturing can be evaluated.This method is also dynamic meaning that how deformation evolves with time as a result of fluid interaction and subsequent sample failure can be monitored.