Using nanoindentation to quantify the mechanical profile of Wufeng-Longmaxi formation shale in Southwest China: Link to sedimentary conditions

Abstract. The fine construction of a continuous mechanical profile for the Wufeng to Longmaxi (WF-LMX) Formation sequence is crucial for designing hydraulic fracturing projects in shale gas development. However, the microscale mechanical profile of WF-LMX formation shale and its relationship to sedimentary conditions are poorly understood. This study investigates the mechanical profile of the WF-LMX Formation sequence using nanoindentation. A total of 27 cutting samples were collected at one to one-half meter intervals from the Sanquan-1 well. Nanoindentation testing, rock mineralogy, major element analysis, and porosimetry were performed. The results showed that the WF-LMX shales in this study region were deposited in a passive continental margin environment, primarily from biogenic silica, in a warm and humid climate in a predominantly freshwater environment. Mechanical properties (hardness, fracture toughness, Young's modulus, and brittle index) varied synchronously with mineral and organic content across the vertical drilling profile, reflecting changes in lithology and sedimentary facies within the WF-LMX Formations. Shales in the upper part of the WF Formation and lower part of the LMX Formation, belonging to the deep-water shelf facies, exhibited high mechanical properties. Quartz and clay play a dominant role in controlling shale mechanics, while the minor rock constituents, nanoporosity have little effect. In particular, biogenic silica (authigenic quartz) plays an important role in increasing the brittleness of shale. The effect of shale constituents on micromechanics is essentially controlled by the sedimentary environment. Additionally, the potential of using nanoindentation to effectively assess shale brittleness was also demonstrated. This study provides a continuous and accurate interpretation profile of mechanical parameters and is helpful in determining favorable intervals for hydraulic fracturing in the design of shale gas development.