Using nanoindentation to quantify the mechanical profile of the Wufeng-Longmaxi Formation in southwestern China: Link to sedimentary conditions

Abstract

The fine construction of a continuous mechanical profile for the Wufeng-Longmaxi (WF-LMX) Formation is crucial for designing hydraulic fracturing projects in shale gas development, which however is generally difficult to achieve by conventional macroscopic mechanical testing methods due to the difficulty in obtaining continuous block shale samples. This study employed the nanoindentation technique to achieve the continuous mechanical characterization of the WF-LMX Formation and correlated the mechanical stratigraphic profile to the sedimentary conditions. A total of 27 cutting samples were collected at 1-1.5 m intervals from the Sanquan-1 (SQ-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. 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 Formation. 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 and 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.