Assessment of deep shale fracture toughness using nanoindentation tests

Abstract

In the past decade, the extraction of shale gas has become economically feasible due to innovations in horizontal drilling and multistage hydraulic fracturing. This has led to increased interest in deeper understanding of the mechanical properties of shale rocks. Currently, the deep Longmaxi shale with a burial depth of more than 3500 m has been receiving increasing interest in the Sichuan Basin. The first task of large scale development of these reservoirs is to understand the mechanical properties of deep shale rocks. Usually, these properties, such as the Young’s elastic modulus E, hardness H, fracture toughness K IC and other parameters are commonly obtained from laboratory tests with core plugs and logging data interpretations. However, these methods have their own limitations. In recent years, the nanoindentation test has been widely used to predict mechanical properties of shale and other rocks. The advantage of this test is the small sample volume required, which means that drilling cuttings or fragments, which are easily obtained during drilling, can be used. In this study, the nanoindentation method is adopted for the accurate measurement of K IC , which is of vital importance for the better understanding of the drilling-induced tensile fractures and for the design of hydraulic fracturing. The specimens retrieved from a deep well in the Sichuan Basin are examined using nanoindentation tests to obtain K IC . The results show that K IC is strongly dependent on mineralogy and is anisotropic. Usually, KIC values measured parallel to the bedding planes are generally higher than the corresponding values measured normal to the bedding planes. The obtained fracture toughness can be used to predict the fracability and brittleness of shale, which have instructional significance for drilling and hydraulic fracturing.