Application of nanoindentation creep tests to characterize viscoelasticity behavior of deep Longmaxi shales

Abstract

The development of deep shale gas reservoirs has become economically and commercially viable through novel horizontal well completion and multistage hydraulic fracturing technologies in recent years. In the Sichuan Basin, the deep Longmaxi shale is a major target formation that has attracted increasing interest. Shale is generally characterized as a sedimentary rock with fine grains, high clay, and organic content, low permeability and porosity, and high heterogeneity and anisotropy. Geomechanical characterization of these organic-rich rocks usually includes an estimation of the elastic modulus, strength properties, hysteresis behavior, and creep deformation. Understanding and characterizing the viscoelastic behaviors of shale is essential because they affect the elastic and fracture properties of rock and cause a series of drilling problems related to the wellbore stability, such as borehole shrinkage, pipe sticking, and casing collapse. The viscoelastic behavior of specimens retrieved from a deep well in the Sichuan Basin was examined using nanoindentation creep tests. Nanoindentation is a technique based on a hard tip, which is pressed into the surface of a sample. The advantage of this test is the use of a small sample volume, such as drilling cuttings or fragments, which are obtained easily during drilling. The results indicated that the creep modulus parallel to bedding and normal to bedding is similar, while the creep modulus of drilling cuttings is the lowest. A linear relationship was both observed between the creep modulus and hardness, Young’s modulus for deep Longmaxi shale. The Burgers creep model showed good agreement with the experimental data. The creep behavior of Longmaxi shale exhibited anisotropy, and the effect of drilling fluids could aggravate the creep displacement. These results have elucidated the creep properties in engineering problems of shale gas formation, drilling, and stimulation.