Experimental study the viscoelastic properties of deep shale gas reservoirs in Sichuan Basin

Abstract

In recent years, the deep shale gas reservoirs has been a major shale gas target in Sichuan Basin. Usually, the burial depth of this target formation is more than 3500m, which is receiving increasing interests to get industrial natural gas. The deep shale gas reservoirs, characterized by fine-grained nature and low matrix permeability as the common shale, require good understanding of geomechanical properties to optimize drilling and fracturing strategies. Meanwhile, the shale reservoirs are anisotropic and inhomogeneous, exhibiting a nonlinear feature under stress loading. However, very limited studies are devoted to characterization of the mechanical properties of deep shale, as opposed to other shale formations. Due to an increasing demand for fundamental properties of this formation to meet the needs of development of this deep shale gas, a comprehensive characterization studies is needed. One of the most important investigation is to obtain the mechanical properties based on an integration of unconfined/triaxial compression tests and creep tests. In this study, the time-dependent, elastic, hysteresis, and strength properties of deep shale specimens, retrieved from a deep well in Sichuan Basin, are evaluating by performing a series of triaxial creep experiments. With respect to mineral composition, the shale cores are found to be organic-rich. The response of these shale specimens are found to be influenced by the planes of weakness and the presence of micro-cracks. The results of creep experiments show that shale tends to significantly creep under the applied stress. The time-dependent axial strain increase linearly with differential stress. And the Power-Law creep model can be used to describe the time-dependent behaviour of shale well. Moreover, the existence of bedding planes results in the creep anisotropy of shale. It is observed that the overall time-dependent axial strain normal to bedding is higher than the parallel to bedding. But the axial strain at 45° angle with respect to the bedding plane is highest. These research results are of great significance for safe and fast drilling in deep shale gas reservoirs. Also, it can be used to guide hydraulic fracturing.