Each shale gas play is unique and the stimulation and completion method should be determined based on individual attributes. Selecting the completion style for an emerging shale play should begin in the laboratory rather than copy successful cases. An understanding of the rock mechanical properties is essential to help understand how the shale reservoir should be completed.
We conducted triaxial compression tests for specimens from North America and South China. The failure modes and mechanical behaviors are analyzed. A new method combines rock mechanical properties and mineralogy to quantify the brittleness was derived and the practical use is demonstrated afterwards. Natural fracture analysis from core and borehole image are conducted and potentially active fractures, which could be most important hydraulic conduits and another index of the fracability, are identified through a three-dimensional Mohr representational method.
The results of this study revealed that black shales are usually more brittle than imagined. Splitting and splitting-shear hybrid failure modes are the main failure modes under low confining pressures, while shear failure modes predominate under higher pressures. Types of shale, burial depth, coring direction and confining pressures are all affecting the mechanical behavior of specimens. The brittleness characterization of shale samples from China is similar to that from America. Compared with North America, shale from South China has higher strength and Young's modulus, while lower Poisson's ratio. A systematic approach of designing completion and stimulation is proposed based on brittleness, closure stress and hydraulic fracture width.
The analyzing results from two shale gas wells verified the value of this method. Brittleness calculating results tallied well with initial production of these wells, indicating good application prospects. Calculation of strength properties, closure stress and fracture width can be practical useful in selecting and designing stimulation measures.