Experimental Study on the Stress Sensitivity and Influence Factors of Shale under Varying Stress

Abstract

Shale reservoirs are characterized by extremely low permeability and high clay content. To further study the stress sensitivity of a shale reservoir, the Lower Cambrian shale in north Guizhou was utilized. Through laboratory testing, the relationships between the shale porosity and permeability and the effective stress were established, and the stress sensitivity of shale was analysed. The mechanical properties and mineral composition of this shale were studied by rock mechanics testing and X-ray diffraction. The main factors affecting the stress sensitivity were analysed. The results show that the porosity and permeability of this shale decrease with increasing effective stress; the shale reservoir permeability damage rate is 61.44 ∼ 73.93%, with an average of 69.92%; the permeability stress sensitivity coefficient is 0.04867 ∼ 0.05485 MPa −1 , with an average of 0.05312 MPa −1 ; and the shale reservoir stress sensitivity is strong. Shale stress sensitivity is related to the rock mineral composition and rock mechanical properties. The higher the clay content in the mineral composition, the lower the elastic modulus of shale, the higher the compressibility, and the greater the stress sensitivity coefficient.

Best Practices for Shale Core Handling: Transportation, Sampling and Storage for Conduction of Analyses

“…During coring and core retrieval, stress unloading and core relaxation can induce a large number of partings, resulting in the formation of micro fractures [24]. Also, as the core is extracted from in-situ conditions, the core expands gradually due to de-compaction or stress relief and other effects over time such as slow gas evolution, leading to inelastic deformation and volumetric expansion sometimes in excess of 6 to 8% [26,27]. This results in an increase of porosity and significant increase in permeability leading to inconsistent results in their measurements.…”

“…However, crushing may eliminate micro Darcy (and higher) permeability, especially in texturally complex, siltier, or sandier unconventional reservoirs [24]. Also, care must be taken for shale where organic matter and clay content exceeds 43% as, due to their matrix-supported fabric, they are likely to have lower elastic modulus, higher compressibility, and a greater stress sensitivity coefficient [27]. This renders them prone to plastic shear and contraction and alteration of their pore structure [34].…”

Study on the Damage Evolution Process and Fractal of Quartz-Filled Shale under Thermal-Mechanical Coupling

“…The shale matrix has the characteristics of small pore throats and extremely low permeability. More than 90% of shale gas wells need to undergo shale reservoir fracturing before they can be directly put into production [12][13][14]. The research on shale gas in my country started late.…”

Acoustic and fractal analyses of the mechanical properties and fracture modes of bedding‐containing shale under different seepage pressures

“…It was found that under certain high normal stress, the fracture permeability decreased with the increase of contact normal stress. Wu et al 36 conducted a rock mechanics test on Lower Cambrian shale in the northern part of China. It was found that the porosity and permeability of the shale decreased with the increase of effective stress, and the mineral composition of the shale had an important influence on the shale stress sensitivity.…”