Effect of water-based working fluid imbibition on static and dynamic compressive properties of anisotropic shale

In the shale gas extraction, drilling and hydrofracturing bring large amounts of water-based working fluid into shale reservoirs, which much affects the wellbore stability and gas production rate. To investigate the effect of water-based working fluid imbibition on static and dynamic compressive properties of anisotropic shale, the INSTRONG 1346 electro-hydraulic servo system and a modified Split Hopkinson pressure bar test system were used to perform the static and dynamic compressive tests on the dry and water-saturated shale specimens with bedding plane inclinations of 0 degrees, 30 degrees, 45., 60 degrees and 90 degrees. Results reveal that the water imbibition process can be divided into three stages, and the chemical water-shale interaction may occur at the second stage. Under static and dynamic compressive loading, water imbibition can seriously weaken the strength and elastic modulus of specimens with the five different bedding plane inclinations, but increase the failure strain. Under the two loading forms, the relationship between compressive parameters of water-saturated specimens and the bedding plane inclination shows the same evolution trend as that of dry specimens, and the compressive parameters are affected the greatest by water imbibition when the bedding plane inclination is 45 degrees or 60 degrees. Furthermore, the mechanism of strength deterioration and the quantitative evaluation on anisotropy of dry and water-saturated specimens under the static and dynamic compressive tests are analyzed and discussed.

Automated summary

In shale gas extraction, the imbibition of water-based working fluid weakens the strength and elastic modulus of shale specimens, but increases the failure strain. The most significant impact is observed at bedding plane inclinations of 45 degrees or 60 degrees. The study analyzes the mechanism of strength deterioration and quantitatively evaluates anisotropy of dry and water-saturated specimens under static and dynamic compressive tests.