An Experimental Study on Porosity and Permeability Stress-Sensitive Behavior of Sandstone Under Hydrostatic Compression: Characteristics, Mechanisms and Controlling Factors

Abstract

Porosity and permeability stress-sensitive behavior of sandstone is investigated through porosity and permeability measurements under hydrostatic compression. An empirical logarithm model is applied to the evaluation of stress sensitivity. Mercury injection, casting thin sections and scanning electron microscope are adopted to discuss the microscopic controlling factors and mechanisms of stress-sensitive behavior of sandstone. Disparities between laboratory-scale and field-scale data are also discussed. Experimental results show that both porosity and permeability decrease with increasing effective stress, and permeability is more sensitive to stress. The evolution of porosity and permeability with increasing effective stress follows the logarithmic model and can be classified into three stages: rapid decline, moderative decline and stable-slight decline stage. Stronger permeability stress sensitivity is observed in specimens with lower initial porosity and permeability, which is fundamentally controlled by the size and shape of void spaces as well as the content and distribution of soft minerals. While porosity stress sensitivity is observed to be discrete. Pore throat with small aspect ratio and irregular shape, fractures and minerals with lower elasticity modulus lead to the increase in permeability stress sensitivity of sandstone. The stress sensitivity characteristics are essentially the result of the dissimilar deformation responses of pores and framework grains under effective stress. Stress sensitivity evaluated in the laboratory usually does not completely represent the stress sensitivity in the field, due to stress release, drilling induced fractures and the difference of experimental and in situ conditions.