Experimental Study on the Anisotropy of Unconventional Tight Reservoirs: Joint Ultrasonic and Electrical Measurements under Pressur

Unconventional tight reservoirs have gained importance in global oil and gas production. The fine-grained tight reservoir rocks are often anisotropic as results of their laminar structures in multiple scales of observation. Anisotropic textures of fine-grained rocks have significant influence on pore structures and fluid transport properties. For successful development of unconventional reservoirs, it is critical to precisely characterize evolutions of pore structures and anisotropic flow properties in depleting tight formations. Common geophysical parameters used for characterization of anisotropy in tight reservoirs include wave velocity, attenuation and electrical conductivity. Previous studies have shown that joint investigations using elastic and electrical properties are necessary for assessment of pore structures and permeability anisotropy in tight reservoirs. Conducting these investigations is always challenged by lack of experimental study on anisotropy mechanisms as well as connections between geophysical anisotropy and permeability anisotropy. Furthermore, the study on connection between direction-dependent attenuation mechanism and pore structure is lacking in tight reservoirs. A comprehensive study on anisotropic velocity, attenuation and conductivity responses would greatly benefit characterization of pore structure and flow properties in tight reservoirs.;In this thesis I first introduce a new experimental design that provides simultaneous, multi-directional ultrasonic and electrical experiments on cores under pressure conditions. The system is tested and validated using standard references and natural rock samples. Then I investigate individual mechanisms and controls of velocity, attenuation and complex conductivity anisotropy in tight rocks under pressure. Results of ultrasonic measurements show that attenuation and attenuation anisotropy are sensitive to the closure of low aspect ratio pores or microcracks. Attenuation anisotropy in clay and organic rich formations correlates well with the presence of compliant materials (clay and organic matter). Evolution of attenuation anisotropy strongly relates to directional pore connectivity in fine-grained samples. Similarly, electrical textural parameters including formation factor (F) and tortuosity (T) tensor also show correlation with pore deformation and pore connectivity change in tight samples. Tortuosity is a very sensitive parameter for preferential pore alignment and directional pore deformation in tight reservoirs.;Finally, this thesis provides results of joint velocity, attenuation and conductivity measurements on sandstone and fine-grained tight rocks under pressure. Results on sandstone imply correlation between rock physics parameters and pore deformation as well as permeability change. For tight reservoir rocks, permeability anisotropy could have completely opposite trends in chalks and shales depending on texture and pore size distribution. Among all elastic, anelastic and electrical anisotropy parameters, imaginary conductivity shows the strongest relationship with preferential pore alignment and directional pore connectivity. While the combination of all three anisotropies provides a comprehensive understanding of textural anisotropy, conductivity anisotropy is the most sensitive parameter for permeability anisotropy assessment and stress monitoring in unconventional tight reservoirs.