Effect of microscopic pore structures on ultrasonic velocity in tight sandstone with different fluid saturation

Abstract

Microcosmic details of pore structure are the essential factors affecting the elastic properties of tight sandstone reservoirs, while the relationships in between are still incompletely clear due to the fact that quantitative or semi-quantitative experiments are hard to achieve. Here, three sets of tight sandstone samples from the Junggar Basin are selected elaborately based on casting thin sections, XRD detection, and petro-physical measurement, and each set is characterized by a single varied microcosmic factor (pore connectedness, pore type, and grain size) of the pore structure. An ultrasonic pulse transmission technique is conducted to study the response of elastic properties to the varied microcosmic details of pore structure in the situation of different pore fluid (gas, brine, and oil) saturation and confining pressure. Observations show samples with less connectedness, inter-granular dominant pores, and smaller grain size showed greater velocities in normal conditions. V p is more sensitive to the variations of pore type, while V s is more sensitive to the variations of grain size. Samples with better connectedness at fluid saturation (oil or brine) show greater sensitivity to the confining pressure than those with gas saturation with a growth rate of 6.9% to 11.9%, and the sensitivity is more likely controlled by connectedness. The pore types (inter-granular or intra-granular) can be distinguished by the sensitivity of velocities to the variation of pore fluid at high confining pressure (> 60 MPa). The samples with small grain sizes tend to be more sensitive to the variations of confining pressure. With this knowledge, we can semi-quantitatively distinguish the complex pore structures with different fluids by the variation of elastic properties, which can help improve the precision of seismic reservoir prediction for tight sandstone reservoirs.