# Influence of Stress Anisotropy on Petrophysical Parameters of Deep and Ultradeep Tight Sandstone
## 1 Introduction  
Deep and ultradeep tight sandstone reservoirs are the main reservoirs contributing to the current oil and gas production capacity in China, and are also the key objects of oil and gas research in China. To improve the understanding of deep and ultradeep tight sandstone reservoirs, it is necessary to explore their physical characteristics. The purpose is to study the specific response of rock physical parameters in reservoirs, indicate the specificity of underground rock structures, and identify their effects on oil and gas exploration. Rock mechanical parameters are the basis for numerical simulations of the stress field, reservoir fracture evaluation, and formation fracturing [1; 2; 3]. The study of the relationship model between rock mechanical parameters and in situ stress has practical significance for the accurate evaluation of rock mechanical parameters and in situ stress prediction [4; 5; 6; 7; 8; 9; 10; 11; 12]. Complex geological structure, surface erosion, multi-stage fracture activity and other factors will influence the complexity and differentiation of in-situ stress distribution. Especially in the vicinity of complex faults, the magnitude and direction of in-situ stress will change to a certain extent. In addition, frequent tectonic activities will cause local faults to produce intricate joints and fissures, which will have a greater impact on the rock properties of the whole local area, thus increasing the difficulty of geological research. Therefore, it is of great theoretical significance to improve the research of in-situ stress for geological exploration. [PERSON] et al. (1958) conducted experimental tests on Berea sandstone and found that the net stress remained unchanged and the sonic velocity changed little with the confiningpressure [13]. As the net stress increased, the sonic velocity increased. [PERSON] (1960) found through experimental studies that the P-wave velocities of igneous and metamorphic rocks increased with increasing confining pressure [14]. Igneous rock enlargement was more pronounced. [PERSON] et al. (1986) considered that the increase in clay content was the main factor for the decrease in porosity and P-wave velocity [15]. [PERSON] et al. (1989) found that the relationship between sonic velocity and effective stress in sandstone changed from a power exponent to a linear exponent [9]. [PERSON] et al. (1994) believed that due to the close contact between tight sandstone grains, a large number of microcracks with a small aspect ratio were formed, resulting in the sensitivity of P-wave velocity and S-wave velocity to pressure changes in tight sandstone compared with ordinary sandstone [16]. [PERSON] et al. (2006) found through experiments that the P-wave and the effective stress have an exponential relationship, the velocity and the effective stress have nonlinear change characteristics when the pressure is low, and the velocity changes tend to be stable when the pressure is high [17]. Sandstone saturated with water and oil has no effect on the change in shear wave velocity, but the shear wave velocity when saturated with gas is small. Lithology affects the variation in sonic wave velocity with shear wave velocity. Sandstone is stronger than shale. Through relevant experiments on volcanic rocks, [PERSON] et al. (2010) found that effective stress and water saturation affected the velocity of the S-wave [18]. When the effective stress is constant, the sonic wave velocity of fully saturated water is large, the sonic wave velocity of bound water saturation is moderate, and the sonic wave velocity of dry rock is small. The variation in the sonic velocity of the rock sample is not controlled by the linear effective stress but increases with a nonlinear increase in the effective stress. [PERSON] et al. (2014) studied the stress sensitivity of fractured reservoirs in the Keshen gas field by using the reservoir fracture numerical simulation method [19].