Xiwu Liu

Research on anisotropy of shale oil reservoir based on rock physics model

Rock physics modeling is implemented for shales in the Luojia area of the Zhanhua topographic depression. In the rock physics model, the clay lamination parameter is introduced into the Backus averaging theory for the description of anisotropy related to the preferred alignment of clay particles, and the Chapman multi-scale fracture theory is used to calculate anisotropy relating to the fracture system. In accordance with geological features of shales in the study area, horizontal fractures are regarded as the dominant factor in the prediction of fracture density and anisotropy parameters for the inversion scheme. Results indicate that the horizontal fracture density obtained has good agreement with horizontal permeability measured from cores, and thus confirms the applicability of the proposed rock physics model and inversion method. Fracture density can thus be regarded as an indicator of reservoir permeability. In addition, the anisotropy parameter of the P-wave is higher than that of the S-wave due to the presence of horizontal fractures. Fracture density has an obvious positive correlation with P-wave anisotropy, and the clay content shows a positive correlation with S-wave anisotropy, which fully shows that fracture density has a negative correlation with clay and quartz contents and a positive relation with carbonate contents.

Anisotropy rock physics model for the Longmaxi shale gas reservoir, Sichuan Basin, China

The preferred orientation of clay minerals dominates the intrinsic anisotropy of shale. We introduce the clay lamination (CL) parameter to the Backus averaging method to describe the intrinsic shale anisotropy induced by the alignment of clay minerals. Then, we perform the inversion of CL and the Thomsen anisotropy parameters. The direct measurement of anisotropy is difficult because of the inability to measure the acoustic velocity in the vertical direction in boreholes and instrument limitations. By introducing the parameter CL, the inversion method provides reasonable estimates of the elastic anisotropy in the Longmaxi shale. The clay content is weakly correlated with the CL parameter. Moreover, the parameter CL is abnormally high at the bottom of the Longmaxi and Wufeng Formations, which are the target reservoirs. Finally, we construct rock physics templates to interpret well logging and reservoir properties.

Fracture prediction approach for oil-bearing reservoirs based on AVAZ attributes in an orthorhombic medium

Fracture systems in nature are complicated. Normally vertical fractures develop in an isotropic background. However, the presence of horizontal fine layering or horizontal fractures in reservoirs makes the vertical fractures develop in a VTI (a transversely isotropic media with a vertical symmetry axis) background. In this case, reservoirs can be described better by using an orthorhombic medium instead of a traditional HTI (a transversely isotropic media with a horizontal symmetry axis) medium. In this paper, we focus on the fracture prediction study within an orthorhombic medium for oil-bearing reservoirs. Firstly, we simplify the reflection coefficient approximation in an orthorhombic medium. Secondly, the impact of horizontal fracturing on the reflection coefficient approximation is analyzed theoretically. Then based on that approximation, we compare and analyze the relative impact of vertical fracturing, horizontal fracturing and fluid indicative factor on traditional ellipse fitting results and the scaled B attributes. We find that scaled B attributes are more sensitive to vertical fractures, so scaled B attributes are proposed to predict vertical fractures. Finally, a test is developed to predict the fracture development intensity of an oil-bearing reservoir. The fracture development observed in cores is used to validate the study method. The findings of both theoretical analyses and practical application reveal that compared with traditional methods, this new approach has improved the prediction of fracture development intensity in oil-bearing reservoirs.

Pre-stack seismic density inversion in marine shale reservoirs in the southern Jiaoshiba area, Sichuan Basin, China

For a typical marine shale reservoir in the Jiaoshiba area, Sichuan Basin of China, P-impedance is sensitive for identifying lithology but not suitable for indicating good shale reservoirs. In comparison, density is an important quantity, which is sensitive for identifying the organic-rich mud shale from non-organic-rich mud shale. Due to the poor data quality and incidence angle range, density cannot be easily inverted by directly solving the ill-posed pre-stack seismic inversion in this area. Meanwhile, the traditional density regularizations implemented by directly using the more robust P-impedance inversion tend to be inaccurate for recovering density for this shale reservoir. In this paper, we combine the P-impedance and the minus uranium to construct the pseudo-P-impedance (PIp) at well locations. The PIp is observed to be sensitive for identifying organic-rich mud shale and has a good correlation with density in this area. We employ the PIp–density relation into the pre-stack inversion framework to estimate density. Three types of regularization are tested on both numerical and field data: These are no regularization, traditional regularization and the proposed approach. It is observed that the proposed method is better for recovering the density of organic-rich mud shale in the Jiaoshiba area.

Azimuthal seismic responses from shale formation based on anisotropic rock physics and reflectivity method: a case study from south-west China

Due to intrinsic anisotropy related to preferred alignment of clay particles and the existence of vertical or high angle fractures, shales usually present orthorhombic anisotropy. The objective of this study was to build anisotropic rock physics models for shales at the seismic scale. Based on the well-log and Formation Micro Imager (FMI) log data from a shale formation in the Sichuan Basin in south-west China, we derive an orthorhombic model at the seismic scale by using Schoenberg and Helbig’s method and generalised Backus averaging method in the rock physics workflow. In order to understand the relationship between physical properties of the rock physics model and seismic wave propagation, we apply the simplified reflectivity method to calculate seismic responses for amplitude variation with azimuth (AVAz) analysis. The method is based on the scheme of anisotropic reflectivity method which is commonly used to simulate the full-wave field in stratified anisotropic media, in analogy with the formula of horizontal slowness components in Schoenberg and Protázio’s method. The AVAz analysis is conducted on the seismograms of PP-wave, radial and transverse components of PS-wave. The results show that overburden effects caused by wave propagation in anisotropic media can’t be ignored. Azimuthal variations in amplitudes of both PP-wave and radial component of PS-wave can be used to indicate strikes of fractures, while PS-wave appears to be more sensitive. We have constructed an anisotropic rock physics model at the seismic scale for shales, and applied it to the Longmaxi Shale in the Sichuan Basin, south-west China. The simplified reflectivity method is proposed to calculate seismic responses for amplitude variation with offset and azimuth (AVAz) analysis.