Multi-scale Fracture Evaluation Method for Shale Reservoirs in Deep Formation Wufeng-Longmaxi in Southern Sichuan

The fractures in the deep shale reservoirs in southern Sichuan are one of the main factors affecting the enrichment of shale gas, fracturing design, and development effectiveness. Therefore, it is necessary to carry out multi-scale joint inversion methods to identify and evaluate fractures at different scales to improve the accuracy of their evaluation and prediction.

To address the above problems, four different scale fracture evaluation methods and their consistency studies have been conducted, including core, logging, remote detection, and seismic. Through the analysis of geological characteristics, core observation, CT scanning and experiments on acoustic anisotropy, the intervals of shale fractures and their anisotropic characteristics are determined. The anisotropy coefficient of shale reservoirs is calculated by dipole cross-wave logging, determining the longitudinal development degree and characteristics of shale reservoir fractures. The acoustic field imaging is used to extract the reflection coefficients of high and low frequency Stoneley wave and equivalent anisotropic coefficients of shear reflection waves, which allowed for the identification and evaluation of the development characteristics and effectiveness of fractures near borehole at a remote acoustic detection scale. Based on high-resolution seismic data with wide azimuth vector offset, the pre-stack seismic anisotropy coefficient is constructed, clarifying the intrinsic relationships between the fracture medium reservoir parameters described at four different scales.

The study shows that the deep shale reservoir fractures of Wufeng-Longmaxi Formation in southern Sichuan are mainly bedding fractures, mostly in a closed state, followed by structural fractures, most of which are filled with calcite. The high-and low-frequency Stoneley wave reflection coefficients in the formations Wufeng-Long11 of the first deep shale gas evaluation well Lu203 in southern Sichuan, show a significant difference, with fracture development and good reservoir connectivity, with an initial production of 1.38×106m3/d. Determining the acoustic anisotropy coefficient is a common parameter for evaluating shale reservoir fractures from core scale to seismic scale, and innovating the acoustic remote detection of shear wave reflection waves equivalent anisotropic coefficients fills the gap in detection range and resolution from logging to seismic scale, achieving high precision prediction of small-scale fractures of 3-20m. The results are generally consistent with core, logging, acoustic remote detection imaging, gas testing results, and production performance, proving the effectiveness of the multi-scale fracture evaluation method of shale reservoirs using cross-scale constraints. Further predictions indicate that the well area Lu 203 is characterized by the development of small fractures/bedding fractures on the basis of relatively stable structures, while the well area Yang101 is the development of structural fractures on the basis of large fault, with multiple calcite fillings, which is one of the main factors contributing to the significant differences in shale gas production between the above two areas. The proposed multi-scale fracture evaluation method based on acoustic anisotropy coefficients provides significant reference value for the comprehensive evaluation of unconventional reservoir fractures.

This research was supported by the National Oil & Gas Major Project (No. 2025ZD1403902) and the CNPC International Science and Technology Cooperation Project (No. 2023DQ0422).

Fig. 1  AVAZ inversion plan along the O₃w layer in L203 Block in Southern Sichuan