Measurements While Fracturing Deep Shale Gas Reservoirs: A Real-Field Comparative Study of Acoustic-Based Monitoring, High-Frequency-Pressure-Based Monitoring, Microseismic Monitoring, and Pressure-Fall-Off-Data-Based Interpretation in the Southern Sichuan Basin

In the Sichuan Basin, over 65% of shale gas resources are stored in deep shale reservoirs (depth>3500m). The complex tectonic evolution in this area and the large burial depth generate extensively-developed and large-scale natural fractures/faults and large principal horizontal stress difference. This condition significantly influences fracture propagation and the resulting geometry. A good understanding of artificial fractures’ location, size, propped length, conductivity, and complexity is pivotal for further fracturing design optimization. However, due to the above geological factors, it is difficult to accurately predict fracture geometry and other properties while fracturing deep shale. To better evaluate the created fracture system and select appropriate fracturing monitoring approaches, four different fracturing monitoring techniques are applied to a single fractured horizontal well in this area, including acoustic-based monitoring (near-field and far-field measurements), high-frequency-pressure-based monitoring, microseismic monitoring, and pressure-fall-off-data-based interpretation. For acoustic-based monitoring, about 5 min to 10 min after the main fracturing is stopped (the water hammer effect ends), 6 sets of active sound waves are emitted to monitor the fracture system. The near-field connectivity index (NFCI), far-field conductivity index (FFCI), fracture length, height, and width can be obtained. As for the high-frequency-pressure-based monitoring, during the fracturing period, the high-frequency pressure fluctuation information is collected at the wellhead, and the fracture initiation is analyzed based on the high-frequency pressure wave cepstrum. For each stage, key information, such as the core stimulated reservoir volume (SRV), SRV region equivalent permeability, perforation cluster opening ratio, fracturing-fluid-reached length, propped fracture length, fracture height, fracture conductivity, and fracture geometry index, can be interpreted. Microseismic monitoring is a fracture geometry monitoring and analyzing method using weak seismic waves induced by subsurface stress changes and fractures. For each stage, it offers the number of seismic events, SRV azimuth, length, height, and width. For the pressure-fall-off-data-based interpretation, by using the pressure responses during the pressure-fall-off period after the main fracturing of a stage, effective fracture half-length, main fracture surface area, secondary fracture surface area, and fracture complexity index (FCI, the surface area ratio of secondary fractures to the main fracture) can be calculated. By comparing 15 stages’ fracture half-length of the analyzed well, we found that, for each stage, the half-length values provided by the four techniques are generally close to each other. Particularly, the data obtained from microseismic monitoring tend to be the largest, while the pressure-fall-off-data-based method's interpretation turns out to be the lowest. Other fracture properties are also documented and their controlling operation parameters are further analyzed. This study can serve as a practice reference for the selection of appropriate fracturing monitoring technique in this region. The pressure-fall-off-data-based interpretation with certain accuracy is a fast and economical fracture monitoring approach.