Research and application of micro-scale fracture prediction technology for deep coalbed methane based on five-dimensional seismic data

Abstract

During the large-scale development of deep coalbed methane in the Daji block on the eastern edge of Ordos Basin, using horizontal wells combined with large-scale volume fracturing, some wells experienced fracturing interference under similar fracturing techniques and technical parameters, significantly affecting the fracturing transformation and gas production outcomes. Through precise 3D seismic interpretation in the early stage, it was found that faults were not developed in the block, suggesting that the development of local micro-scale fractures might be a significant factor causing fracturing interference in horizontal wells. Focusing on the deep No. 8 coal seam in the study area, we rely on high-quality 3D seismic data characterized by “two wide and one high” features. Through OVT （Offset Vector Tile）domain processing, we obtained five-dimensional seismic data encompassing time, space (3D coordinates), offset (or source-receiver distance), and azimuth. For the first time, we employed azimuthal statistics to investigate the development degree, orientation, and developmental stages of micro-scale fractures. The research findings indicate that micro-scale fractures are generally well-developed in the study area, with five major fracture zones observed, and the degree of micro-scale fracture development is higher in the southern part compared to the northern part. The development of fractures is controlled by two phases of regional tectonic activity, exhibiting two distinct planar distribution directions. Specifically, fractures oriented in the near EW and NW directions were formed during the Yanshanian tectonic activity, while fractures oriented in the near SN and NE directions were formed during the Himalayan tectonic activity. By integrating regional tectonic stress fields from different periods, field outcrops, imaging logging, and array sonic logging data, we verified the reliability of our research findings on the degree and direction of fracture development. Applying these micro-fracture prediction results guided the optimization and implementation of the horizontal well fracturing plan in the study area in later stages, leading to a significant reduction in the interference ratio from 14.58% in 2023 to 5.23%. Meanwhile, statistics show that the average daily gas production for wells put into production in 2023 was 6.7×104 m3, and for wells put into production in 2024, it was 7.5×104 m3, indicating a significant reduction in fracturing interference and a continuous improvement in development effectiveness. This prediction method can provide valuable insights for predicting micro-scale fractures in deep coalbed methane blocks with similar geological conditions.

Keywords

• daji block
• deep-seated no. 8 coal
• coalbed methane
• microscale fracture prediction
• ovt
• anisotropism