A Comparative Study of Pre-Stack Fracture Prediction Methods for Coal Bed Methane

The No.8 coal seam in the Taiyuan Formation of the lower Permian with a burial depth greater than 2,000 m in the XX block of the eastern margin of the Ordos Basin has a confirmed reserve of over 100 billion cubic meters. It is currently in the pilot development stage using horizontal wells and large-scale hydraulic fracturing. Natural fractures are the foundation for the formation of complex fracture networks through hydraulic fracturing. Therefore, researching natural fractures is crucial for the transformation of horizontal wells and gas production efficiency. This article takes the No.8 coal seam in this area as an example, using high-quality five-dimensional seismic data with “two widths and one height,” and calculating the anisotropy intensity by both ellipse fitting and azimuth statistics methods. Combined with analysis of structural stress fields, outcrop data, imaging logging, and array acoustic logging at different stages, the adaptability of these two methods to deep coal bed gas is analyzed based on the results, and the limitations of the ellipse fitting method are analyzed from a theoretical perspective. The research results show that the degree of fracture development in the southern part of this area is higher than that in the northern part, mainly consisting of two sets of fractures in different directions controlled by two periods of tectonic activity in this area. The Yan Mountain period formed fractures close to east-west and northwest directions, while the Xishan period formed fractures close to south-north and northeast directions. The above observations are consistent with the interpretation results of imaging logging, array acoustic logging, and reservoir fracture development reflected by fracturing wells, which verifies the rationality of the azimuth statistics method for fracture prediction. However, the traditional ellipse fitting method cannot describe multiple sets of fractures due to its inherent limitations, making it unsuitable for this area. The fracture prediction results of the azimuth statistics method in this article play an important role in optimizing horizontal well fracturing technology and improving gas production efficiency, providing important references for the prediction of coal bed gas fractures in other blocks.