A Logging Evaluation Method for Cleat Development in Deep Coal Rocks Based on Acoustic Attenuation

Compared to shallow coal seams, deep coal bed methane (CBM) is characterized by a higher content of free gas, which typically exhibits the traits of “immediate gas production upon well startup and high output upon gas production.” It has become an important natural gas resource for China to optimize its energy structure and achieve carbon peaking and carbon neutrality goals. Cleats serve as the primary pathways for gas desorption and migration in deep coal seams and act as weak structural planes of coal rocks, playing a crucial role in controlling both wellbore wall stability during drilling and fracture propagation during hydraulic fracturing. Therefore, the degree of Cleat development is regarded as a critical indicator for the efficient development of deep CBM. At present, most Cleat evaluation methods in coal rocks are derived from shallow coal seam studies, which primarily rely on compressional wave velocity and shear wave velocity characteristics. However, in deep coal seams subjected to high stress and strong compaction, Cleats often exhibit low aperture, resulting in weak logging responses. This makes the methods developed for shallow coal seams inapplicable, highlighting the lack of effective approaches for evaluating Cleats in deep coal seams. This study begins by conducting experiments on 70 coal rock samples with varying degrees of Cleat development. Combined with numerical simulations of coal rocks with different Cleat apertures, the experimental and simulation analyses Cleatly confirm the effectiveness of acoustic attenuation in evaluating Cleat development in deep coal seams. Compared to velocity and anisotropy, attenuation is more sensitive to Cleats with low aperture or partial closure. Based on dual-porosity medium theory and squirt flow theory, a rock physics characterization model of Cleat-related acoustic attenuation in deep coal seams is established. Subsequently, an inversion method for Cleat development degree based on acoustic attenuation is developed using an iterative method and applied to actual well evaluations. The results demonstrate that the proposed logging evaluation method for Cleat development based on acoustic attenuation more accurately reflects the degree of Cleat development. Compared to the velocity-based methods for shallow coal seams, the relative error is reduced from 27% to 18%, indicating that attenuation is more effective than velocity in assessing Cleat development in deep coal seams.

fig1. Comparison of Logging Evaluation Results for Cleat Development in Deep Coal Rocks

fig2. Comparison of Relative Errors in Logging Evaluation of Cleat Development in Deep Coal Rocks