Unlocking Coal Seam Fractures: Enhancing Structural Analysis of Cleats with Horizontal Borehole Image Logs

Abstract

Cleat identification is crucial for Coal Bed Methane (CBM) production and present-day stress regimes. Cleats which are fractures present in coal seams, typically classified as face and butt cleats, perpendicular to each other and to the coal bedding. The straight-line visualization of cleats in vertical boreholes presents challenges for accurate interpretation. Therefore, the optimal approach is to characterize cleats in horizontal boreholes, allowing more accurate analysis of cleats interpretation.

In this study, an 8-pad, 2.4" Slim Micro Resistivity Imager Tool (SMRIT) was deployed in 10 horizontal Water-Based Mud (WBM) boreholes to acquire high-resolution borehole images. Along with the microresistivity data, petrophysical logs such as gamma ray, resistivity, density and caliper logs were integrated to provide a comprehensive understanding of the subsurface. Data processing was performed, including corrections for magnetic declination and speed correction to ensure accurate image representation. These corrections were crucial for generating both static and dynamic 360° resistivity-based borehole images. Cleats, identified based on their high dip angles (>75°), were manually delineated from dynamic images.

The study processed data from 10 coal seam horizontal boreholes, utilizing dynamic images to clearly distinguish and verify cleats along coal seams. Cleats and other fractures were differentiated by dip angle, as formation fractures (either conductive, resistive, or mixed) appeared similar. In relationship between cleat density and cleat spacing, most of the boreholes show high density of cleats and low cleat spacing. This characterization of cleats significantly improves the effective permeability of otherwise low-permeability coal and improves CBM production. All boreholes showed two distinct cleat orientations (major and minor directions), with an average of 810 cleats per average of 500 m section. Horizontal boreholes provided clearer identification of cleat features compared to vertical boreholes, where orientations were more scattered. The dominant face cleat aligned with the maximum horizontal stress (σ_Hmax) in a NW-SE direction, while the subordinate butt cleat, oriented perpendicular to the face cleat, aligned with the minimum stress (σ_Hmin) in a NE-SW direction.

The SMRIT proved to be a valuable tool for identifying geological structures in horizontal boreholes, especially cleats within coal seams. In addition to structural interpretation, the tool enabled statistical analysis of cleats and reveals the CBM production and horizontal stresses of coals in the study area. This approach supports multiscale studies of cleat systems, enhancing the understanding of coal seam fracture characteristics and field stress regimes.