Bed Boundary Mapping Technology Improves Coal Mining by Revealing Its Complex Geological Structures

Drilling horizontal wells in the coal mines in Central Queensland, Australia, is key to understanding the complex nature of the coal seams and their lateral extension prior to underground longwall mining. These coal seams are geologically complex, with faulting, varying dips, and bed thinning frequently encountered. Consequently, the requirement arose for an advanced logging tool with the capacity to provide accurate geosteering, reveal complex geology, and ultimately improve footage within the coal seams. Conventional geosteering using gamma ray correlation with offset wells has been widely used in geosteering the horizontal section of these wells. The process of confirming coal seam structures involved a risky and time-consuming process. It required branching out, logging shoulder beds, and then pulling back into the main bore to drill ahead, repeated a few times throughout drilling the horizontal section. Even though this might work in certain applications, the approach has some limitations when drilling these zones. It is reactive due to the shallow depth of investigation of gamma ray measurements and the fact that the sensor may be located too far behind the bit to aid efficiency. More importantly, it involves the risk of drilling into the hazardous shoulder shale and wasting drilling time and footage out of target. For the last 3 years, an advanced geosteering technique utilizing the deep-reading directional resistivity tool has been used for bed boundary mapping in this high-resistivity environment. The tool provides conventional propagation resistivity, azimuthal gamma ray, and directional resistivity with a greater depth of detection than other tools in its class through the use of longer transmitter-receiver spacing for directional antennas. Importantly, these directional measurements are available in all three frequencies (125 kHz, 500 kHz, and 2 Mhz) to allow a greater selection of measurements for structure inversion/interpretation optimal to the particular geology and application. Incorporating long-spacing 2-Mhz directional resistivity measurements, the multilayer bed mapping technology confirmed and accurately interpreted structural changes in the coal seams where the top and bottom of the coal seam were mapped. The traditional method employed by the coal mining industry of detecting coal seams geometry relies on reactive steering, driving the need for multiple openhole sidetracks throughout the well. The bed boundary mapping technique has enabled the operator to overcome the limitations of the conventional geosteering technique. The outstanding result and gained experience gave the operator the confidence to run the tool and geosteering services in more wells to resolve the coal seams’ complex structures and map their boundaries. The inversion result provided geological insights into the coal seams’ structures. The improved geological model based on the inversion has shown that the coal seam is geologically complex, and the correct delineation of its boundaries and identifying their precise true vertical depth can add significant value to the planning, evaluation, and execution of mines.