Horizontal borehole azimuth optimization for enhanced stability and coal seam gas production

Horizontal boreholes have been widely used to extract natural gas from coal seams. However, these boreholes can encounter severe instability issues leading to production interruption. Optimizing drilling azimuth is a potential solution for enhancing borehole stability while considering gas production. In this work, we improved and implemented a dual-porosity, fully coupled geomechanical-hydraulic numerical model into COMSOL Multiphysics to investigate into this factor. The sophisticated numerical model incorporates various critical factors, including desorption-induced matrix shrinkage, stress-dependent anisotropic fracture permeability, and the interactions of gas flow and reservoir deformation in matrices and fractures.

A suite of simulation scenarios (e.g., varying coal strength) was carried out to quantify the impact of drilling azimuth on coal permeability evolution, cumulative gas production, and the borehole break-out width for Goonyella Middle Seam of Bowen Basin, Australia. The model was calibrated against both theoretical permeability values and field gas production data. Due to the lack of directly measured matrix permeability data, the actual gas production was used to back calculate the best-matched matrix permeability, which is 0.65 μD for this particular work. Moreover, based on the breakout shape and induced volumetric strains around the borehole, drilling along the maximum horizontal stress does not necessarily lead to the best stability of the borehole, as generally believed. A drilling azimuth between 0° and 60° results in similar breakout width, whereas a drilling azimuth between 60° and 90° achieves the most efficient gas production. By considering both gas production efficiency and borehole stability, for this particular reservoir condition, the optimum drilling azimuth is determined to be between 45° and 60°.

This study presents a practical approach for determining the optimum drilling azimuth in coal seam gas extraction through in seam boreholes.