Co-Application of Indirect Hydraulic Fracturing and Micro-Proppants with Existing Surface-to-Inseam Wells to Improve Pre-Drainage of Low Permeability Coals in Mining Areas

Abstract

Surface-to-inseam (SIS) pre-drainage has become the most common method to pre-drain coal mine methane (CMM) for safer mining operations, improve gas recovery in areas of overlapping coal seam gas (CSG or CBM) and mining tenures, and provide beneficial use methane for mining operations. Coal permeability is the key discriminator in gas recovery and SIS wells have been less successful in draining tight low-permeability coals. To improve well productivity, fracture stimulation using indirect hydraulic fracturing (IHF) from horizontal wells deployed below potentially mineable coal seams has been applied using 23 stages in a 1000-meter lateral on the first application for mining pre-drainage. To improve IHF treatments, micro-proppants could be injected to increase the stimulated reservoir volume (SRV) by maintaining conductive fluid flow paths in natural fractures and cleats, countering detrimental pressure-dependent permeability (PDP) effects. This paper demonstrates key aspects to the co-application of lateral, multi-stage IHF and micro-proppant injection in coal seam gas drainage. Successful IHF cases applied to date in the Cooper and Bowen Basins, Australia, have provided valuable insights on key considerations in planning and deploying IHF treatments. In addition to increased production from the IHF wellbore relative to a standalone SIS well, intersections of the IHF treatments with offsetting SIS wells are observable and when properly designed and are a method to improve the surrounding SIS well productivity. Key reservoir data and a multidisciplinary approach integrating well test analyses, hydraulic fracturing modelling, and reservoir simulation are used to quantify the potential benefits of the IHF/SIS co-application process. Finally, past laboratory and modelling studies provide insight into the potential benefits of the additional co-application of micro-proppants in IHF treatments. This paper primarily demonstrates the benefits of co-application of IHF and SIS wells based on varying SIS well drainage and interconnecting IHF wells for a Permian coal case in the Bowen Basin. Recovery predictions for the SIS wells are made using various permeability and spacing patterns, then compared with the recovery of IHF deployed within the pattern of SIS wells. This paper highlights the optimal well placement/spacing between the IHF and SIS wells, number of fracture stages along the IHF well, and optimised fracture treatment schedules (e.g., injection rates, fluid parameters, proppant considerations, etc.), based on permeability, permeability anisotropy, and current Australian economic parameters. Finally, investigations regarding the co-application of micro-proppants indicate that an additional benefit is most apparent in low permeability applications. This paper provides workflows detailing the practical aspects of the design, execution, and evaluation of these technologies for both CSG and CMM applications. This technology is particularly suited to other Eastern Hemisphere areas (e.g., China, India, Africa, North America) where: strike-slip regimes complicate hydraulic fracturing; overlapping tenures prevent the placement of steel-based completions in potentially mineable seams; or inseam drilling is unstable or risky based on geomechanics.