Approaches to Fines Production in the Walloons Subgroup, Surat Basin, Queensland

Abstract

This study characterises the root causes for fines generation in coal seam gas wells in the Walloons Subgroup of the Surat Basin, southeast Queensland, Eastern Australia. Fines production can be critical in causing erosion in downhole pumping equipment and disruption to surface facilities. The Surat Basin has, in places, exceptional coal permeabilities (>1 Darcy), and most high permeability wells are completed with pre-perforated liners. Fines that are captured in separators are usually generated from the interburden lithologies, not from coals (reservoir rocks). Fines production characterisation and mitigation generically requires identification of the processes that lead to breakdown of cohesion in rocks that generate fines. Conventionally, fines production is dealt with using a geomechanical approach to understand the interaction of rock strength and in situ stresses in the context of reservoir production conditions. Significant factors that may control fines production are in-situ stresses, rock strength, drawdown and depletion as well as completion type and geometry. Geomechanical models were developed from log based, strain derived stress models, which are calibrated to rock strength testing of the core samples. Interestingly, fines production remains prevalent in areas with low differential stress and little variability in other parameters, such as flowing bottom hole pressure (FBHP). Tests exposing interburden rocks to produced formation water were used to understand rock weakening, as well as a comprehensive program of mineralogy (XRD) and rock fluid sensitivity testing on selected sandstone, siltstone and mudstone core samples. Each sample was photographed initially dry (i.e., in its original state), and then in contact with synthetic brine or various clay stabiliser solutions, in order to qualitatively evaluate the change in both rock strength and stability over the duration of fluid exposure. Results suggest that volcanogenic sediments contribute to fines production in fields in the Walloons Subgroup. The results of the work presented here help to identify completion strategies, go forward development and future production optimisation opportunities.