Approaches To Fines Production In The Walloons Subgroup, Surat Basin, Queensland

Abstract

The root cause of fines generation is characterised for coal seam gas wells in the middle Jurassic Walloons Subgroup (WSG) in the Surat Basin, Queensland, Australia. Coal Seam Gas (CSG) has been produced for around 10 years from coal measures in the WSG. The Surat Basin has, in places, exceptional coal permeabilities (>1 Darcy).

Fines production characterisation and mitigation generically require identification of the processes that lead to breakdown of cohesion in rocks. Conventionally, fines production is dealt with using a geomechanical approach to understand the interaction of rock strength and in situ stresses under reservoir production conditions. Significant factors that may control fines production are in-situ stress, rock strength, rate of reservoir 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 was observed 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 conducted to understand the extent of rock weakening. A comprehensive analysis of mineralogy (XRD) and rock fluid sensitivity testing on selected sandstone, siltstone and mudstone core samples were also conducted. Each sample was photographed initially at surface after coring and then again after exposing to synthetic brine or various clay stabiliser solutions, to qualitatively evaluate the change in both rock strength and stability over the duration of fluid exposure. Petrophysical interpretation of wireline logs were compared with XRD and the results suggest volcanogenic sediments contribute to fines production in the WSG.

Essentially fines production appears to be a result of produced fluid – rock interaction, leading to a deterioration of interburden rock strength. The results of the work presented here underpins future production optimisation opportunities.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018