Influence of in Situ Stresses on Fracture Stimulation in the Surat Basin, Southeast Queensland

Abstract

The contribution investigates the relationship between in situ stress regimes, natural fracture systems and the propagation of induced hydraulic fractures in APLNG's (Australia Pacific Liquid Natural Gas) acreage within the Jurassic to Cretaceous Surat Basin in southeast Queensland. On a regional scale the data suggest that large basement fault systems have significant influence on the lateral and vertical interplay between geomechanical components which ultimately control permeability distribution in the area. At a local scale we show several case studies of significant in-situ stress variations (changes in tectonic regime from reverse to strike-slip, changes in horizontal stress orientation as well as changes in differential horizontal stress magnitude) which are identified from wireline image log interpretations and geomechanical models constructed from wireline sonic and density data. These variations are reflected in hydraulic fracture propagation, which is monitored through microseismic monitoring, tiltmeter monitoring. Reverse stress regimes result in the propagation of horizontal fractures; in areas of higher differential stress linear hydraulic fracture orientations are common, whereas in regions of lower differential stress the orientation of hydraulic fractures appears influenced by both stress and pre-existing fractures. The paper is relevant for fracture simulation in areas with complex in-situ stress regimes. The major technical contribution of the study is the use of geomechanical modelling for predicting hydraulic fracture propagation styles.

Introduction

Coal seam gas production depends on effective connectivity of the coal-specific (reservoir) permeability and the wellbore. This permeability connectivity between coal matrix and wellbore is provided via natural coal fractures and/or coal cleats. In the Walloons coals permeability is dominated by natural, tectonic fractures which can be identified on image logs. Cleat orientations and density (which may be observed in drill core, but not in image logs) follow broadly the orientation of natural fractures. Our analyses of the permeability system are based on the interpretation of natural fractures in coals from image logs.