Investigation of the subsurface structural behaviour in the Surat Basin using SAR interferometry deformation maps
2017-03-03T00:47:51Z (GMT) by
Mapping subsurface structural behaviour and volume change in the underlying reservoir due to resource extraction are of prime importance in both conventional and unconventional hydrocarbon reservoirs. The structurally fractured or faulted zones that often occur around collapsed features are permeable zones for hydrocarbon accumulations and weak zones for infrastructure development. Therefore, knowledge of subsurface structural behaviour is a key priority for both geoscientists and reservoir engineers. In-situ geophysical observations such as seismic testing and well-logging are the most commonly used methods for determining subsurface structure, by marrying time and depth measurements of seismic surveys and wells. Volumetric techniques are also able to indirectly estimate the hydrocarbon in place based on a geological model, but due to technical limitations such as sparse sampling in time and a need for detailed down-hole information, enhanced mapping of subsurface behaviour requires costly data and software, and expertise in geological modelling and interpretation. Interferometric SAR remote sensing provides a non-contact observation technique for monitoring large hydrocarbon basins with much higher spatial/temporal coverage and lower cost than traditional methods. While it allows for measuring the ground surface deformation with high vertical accuracy, it does not give any direct information on subsurface structure. Consequently, this thesis investigates the novel idea of using InSAR deformation maps, complemented with geological modelling to extract reservoir volume change and to infer the structural behaviour of the subsurface for an unconventional hydrocarbon field with no access to a dynamic model. This study first presents the outcome of two different InSAR processing algorithms using a unique combination of satellite acquisitions to detect and analyse ground surface deformation due to man-made interactions in an operational hydrocarbon extraction basin located in eastern Australian. For the last five years, the north-eastern part of this basin, containing coal seam gas mining operations and accompanied groundwater extraction, has drawn the attention of local operating companies and water commissions for its gradual depressurization and subsequent land surface deformation. Contrary to a previous study, which used a different interferometric technique, three regions above coal seam gas mining districts were identified as having an ongoing downward motion. As a proof-of-concept study, the sub-basin with a maximum settlement risk and limited seismic measurements was selected for evaluating its subsurface structural behaviour and reservoir volume change. This research presents a unique approach to comprehensively assess the viscoelastic multi-layer source model for the selected area with significant deformation overlaying an unconventional reservoir. This model was previously tested on conventional hydrocarbon resources with no access to 3D geological modelling. Tuning elastic properties of underlying formations in a stratified coal seam gas reservoir with more than approximately 300m thickness of overburden and several fresh-water aquifers was conducted by integrating down-hole logs and seismic interpretation through property modelling and source inversion. The outcomes of the inverse modelling in this coal seam gas reservoir include the retrieval of stress components and fractional volume change. Analysing volume change results revealed that the dense network of extraction wells was the main cause for ground surface deformation and subsequent volume change. Moreover, the two-lobe pattern and NW-SE trend of volumetric change were found to be the controlling effect of an underlying structure, such as an aperture or a fault that affects subsurface behaviour and was not identified in the 3D geological model due to the absence of seismic acquisition in this area.