Geoteric | February 5, 2016

Shallow Hazard and Gas Escape Systems Modelling from 3D Seismic

Shallow Hazard and Gas Escape Systems Modelling from 3D Seismic

Geoteric on 05 Feb 2016
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Shallow Hazard and Gas Escape Systems Modelling from 3D Seismic
By Tom Wooltorton, GeoTeric
 
Shallow hazards in offshore oilfield developments often come in the form of gas chimneys and shallow  gas emplacements, and can endanger the integrity of  rig or platform foundations as well as ongoing drilling  operations. Therefore, mapping them accurately prior
to any drilling or development operations can be critical  for safety and reducing costs due to interruptions.  An analysis was carried out to determine if 3D seismic,  interrogated in a data driven but interpreter guided  fashion, could be used to build a subsurface model  revealing in detail where gas escape chimneys feed  shallow gas accumulations, allowing the shallow hazards  to not only be mapped accurately and risk reduced,  but also inform the risk of trap leakage. Modern highresolution  3D seismic is often more than adequate to  locate such features and capture their extents, and  is available from the main reservoir interpretation at
no extra cost. Its large areal coverage can locate and  prioritize targets for ultra-high resolution shallow hazard  surveys. Additionally, complex feature morphologies can
be captured using semi-automated techniques that are  impossible to define using manual interpretation on a  vertical slice-based approach.  A dataset over the Maari field, Taranaki basin, offshore  New Zealand was used for the analysis. The survey has  a lateral resolution of 25m by 12.5m and a vertical  sample rate of 4ms. The reservoir is composed of  several Miocene and Eocene pay sands layered in a  four-way closure, and although the field was discovered  in 1983, it was not produced from until 2009, due  to complexities of commercialization requiring many  injection and production wells. Issues included the
relatively shallow depth of the reservoir (1300m TVD),  cold temperatures and waxy oil. In the Moki sandstone  alone six horizontal production wells with ~11,000m of  section have been drilled, accompanied by three deviated  water injectors. This complexity increases the importance  of a thorough subsurface understanding of hazards and  their geometries, and the significance of shallow hazards  has been identified throughout.
 
Download full article here.
 
(from Geophysical Society of Houston's February journal issue)
 
 
Horizon slice at the base-Pliocene unconformity (600-700m TVDss) displaying a frequency decomposition RGB (Red Green Blue) blend, highlighting several features. The interplay of colour reflects the relative contributions of the individual frequency magnitude responses. Three gas chimneys are identified by low magnitude, chaotic responses (circled), and these lie adjacent to several fault systems that facilitate gas migration into shallow, permeable sands. 

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