Shallow Hazard and Gas Escape Systems Modelling from 3D Seismic Data

Abstract

Shallow hazards in offshore oilfield developments often come in the form of gas chimneys and shallow gas accumulations, and can endanger the integrity of rig or platform foundations as well as ongoing drilling operations. The presence of different fluids and different fluid phases can result in local pressure differentials that lead to instability of shallow sediments, and can make them unpredictable in behaviour when loaded or drilled through. Mapping these accumulations accurately, and understanding the systems that feed them, prior to any drilling or development operations can be critical for safety, and reducing costs due to disruption and delay of the drilling program.

A test-case 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 high-resolution 3D seismic is often more than adequate to locate such features and capture their extents, is available from the main reservoir interpretation at no extra cost, and with 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.

A dataset over the Maari Field, Taranaki Basin, offshore New Zealand, was used for the analysis. The seismic bin spacing is 12.5 m × 25 m and the available data coverage was approximately 120 km². The vintage made available was shot in 1999, and superseded by a later higher resolution, longer offset survey in 2012. All of the results discussed here were extracted from the 1999 survey. The reservoir is composed of several Miocene pay sands at the top of a dome structure, 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 (1300 m true vertical depth [TVD]), cold temperatures and heavy oil. This complexity increases the importance of a thorough subsurface understanding of hazards and their geometries, to facilitate the drilling of wells as safely and effectively as possible.

AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015