Shallow Hazard and Gas Escape Systems Modelling From 3-D Seismic

Abstract

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 reduce risk, 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, it 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. A dataset over the Maari field, Taranaki basin, offshore New Zealand was used for the analysis. Although the field was discovered in 1983, it was not produced until 2009, due to complexities of commercialization requiring many injection and production wells. This complexity increases the importance of a thorough subsurface understanding of hazards and their geometries. The method followed was to initially condition the data using structurally oriented, edge preserving noise attenuation filters. This increased the signal to noise ratio, and helped discriminate genuinely chaotic gas signatures from the background seismic. Attribute analyses, including frequency decomposition and RGB blending, clearly revealed the presence of three main gas chimneys, and associated shallow gas accumulations. Crucially these gas chimneys were traced to specific leakage points in the reservoir units. Finally, these results were used as the basis for geobody extractions that when combined together, comprised a subsurface model that accurately described the movement of gas from depth to accumulation points in the shallow zone. The analysis proved an effective and accurate method for quickly building a subsurface model that not only located shallow hazard risk and provided targets for special survey analysis, but also tied that information back to the reservoir itself, providing an insight into the post-trap migration of hydrocarbons and trap effectiveness.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015