Shell and venture partners encountered numerous seafloor and near seafloor geohazards at the deepwater Gumusut-Kakap field on the continental slope offshore Sabah, Malaysia. The geohazards included subsurface gas hydrates, steep seafloor slopes, a history of mass transport activity, seafloor expulsion, shallow faulting, low seismicity, shallow gas and shallow water flow.
Early identification of the geohazards, by interpretation of exploration 3D seismic data and evaluation of discovery and appraisal well logs, provided the impetus necessary to undertake an extensive data acquisition program to determine the extent of the hazards and the mitigation needed for a successful development. A deeptow survey, high-resolution 3D seismic survey and a geohazard investigation employing geotechnical methods and oilfield well logging techniques, were executed to characterise the geohazards. Numerical modeling was employed to further evaluate the hazards of gas hydrate dissociation and slope instability. Characterization of gas hydrate saturations and analysis of gas hydrate dissociation, caused by heat flux from production wells, was novel work, which is not commonly performed within the deepwater oil industry.
This paper describes the characterization of the near seafloor geohazards encountered within the Gumusut-Kakap field and presents sample data from the site surveys and results from analytical models used to determine the potential impacts of the geohazards. The geohazards are likely to be common to other deepwater prospects within the basin. The site survey activities and analytical techniques used at Gumusut-Kakap can serve as a model for future developments. As a result of the geohazard risks, as well as other project drivers, a field development plan with subsea drill centers and an FPS host was selected over a centralised development with direct vertical access wells.
The Gumusut-Kakap field in the Malaysian deepwater province offshore northwest Borneo was discovered in 2003, with the drilling of the Gumusut-1B well. The field is located 120km (75 miles) offshore, in the southern portion of Sabah Block J and Block K (Figure 1). Four appraisal wells, Gumusut-2 and Gumusut-3 in Block J and Kakap-1 and Kakap-2 in Block K, were drilled between 2004 and 2005.
The seafloor slopes regionally to the southwest, with a gradient of approximately 5 degrees and water depths within the development area vary from 900m to 1,200m (2,950 to 3,940 ft). Seafloor and near seafloor sediments are hemipelagic, interbedded with turbidites and mass transport deposits.
A number of geohazards were identified:
Gas hydrates
Steep seafloor slopes
Mass transport activity
Seafloor expulsion features
Shallow faulting
Low seismicity
Shallow gas
Shallow water flow
This paper describes the characterization of the above geohazards, with the exception of the shallow gas and shallow water flow hazards, which were prevalent throughout the field and did not significantly influence field development plans.
