Geohazards: Are We Looking at Them the Right Way? View From Down Under
Authors: Lane Andy , Taylor Alex 
Published: October 2008

The geohazard evaluation of an exploration permit should start early in the exploration process and generally prior to finalising leads and drilling locations. Two key resources that are employed are exploration seismic and offset wells. Some exploration permits may only have limited 2D exploration seismic when the geohazard evaluation commences or may have 3D exploration seismic.

Frequently the evaluation commences using 2D and the subsequent acquisition of 3D data refines the hazard assessment prior to selection of the final drilling location. Correlation of shallow seismic data to well data may provide poor results however on a positive side this can prove that the shallow section is drillable and depending on the detail may provide invaluable data for future anchoring, guide base settlement and conductor support.

Depending on results and certainty / risk a decision is made to continue the geohazard assessment with further data acquisition and analysis. Data could be engineering seismic, high resolution or ultra high resolution multi channel seismic, geotechnical coring, geological modeling, geochemical coring, and remote sensing in addition to analysis of measurements made while drilling offset wells.

This paper provides an overview of a systematic approach to planning and managing geohazard assessments and field activities from an Australian perspective and highlights the lessons learnt from various operations within Australia and internationally.

Introduction

Geohazards are defined as seafloor and substrate soil / rock conditions having a potential of developing into a failure event causing loss of life or investments. They include slumps and slides, gas hydrates, gas charged sands and shallow water flows. The definition is expanded to include geological features having an impact on drilling operations; abnormal pore / formation pressure and shallow water flows, shallow faults, authigenic carbonates, infilled channels, palaeos lumps, reefs / bioherms, pockmarks, uneven seafloor and salt domes. Potential consequences of geohazards to drilling operations include loss of rig from blowout / punch through/ foundation scour and loss of well from shallow water flow. Figure 1 graphically illustrates why geohazards are investigated. The potential for geohazard impact increases in the production stage due to longer timeframe of occupancy and increased exposure to the seafloor.

The complete identification of potential geohazards has to be based on comprehensive site survey and geotechnical investigation campaign data, which has significant HSE, planning, logistics and cost implications. Routine completion of such site specific studies for each and every well is a simplistic approach requiring a significant effort. Furthermore although defining the potential for geohazards, they do not make the problem go away! Surveys are neither a magic wand nor certainly a tick in the box solution!

Engineering programmes can be designed to avoid or withstand the impact of predicted geohazards. However, these programmes have a cost impact and if the geohazards are poorly understood or incorrectly evaluated the cost impact of a flawed programme can be even greater.