Logging, Sampling, and Testing for Offshore Geohazards

Abstract The growth in deepwater hydrocarbon field development has increased the demand for assessments of shallow geohazards. Geohazard assessments rely on geophysical surveying which is showing rapid advancements in acquisition and interpretation technologies. This paper describes recent developments in deepwater logging, sampling and testing performed to ground-truth and augment geophysical interpretation. The focus is on improvements in the operation and interpretation of geophysical logging, pressure soil sampling, pressure water sampling and in-situ testing for equilibrium pore water pressures. Figure 1 Examples of shallow geohazards (after Campbell et al., 1986) (available in full paper) These technologies can be used to profile, with depth,geotechnical/geological soil characteristics,the geochemical character of soil/water/gas systems, andgeostress conditions including in-situ pore water pressures other than hydrostatic pressure. Introduction Geohazard assessments provide input for risk-based decisions regarding endeavors such as hydrocarbon field development and scientific research of the ocean environment (Power et al., 2005). The principal stakeholders are industry, governments and communities at large. Figure 1 shows examples of shallow geohazards (Campbell et al., 1986). "Shallow" in this paper is defined as less than 400 m below seafloor. Figure 2 provides context for gathering and interpreting data, including geophysical survey and time-based monitoring measurements. This context is important, as geohazard assessments require an integrated approach (for example Galavazi et al., 2006). Figure 2 Paper focus (available in full paper) The focus of this paper, the logging, sampling and testing technologies, particularly obtaining vertical profiles of:geotechnical/geological soil characteristicschemical character of soil/water/gas systemsin-situ geo-stress conditions including pore water pressures differing from hydrostatic. Item (1) may be described as ground truthing geophysical survey information. Items (2) and (3) provide parameter values that cannot be accurately inferred from geophysical data. It is evident that this paper cannot cover all of the hazards shown in Figure 1. It should be recognised that timebased processes can be monitored by incidental, discontinuous survey measurements, but are more readily captured by long-term monitoring not described here. Historical Perspective Logging, sampling and testing for shallow offshore geohazards developed along two parallel tracks:offshore scientific expeditions andindustry-led offshore geotechnical investigations. The scientific track may be traced to 1968 when JOIDES (Joint Oceanographic Institutions for Deep Earth Sampling) obtained a long-term lease for a custom drillship named Glomar Challenger. This drillship was operated as part of the Deep Sea Drilling Project DSDP. In 1983, the Glomar Challenger was replaced by a more advanced drillship named Joides Resolution operated under the Ocean Drilling Program ODP until 2003. The Integrated Ocean Drilling Program IODP started in 2003 (IODP, 2001). The scientific program includes a second scientific drillship named Chikyu, which allows riser drilling.