The growth in deepwater hydrocarbon field developments has increased the demand for geohazard assessment. Proper assessment requires a suite of complementary tools for in situ sampling and testing. These tools are rapidly improving in terms of technology and robustness.
The focus of this paper is on the Fugro Pore Water Sampler (FPWS), an improved downhole tool to sample pore water and gas in situ at water depths as great as 3000m. the samples are kept pressurised during retrieval to detect gas in soil.
Features of the FPWS include optional add-ons for in situ pore pressure and temperature measurement within a single deployment round, and for real-time monitoring of sampling and testing. The FPWS is attached to a new WISON EP system, which provides the thrust to push the tool in the ground with a maximum stroke of 4.5m. the analyses of gas in soil have been significantly improved by the addition of a tracer to the filter water the dilution of the tracer is measure for the dilution of pore water by filter water during sampling. The dilution is applied to determine the in situ pore water salinity. The quality of the measurements from the in situ pore water samples has been validated by independent measurements from pressure (core) samples.
As the offshore exploration moves to deeper water, more geologically complex and geohazard prone areas are encountered. Furthermore, scientific research expeditions are now considering gas hydrate in the soil as a potential hydrocarbon resource. These developments place new demands on offshore site investigations. The demands have led to a suite of new and improved tools for deepwater in situ testing and sampling 1,2.
This paper describes the Fugro Pore Water Sampler(FPWS), an improved downhole tool that collects pressurized in situ pore water samples and, optionally, determines in situ pore were pressure and in situ temperature (Figure 1). Water samples are kept pressurized to preserve any dissolved or free gas.
To date, the FPWS has been utilised in deepwater (> 1000m water depth) site investigation conducted in 2006 and 2007, with a success rate of 75 %. The results from the in situ pore water sampler were complementary to the analysis of pressure (core) samples. The analysis of the pore water from these pressure samples validated the results from the pore water sampler. Further use of the FPWS is commissioned for 2007.
Several in situ pore water samplers have been developed to collect gas in soil 3,4,5. The main purpose of these samplers is to determine whether free gas is present in the soil at in situ condition. Tables 1 present an overview of pore water samplers that collect in situ pore water and gas.
All these pore water samplers in Table 1 require saturation of an inlet filter with de-ionised (and de-gassed) water.
Figure 1: Schematic drawing of the Fugro Pore Water Sampler (FPWS) (available in full paper)
Table 1: Overview of tools for in situ pore water and gas sampling (available in full paper)