In the course of the preparation for a major offshore soil investigation in potentially gas-charged clayey sediments in the Norwegian Trench in the North Sea, a special sampler was built. The tool can retrieve soil samples under ambient (downhole) pressure to allow a quantitative assessment of the amount of gas in the sediment. This "Ambient Pressure Sampler" forms part of the routinely available soil testing package and allows the quick retrieval of a 65 cm3 pressurized sample. The sampler consists of a ball-valve placed at the lower end of a remote-controlled hydraulic push-sampling device. The tool can be operated on a routine basis in the course of a standard soil investigation program. This paper describes the design parameters of the tool and the actual design and the working procedures. Operational experience offshore is discussed as well as the general test results.
In 1983 A/S Norske Shell awarded Fugro B.V. a soil investigation program in the Troll field offshore Norway. The site is located in the Norwegian Trench, which has a water depth of about 330 meters. The soil stratigraphy consists of 60 meters of normally or slightly over consolidated clays overlying hard clays, sand, and gravels.
The general area is covered with pockmarks and geophysical records showed typical anomalies. Both of these facts led the client to believe that shallow gas could be present, which may have migrated through the soft top layers. They requested a study to determine whether it would be possible to sample the soft clay in such a way that gas, potentially entrapped in the clay, could be preserved during retrieval and storage of the samples. The total soil investigation program and its objectives have been described. In this paper, the specific problem of gaseous soil sampling and its solution in terms of hardware and procedures is discussed. The sampler that was newly designed and constructed has been named the Ambient Pressure Sampler (APS).
If gas molecules or free gas is present in the soil, it has either been generated locally by the decomposition of embedded organic matter or it has migrated from a petrogenic reservoir. Soil engineers, geologists, and geochemists are interested to know about the origin, nature, and quantity of the gas for various reasons: The local generation or migration of gas, if present in enough quantity, can pose a hazard to a platform.
Gas present in the soil can affect the soil behavior and results of a soil investigation.
Normal soil samples are subject to decompression during retrieval to the surface. Vaporization of the gas molecules in the pore water and expansion of the gas during decompression will disturb the soil samples, thus affecting the quality of subsequent laboratory tests.
