ABSTRACT

An analysis and transfer system(ATS), which can subsample cores and subsequently transfer subsamples into specialized chambers before they are sent to the laboratory for further measurements, is able to keep the in-situ temperature (2∼4°C) and pressure (30MPa) during the transfer process, which can prevent the changing of physicochemical properties of the cores. The system has been tested in the South China Sea twice and shows an excellent performance on pressure maintenance, the amplitude of the pressure fluctuation caused by the movement of catcher and cutting unit is less than 20%.

INTRODUCTION

Natural gas hydrate (NGH) is a kind of ice-like solid compound composed of gas (mainly methane) and water, which can be formed in nature and man-made environment under low temperature and high pressure (Makogon, et al., 2007). Hydrate in the crust is a potential unconventional natural gas resource with abundant deposits, mainly distributed in the continental permafrost sedimentary layer, the ocean bottom at the outer edge of the continent and the deep seabed plain sedimentary layer. The gas content of natural gas hydrate is quite high; the unit volume of natural gas hydrate contains up to 180 times the volume of gas. The natural gas hydrate coring from the seafloor provides samples for the laboratory studying on physicochemical properties of natural gas hydrate and the condition of mineral reserves, which are of great importance for the development of the mining of the seabed resource, as well as the environmental protection.

Development of the pressure corer has involved teams around the world, including the International Ocean Drilling Program (IODP), the International Deep-sea Drilling Program (DSDP) and the European Union's Marine Science and Technology Program. The type and service conditions of major pressure-retained corers in use are shown in Table 1. However, it is still difficult to gain a single columnar pressure-retained core with a length greater than 3 m. As for China, the long gravity-piston pressure-retained corer developed by Zhejiang university (shown in Fig. 2) was tested on the HY6-11-03 voyage and obtained a single column of pressure-retained core of 14.15 m, which is the longest single column core with in situ pressure in the world so far. Physical characteristics of NGH include shear strength, compressibility, porosity, permeability, thermal conductivity, pressure and temperature. Some features can only be measured in the laboratory. Therefore, the system which can subsample cores and subsequently transfer subsamples into specialized chambers for further measurements is the major objectives of researchers (Zhao, J., et al., 2011; 2012). PCATS, the Pressure Core Analysis and Transfer System shown in Fig. 1, is the interface between the pressure corer and investigators using pressure core samples (Schultheiss, P, et al., 2006; 2009; 2011). Pressure cores can be transferred from compatible pressure corer into a specialized chamber (Santamarina, J. C, et al., 2012). Nondestructive analysis includes gamma density measurements, P-wave velocity measurements, and both linear X-ray images and 3-D X-ray computed tomography, providing data that has intrinsic scientific utility and is critical for the selection of subsamples (Chen, J. W, et al., 2013).

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