Abstract

The exploration and sampling efficiency of marine gas hydrate formation can be increased by using the wire-line retrieving drilling tools that includes both an advancing detection tool and a hydrate pressure core sampler. During drilling, formation resistivity is obtained using the advancing drilling tool in front of the coring bit. This data is also used to identify hydrate layers during drilling. It has been successfully demonstrated in design and laboratory tests that the advancing detection tool provides a rapid approach to measure, store and transmit formation resistivity data. If high resistivity is detected while drilling, the pressure core sampler can be deployed from a wireline fishing tool instead of the advancing detection tool. The pressure core sampler has also shown a high efficacy during simulation and field drilling experiments, where it can maintain a maximum pressure of 30 MPa with a core diameter of 50 mm, with an average success rate of retaining pressure of 75% and an average core recovery of 64.5%.

Introduction

Natural gas hydrate is a high energy density and clean resource (Bohrmann and Torres, 2006; Makogon and Holditch, 2007). It has a high combustion rate per unit volume, producing more heat than coal, oil and natural gas under similar conditions, albeit with lower levels of pollution than conventional fuels. Moreover, natural gas hydrate resources are abundant. It is estimated that the total amount of natural gas hydrate in the world is equal to twice of conventional energy in the world (Kvenvolden, 1999). At present, 90% of the total proven natural gas hydrate is distributed in seabed sediments of the continental shelf, mid-ocean ridge, trenches and ridge in the water depth of 1000 ~ 1500m (Johnson, 2011). China has a vast sea area, and the reserves of gas hydrate are also considerable (Yu et al, 2014). In the investigation and evaluation of gas hydrate resources, geophysical methods are often used to detect bottom simulating reflector (BSR) for determining hydrate distribution area and buried depth (Rutherford and Williams, 1989; Zhao et al, 2013). However, sometimes BSR can not accurately reflect hydrate burial depth. At last, the way of drilling sampling is very necessary to obtain core for analyzing physical-chemical properties of strata. During the Ocean Drilling Program (ODP), the pressure core sampler (PCS) and Fugro pressure corer (FPC) was developed for drilling vessel JOIDES Revolution (Pettigrew, 1992; Graber et al, 2002), resulting in many successful pressure cores retrieving from gas-rich or hydrate-bearing sediments (Dickens et al, 1997; Expedition 311 Scientists, 2005). At present, hydrate sampling technology used in China relies on JOIDES Revolution and other drilling vessel (Schultheiss et al, 2009). But the rent is very high, which greatly limits the progress of hydrate exploration and development. Therefore, on the basis of the previous research about hydrate pressure coring technology and according to technical problems encountered in sampling process of Marine gas hydrate, our team develops the wire-line retrieving drilling tools including advancing detection tool and pressure core sampler, aiming to obtain high fidelity hydrate core efficiently and further improve development progress of marine hydrate.

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