Study On The Pressure Retaining Capacity Of Ice Valve Based Pressure Corer For Natural Gas Hydrate Sampling

Pressure corer is the key equipment for natural gas hydrate investigation.However,since 2002,in the world's major hydrate exploration voyages,the average success rate of pressure coring is less than 60%,which seriously limits to study on natural gas hydrate stratum.Pressure coring techniques need to be improved urgently.At present,nearly all the pressure corers are based on mechanical valve,e.g.,a ball valve or a flap valve,to close the bottom of coring barrel,thus sealing and maintaining the in situ pressure.However,a mechanical valve works in the fluids laden with abundant solid particles in the borehole drilled.Its sealing performance can be easily deteriorated by the solid particles and drilling cuttings.In the paper,an ice-valve-based pressure-coring system using an ice valve made from the in situ drilling fluid in the borehole drilled to retain pressure was proposed.The drilling fluid in the bottom of the sample tube is frozen as an ice block,then the mouth of the sample tube were sealed by the ice block,which is called as the ice valve.Compare to the conventional pressure corers which are based on the mechanical valves,the ice-valve-based pressure corer has several advantages: no moving parts and no precise matching surface are needed,other conventional sealing elements are also in absence.Therefore,the ice-valve-based pressure corer seems to be a promising solution to develop the new generation of pressure corers with high performance for investigating natural gas hydrates.This study was carried out in several aspects of research content as followed :Firstly,three different structured ice valve tubes were designed and manufactured to study the influence of the ice valve tube structure on the ice valve pressure retaining capacity.Experimental results show that the ice valve tube with several grooves can improve the pressure retaining capacity of the ice valve significantly.Secondly,the effect of ice valve length,the size and concentration of solid particles in the drilling fluid,the rust on the inner surface of the ice tube,and the freezing temperature on the pressure retain capacity of ice valve formed from natural gas hydrate drilling fluid were studied experimentally.Results show that the ice valve with diameter of 58 mm and length of 85 mm can retain 25 MPa.And increasing the ice valve length can improve its pressure retaining capacity significantly.Instead of weakening the pressure capacity,the solid particles can enhance the ice valve pressure retaining capacity.The rust on the inner wall of the ice valve tube have little influence on the pressure retain capacity of the ice valve.Low temperature is beneficial to the ice valve pressure retaining capacity,and the lower the freezing temperature,the higher pressure the ice valve can retain.Thirdly,The pressure retaining capacity of the ice valves made from seawater mixed with different mass concentrations of bentonite was tested;The influence of the ice valve length and freezing temperature on the pressure retaining capacity was also studied.Results show that the ice valve made from pure seawater could not retain pressure for the fissures caused by stress produced by the temperature difference and the incompletely freezing of the ice valve caused by the concentrated inorganic salts in the core of the ice valve.The ice valve made from seawater mixed with 7% mass concentration of bentonite can sustain pressures of up to 47 MPa.Both high ice valve length and low freezing temperature are beneficial to improving the pressure sustaining capacity of the ice valve.Results indicate that using the ice valve made from seawater mixed with bentonite as an option to substitute mechanical valves in the pressure corer can be a potential solution to improve the recovery of pressurized hydrate-bearing sediment core.In order to shorten the ice valve formation time and ensure the cold source can freeze the ice valve continually,the cold source based on evaporative refrigeration was proposed,and relevant experimental system was also built.The ice valve tube with spiral groove in the outer wall was employed as an evaporator.And the refrigeration compressor driven by power was employed as a refrigerator.The insulation layer was adopted to avoid the heat exchange between the sampler and drilling fluid in the annul space between the sampler and the borehole being drilled.The experimental results show that it's an efficient method to employ the evaporative refrigeration system as the cold source,and the insulation layer with air can prevent the freezing of drilling fluid in the annul space between the sampler and borehole efficiently.