| The polar region has rich natural resources and military value.The climate change caused by the melting of snow and ice,the abundant marine resources and biological genetic resources are closely related to our life.Scientifically speaking,the polar ice sheet and the sub-ice environment have recorded the information of the earth's climate change and the history of climate evolution.Because of this,it is known as the“time container”for recording the earth's environmental data.At the same time,this information can clarify the mechanism of earth's climate change.Ice core is the best medium for obtaining effective information and studying environmental,biological,chemical and physical processes in the earth system because of its high resolution,large amount of information,strong fidelity,long time series and high cleanliness.Ice drilling is the most common of obtaining the ice core in order to meet a large number of such needs.Although most of the ice borehole are equipped with appropriate drilling fluids,several accidents related to the stability of the borehole wall still occur.For example,in1996,the ice borehole located in Dome F,Antarctic was stuck at a depth of 2,503 m.In1997,the drilling fluid pressure exceeds the confining pressure for about 0.25MPa in Dye-3 hole at a depth of 2037.63m in Greenland,Arctic,conducted by Denmark NGRIP,resulting in a big expansion on the bottom of the borehole.In 1999,the same stuck accident occurred in Dome C borehole at a depth of 740m which was conducted by Italy-France's Antarctic team.During 5G borehole drilling period,there were several different types of hole wall instability at depths of 550m,2502m and 3668m in Russian Vostok Station,Antarctic.In 2016-2017,the drilling fluid was suddenly lost at the interface of ice and rock,which caused the stop of Pirrit Hills program in southwest of Antarctic.In order to effectively solve the stability problem in ice borehole under brittle behavior,we first combine the different failure criterion of ice and different borehole state with stress distribution around the borehole to obtain critical safety drilling fluid pressure window.The theoretical results indicated that:For an unbroken borehole wall,the safety drilling fluid pressure window calculated by Mogi-Coulomb,teardrop and Derradji-Aouat failure criterion came to be approximate at a strain rate of around 10-3/s.And no borehole collapses or fracturing occurred under the common drilling fluid pressure,but when drilling in high ice flow areas on ice sheets or glaciers,the instability of the borehole would increase.For a fissured borehole wall,the fracture toughness,ice friction coefficient and the fracture state in the ice played important roles in determining the borehole critical failure pressure and a maximum reduction of about 55%of the safety drilling fluid pressure window was calculated when comparing the fissured borehole wall to the unbroken one.We studied the crack propagation on the borehole wall under the interaction of ice pressure and drilling fluid pressure based on the fracture mechanics and the principle of“quasi-static expansion”mechanics.The crack propagation calculated by fracture mechanics indicated that when considering the single crack of a hole wall,the crack will expand rapidly when the length of the crack exceeds a critical value.At the same time,the critical value of the crack length is closely related to the fracture toughness of the ice and the stress distribution around the crack surface.When considering the presence of dense cracks on the borehole walls which are not far with each other,those multi-crack interaction reduces the critical condition for rapid crack propagation.The theoretical assignment of"quasi-static expansion"principle indicates that the crack at that location will still rapidly expand at a certain moment under the action of time accumulation even if the stress intensity factor at the crack tip does not reach the fracture toughness of the ice.We used the CZM model in ABAQUS numerical simulation to study the crack propagation after the crack occur on the ice borehole wall,focusing on how those factors like ice stress state,crack intersection angle,infiltration and viscosity of drilling fluid,influence the interaction relationship between hydraulic crack and natural crack.The simulation results show that:under the normal borehole stress state,when the intersection angle between hydraulic crack and natural crack is larger than 75°,the hydraulic crack is easy to penetrate the natural crack surface,at the same time,higher intersection angles will reduce the critical drilling fluid pressure in the borehole,resulting in the instability of the borehole wall.When the intersection angle is less than75,the hydraulic crack turns and opens the natural crack.The interaction process showed that it is often the first to open the intersection to form an obtuse angle in semi-natural cracks and then in turn to open the entire natural crack.We also concluded that the higher stress difference will obviously increase the displacement of the intersection of the cracks,and help the hydraulic cracks to smoothly penetrate the natural cracks.At the same time,high infiltration rates increase the chance of hydraulic fractures penetrating through natural fractures,but concurrently require higher drilling fluid pressure to maintain crack propagation.The size and growth of artificial columnar ice crystals were studied from the microscopic point of view by using G50 fabric analyzer.On the basis of this,we tested the tensile strength of artificial columnar ice of larger ice crystals under different temperature and loading rate and the friction coefficient between ice and ice,to provide theoretical basis for the stability study of ice borehole wall under brittle behavior.The results of the experiments showed that the average particle size of the transverse ice crystals produced by the low-temperature incubator increases with the decrease of temperature,while the longitudinal grain size does not change significantly.The average grain size of the ice crystals varied between 13mm and 20mm.As the temperature decreases,the ice crystals gradually change from a diverging manner to a mutual occlusion,which also leads to an increase the strength.The maximum tensile strength of the artificial columnar ice can reach to about 0.8MPa.At the same time,ice tensile strength decreases with the increase of the strain rate in the brittle regime.The friction coefficient test between ice and ice showed that when there is no filler between ice bodies,the value of friction coefficient varied from 0.3 to 0.5,and under our tests speed,the friction coefficient increasing first followed by an obviously decreasing.The filling of different drilling fluids between ice and ice,has a great influence on the friction coefficient and the friction coefficient values are larger than those without filling.Our test results indicated that the drilling fluid inhibit the occurrence of shear failure on the ice borehole wall.We used true triaxial hydraulic fracturing experimental system to carry out hydraulic fracturing simulation tests on artificial columnar ice borehole under different borehole wall and stress state.The results showed that:For a complete borehole wall,the initial fracturing and crack propagation pressures are quite different freezing at different temperatures.As the temperature decreases,the internal strength of the ice became greater.At the same time,we confirmed that when using D-A criterion is selected to predict the critical fracturing pressure of complete borehole,the parameter n is between 2.4 and 2.6.Also,when the difference between the three-dimensional stress was not large,the crack would propagate basically perpendicular to the minimum horizontal principal stress direction.For a fissured borehole with stratified properties,when the value of?-??/is less than about 0.75,the fracturing cracks all extended along the interface of the layered frozen,where the expansion pressure is approximately equal to the vertical pressure.the value of?-??/is greater than about 0.75,the crack would propagate basically perpendicular to the minimum horizontal principal stress direction.At this time,the corresponding fracturing and expansion pressure are much smaller than the case of stratified one's.For the borehole wall caused by cold and heat effect,the crack initiation and expansion form were uniform,the expansion pressure is approximately equal to the minimum horizontal principal stress.In the whole tests,the cracks inside the ice sample were mutually communicate with each other.We can conclude that,in the actual drilling process,the difference of the three-dimensional stress is small,when drilling through the fissured ice layer,the drilling fluid pressure should be kept as low as that overburden pressure and special attention about the change of the liquid level should be taken to ensure borehole stable. |
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