Theoretical And Experimental Study On Fragmentation Mechanism During Polar Debris-Rich Ice Drilling

Polar ice caps and geology conditions is being a desired exploration repository for human beings. Nowadays, more and more countries are developing deep ice and bedrock drilling equipment to go through ice sheet and the ice- rock interface and get subglacial bedrock samples in order to know the geomorphology formation and evolution of this region and understand the ice- rock interface environment, which plays key roles on researching the formation of ice sheets, glaciers dynamics and climate change.Debris-rich ice locates near the ice- rock interface which contains few tens of meters’ basement ice with rocks particles, basal meltwater and glacial till. The hardness and abrasiveness of debris-rich ice is far more than that of ice. It is worth to note that the conventional steel tooth bit is no longer capable to drill through such formation. Cemented carbide drilling and diamond drilling are both applicable but no matter which method the required torque and energy of breaking debris-rich ice torque are much higher than that for ice drilling. On one hand it needs more powerful down hole motor and anti-torque system, on the other hand, conventional impregnated diamond bit and surface impregnated diamond bit requires high weight on bit when drilling ice and ice is relatively too soft to wear carcass diamond particles being exposed, which obstacle its application in debris-rich ice.This paper presents the research on the relationship between drilling torque, rate of penetration(ROP) and weight on bit(WOB), rotational speed, parameters of cutting tools and physical and mechanical properties of rock and ice in debris-rich ice drilling process and the establishment of mathematical model. According to the mathematical model, we can conclude that some drilling parameters for instance ROP, drilling torque and power consumption are effected by ice and rock physical and mechanical properties such as internal friction angle, compressive strength and shear strength, cutting tools parameter(rake angle and relief angle) and the coefficient of friction between cutting tools and samples, which provides theoretical foundation for polar debris-rich ice drillingThe theoretical flow rate of drilling fluid for drilling debris-rich ice is concluded using existing armored cable electro mechanical coring technology. The minimum pump output should be at least 20.2L/min and 45L/min with richer rock formation assuming that the outer and inner diameter of bit is 136 mm and 104 mm, respectively and inner diameter of core barrel is 117 mm. Besides, the simulation of bottom-hole reverse circulation of IBED drill shows that IBED achieve down-hole local reverse drilling fluid circulation and ice cuttings and rock cuttings can get into the ice cuttings chamber.A multiple function test stand was designed to drill ice, debris-rich ice and rock in order to compare different drill bits. Cemented carbide bit, round PDC bit and square PDC bit with 60 mm outer diameter were tested to drill debris-rich ice sample with 10% and 30% granite rock. The results show:a. Cemented carbide bit and PDC bit both can drill debris-rich ice effectively. The instantaneous ROP of cemented carbide bit will occur waves bouncing during drilling in debris-rich ice and the ROP of PDC bit is much faster than that of cemented carbide bit. ROP of round PDC bit is around 5 mm/s(18 m/h) consuming 0.25 kw while that of cemented carbide bit is 2 mm/s(7.2 m/h) consuming 0.18 kw with 1000 N WOB and 30% rock sample tested.b. The mathematic model is proved to be reasonable by the test and theory results comparison on the influence of WOB and rotation speed to ROP and power consumption. Not only do the theoretical and test plot have similar trend but also the calculated and test power consumption of cemented carbide bit and PDC bit is quite close to each other. Taking PDC bit as an example, the calculated and test ROP increases from 2.8 m/s and 4.12 m/s to 11.3 m/s and 8.82 m/s, respectively, under increasing rotation speed from 50 rpm to 200 rpm. The calculated and test ROP increases from 2.8 m/s and 1.6 m/s to 7.1 m/s and 8.82 m/s, respectively, under increasing WOB from 500 N to 1250 N.Since 2011, China has undertaken its own Antarctic deep ice core drilling project. Jilin University is also developing a non-pipe deep drilling equipment for bedrock coring beneath ice in Antarctic- IBED(Ice and Bedrock Electromechanical Drill). This paper using pilot research designed and tested the PDC bit for IBED to drill in debris-rich ice. Although the test stand needs to be improved and the test results are not comprehensive, some results still can conclude: the designed drill bit can effectively drill debris-rich ice and break ice core successfully using core catcher system with pre-rotation before lifting drill, which provides scientific reference for designing of drill equipment for debris- rich ice and rock in polar region.In conclusion, PDC drill bit is efficient for drilling in debris-rich ice with high development and application prospects. This study provides scientific theory for polar drilling and presents theoretical meaning and application value for new type of polar drilling equipment.