| The polar regions abound in mineral,biological,and marine resources,serving as a vital supplement to global energy demands.Concurrently,the climate change,extreme environments,and intricate ecosystems in these regions are pivotal for advancing human comprehension of the Earth’s environment and forecasting future alterations.Polar ice sheets are renowned as "time capsules," harboring invaluable scientific data on climate change,geology,and biology spanning tens of thousands of years.This knowledge profoundly enhances our understanding of climatic variations and reveals the intricate processes of biological evolution.Polar drilling represents the most direct method for acquiring ice cores,and in response to scientific research imperatives,nations worldwide have initiated ice core drilling initiatives in the polar regions.Despite the employment of specific technological measures to sustain the stability of ice holes,incidents pertaining to their destabilization frequently occur.Due to the limitations of polar geographic location and scientific research timelines,the annual pure drilling time for polar drilling is comparatively limited,and the management of in-hole accidents significantly impedes the overall progress of such projects.Currently,a preponderance of common in-hole accidents stems from the deformation of the ice hole and the cracking of the hole wall,primarily attributed to pressure differentials within the hole.However,there is a dearth of in-depth research and universally recognized effective measures to address the instability of ice hole walls.To address this issue,this paper delves into the mechanisms of ice hole wall deformation and hydraulic fracturing behavior in polar ice holes.In this paper,a viscoelastic-plastic stress field distribution model for ice holes is formulated based on the viscoelastic-plastic constitutive equation of the ice body.The model analyzes the impact of varying constitutive model parameters and geoengineering factors on the stress field distribution within the ice holes.Subsequently,the maximum tensile stress criterion,the Mohr-Coulomb criterion,and the DerradjiAouat criterion,are employed to simulate the viscoelastic-plastic crack initiation in the ice hole wall.The theoretical analysis reveals that an increase in the horizontal ground stress difference and drilling fluid column pressure leads to an enlargement of the viscoelastic radius and a reduction in the circumferential stress at the hole wall.Specifically,when the maximum horizontal ground stress rises from 9 MPa to 11 MPa,the plastic radius expands from 0.49 m to 0.54 m.Within the visco-plastic zone,both the radial and circumferential stresses of the ice hole decrease with the augmentation of the maximum horizontal ground stress.Conversely,in the viscoelastic region,near the wellbore(ρ<0.7 m),both stresses decrease as the maximum horizontal ground stress rises,while in the distal region(ρ>0.7 m),both stresses increase.The established model for hydraulic fracturing initiation in ice holes,coupled with experimental data from ice body hydraulic fracturing tests,indicates that when the ice hole wall exhibits viscoelastic properties,the Derradji-Aouat criterion accurately predicts shear damage as the initiation mode.However,for a visco-plastic ice hole wall,the maximum tensile stress criterion is preferred,indicating tensile damage as the initiation mode.Based on the glacier physical mechanics theory and the established ice hole stress field distribution model,a comprehensive numerical calculation model for analyzing ice hole deformation during polar drilling has been developed.Subsequently,an indepth analysis was conducted to examine the factors that influence ice hole deformation,leveraging the context of the Chinese deep ice core drilling project DK-1 in the Antarctic Dome A region.Through normalization of these factors,the principal controlling elements of ice hole deformation were identified.The study reveals that throughout the entire hole depth range,closure,expansion,and coexistence of these phenomena occur concurrently.Notably,the polar surface temperature exerts the most significant influence on ice hole deformation.Beyond a critical hole depth,however,the density of the drilling fluid emerges as the primary governing factor.Specifically,for the DK-1 ice hole,closure is observed in all directions above 1000 m.Between 1000 and 1600 m,closure is evident in the direction of maximum horizontal ground stress,whereas dilation occurs in the direction of minimum horizontal ground stress.Below1600 m,dilation is observed in all directions.Among the various factors affecting ice hole deformation,the polar surface temperature stands out as the most influential,exhibiting a nonlinear relationship with the rate of change in pore diameter.As the surface temperature undergoes a linear decline from-27.2°C to-58.5°C,the respective rates of change in pore diameter at a depth of 800 m in the direction of minimum horizontal ground stress are 41.48%,11.37%,2.10%,1.09%,and 0.02%,Conversely,at the same depth but in the direction of maximum horizontal ground stress,the corresponding pore diameter change rates are 82.37%,38.23%,10.92%,2.51%,0.45%,respectively.Regarding the relationship between drilling fluid density and the rate of change in pore diameter,a critical depth emerges.Above this depth,the relationship is linear,while below it,the relationship becomes nonlinear.Specifically,for the DK-1ice hole,the critical depth in the direction of minimum horizontal ground stress is 1200 m.Beneath this depth,an increase in drilling fluid density correlates with a higher rate of change in hole diameter.Conversely,in the direction of maximum horizontal ground stress,the critical depth is 800 m.Below this depth,a higher density of drilling fluid results in a reduced rate of change in pore diameter.An independently designed low-temperature true triaxial hydraulic fracturing experimental system was employed to conduct comprehensive hydraulic fracturing experiments on ice borehole walls.The experiments investigated the intricate effects of ice body temperature,drilling fluid properties,and ground stress conditions on the initiation and extension patterns of fractures within the ice.Subsequently,the accuracy and validity of the previously formulated pore wall initiation model were experimentally verified,thereby assessing its applicability.Finally,an analysis was conducted utilizing the multilayer perceptron neural network method to evaluate the factors that influence water pressure fracturing of the ice borehole wall.The results of this rigorous study indicate that ice temperature is the most pivotal factor affecting the fracturing behavior of ice borehole walls,with a lower temperature being associated with a heightened likelihood of multiple crack formation due to the increased internal energy accumulation resulting from the elevated crack extension pressure.The triaxial hydraulic fracturing test reveals a distinct correlation between hydraulic fracturing pressure and various parameters.Specifically,a decrease in ice temperature and an increase in vertical ground stress both lead to an increase in hydraulic fracturing pressure.As the temperature was reduced from-10°C to-30°C,the hydraulic fracturing pressure increased from 3.77 MPa to 6.96 MPa.Similarly,an increase in vertical stress from 5 MPa to 7 MPa resulted in an elevation of the fracturing pressure from 12.59 MPa to 16.54 MPa.Conversely,an enlargement in sample size and an augmentation in horizontal stress difference led to a decrease in hydraulic fracturing pressure.Regarding the crack patterns observed after hydraulic fracturing,the ice samples exhibited random cracking patterns,with single and double cracks forming in the laboratory-scale specimens.Notably,larger-sized samples tended to develop cracks at a 45° angle under no-perimeter-pressure conditions.In the presence of perimeter pressure,where the horizontal ground stress was non-uniform,the cracks in the ice samples were primarily perpendicular to the direction of minimum ground stress and propagated along the direction of maximum ground stress.A thorough analysis of the importance level of factors within the multilayer perceptron model indicates that ice sample temperature is the most significant factor affecting hydraulic fracturing pressure,followed closely by minimum horizontal principal stress.Among the factors considered,drilling fluid viscosity exhibited the least influence.A rigorous numerical model,leveraging the Extended Finite Element Method(XFEM),was formulated to investigate the three-dimensional hydraulic fracturing process and the simultaneous expansion of two-dimensional double-fractures within the ice borehole wall.The study incorporated factors such as the viscosity of the drilling fluid,its potential leakage,and the spacing between fractures to analyze the expansion behavior of hydraulic fractures within the ice body.The integration of indoor experimental data and simulation outcomes reveals that the viscosity of the drilling fluid exerts a marginal influence on the hydraulic fracturing behavior of ice borehole walls.Conversely,the leakage of the drilling fluid substantially enhances the extension distance and width of the fractures.Furthermore,in the event of hydraulic fracturing accidents resulting in the formation of multiple fractures with narrow spacing,the extension of these fractures is subject to mutual shielding effects,thereby leading to a reduced extension distance.Indoor experiments have demonstrated that as the viscosity of the drilling fluid increases from 10 m Pa·s to 100 m Pa·s,the corresponding hydraulic fracturing pressures are 4.63 MPa and 4.55 MPa,respectively.This slight variation suggests a minor influence of viscosity on the fracturing process.Numerical simulations further reveal that as the viscosity increases from 1 m Pa·s to 50 m Pa·s,the maximum fracture width increases marginally,from 4.14 mm to 4.17 mm at the initial stage of hydraulic fracturing,and from 9.33 mm to 9.35 mm at the later stage.This indicates that the effect of drilling fluid viscosity on the fracture extension distance and width in drilling wells is negligible.However,when the leakage volume of the drilling fluid increases from 0.001 m3/s to 0.003 m3/s,a significant impact is observed.Specifically,the maximum fracture width increases substantially,from 4.14 mm to 6.26 mm during the early fracturing phase,and from 9.32 mm to 15.4 mm in the later stage.This significant increase highlights the substantial role of drilling fluid leakage in enhancing the severity of hydraulic fracturing accidents.In scenarios where multiple cracks are induced by hydraulic fracturing and the spacing between these cracks is narrow,the expansion of these cracks alters the stress field in the vicinity,reducing the vertical stress.Consequently,the expansion speed increases.However,due to mutual shielding effects,the cracks tend to propagate in directions that are further apart from each other.This paper reveals the deformation mechanism of ice holes and identifies key controlling factors that influence the crack initiation and expansion during hydraulic fracture.This research enhances the stability theory of ice holes for polar drilling,and provides relevant theoretical support for the safe and efficient drilling of polar ice holes. |
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