Study On Saturation Mode And Saturation Effect Evaluation Of Calcareous Sand

In recent years,with the smooth progress of the construction of reef islands,research related to calcareous sand has attracted the attention of more and more scientific researchers.Due to the unique biogenesis and deposition process,most of the calcareous materials have not been transported over long distances,retaining the original biological skeleton,which is porous and contains internal pores,irregular shapes,low strength,and easy breakage,resulting in different engineering mechanical properties from the general terrigenous sediments.In-depth study of the physical and mechanical properties of calcareous sand is of great significance to the exploration,evaluation and development of oil and gas resources in the South China Sea.In the conventional soil mechanics test,the vacuum combined with back pressure saturation method is generally used to increase the saturation of sample,but this method is easy to cause the particle breakage of the sample on the calcareous sand,which in turn changes the initial structure of the sample and increases the test error.To this end,this thesis combines macro and mesoscopic studies on the saturation mechanism and saturation effects of calcareous sands,through the triaxial shear test of different saturation methods(hydraulic saturation,carbon dioxide saturation,back pressure saturation)and saturation(B value 0.95,0.90,0.85),analyzes the influence of different saturation modes and degrees of saturation on the mechanical properties of calcareous sand;and from the microscopic aspect,measures the internal pore content of calcareous sands,and analyzes the difference of pore water distribution of calcareous sands under different saturation methods.It provides a scientific basis for the selection of suitable saturation methods for calcareous sand and the evaluation of saturation effects.The main conclusions are as follows:(1)The carbon dioxide method can make the saturation of the sample reach more than92%,and it is basically consistent with the pore water distribution of the back pressure saturation method;it shows that only the use of carbon dioxide saturation can make the sample reach a higher saturation.The ratio of internal pores to intergranular pores in the calcareous sand used in the test generally does not exceed 1%,and the ratio reaches the maximum(0.7%)in the sample with a relative density of 90%,which can be considered In this state,the influence of internal pores on the saturation effect of calcareous sand is minimal.(2)The influence of the saturation method on the calcareous sand has different characteristics due to the difference in particle size.For the calcareous coarse sand,the mechanical properties of the three saturation modes are relatively small.While for the calcareous medium sand and the calcareous fine sand,the strength and deformation characteristics of the three saturation mode are different;the peak strength and sliding friction angle of the sample when the hydraulic saturation is saturated is the largest,followed by carbon dioxide saturation,and the back pressure saturation is the smallest;and the difference in deformation characteristics between the three saturation methods decreases with the increase of the particle size.Therefore,it is recommended to adopt the combined saturation method of carbon dioxide and back pressure;by increasing the purity of carbon dioxide or the aeration time,the initial saturation of sample before the back pressure saturation is increased,the back pressure is reduced,and the particle crushing of the calcareous sand is avoided during the saturation process.(3)When the pore water pressure coefficient B is greater than 0.85,the deviation of the peak strength of calcareous sand at different saturations is not more than 3%,and the difference between the sliding friction angles is 0.5° Within;the change law of the volume strain ～ axial strain curve is basically the same,and most of the curves basically coincide;the stress-strain curve and stress path of calcareous sand have very little difference at different saturations;the cumulative distribution curves of particle size of calcareous sands with different saturations after the test basically coincide,and the deviation between the relative crushing rates is small.Therefore,it is recommended that the pore water pressure coefficient B value is greater than0.85 as the saturation evaluation standard for calcareous sand;when the carbon dioxide method is used to make the sample have reached a higher saturation,it is effective to apply an appropriate and smaller back pressure increasing the saturation can reduce or even avoid the occurrence of particle breakage.