Study On Mechanical Properties Of Weakly Consolidated Deep Water Shallow Sediment And Its Application In Engineering

Compared with land or shallow sea,strata(<100m)in deep water is generally weakly consolidated due to low overburden pressure.The mechanical properties of shallow sediment in deep water are significantly different from those of conventional strata.In addition,because of high pressure and low temperature,the scope of safety mud density for shallow sediment is extremely narrow.Then,well structure will be complex and accidents(collapse,leakage)will happen if the mud density is inappropriate.In this paper,the prediction methods of mechanical properties of deep water shallow sediment are put forward through laboratory test and numerical calculation.And a sedimentary compaction model and borehole stability analysis model of deep water shallow sediment are established.The main achievements are as follows:A method of making artificial sample is proposed to study the mechanical properties of deep water shallow sediment.The basic theoretical of making artificial sample is: deep water shallow sediment is mainly affected by compaction in the deposition process.The specific step is: making samples with the same physical properties of deep water shallow sediment within a short time(24h)by increasing the effective stress of consolidation appropriately in the modified experimental instrument;the mechanical properties of artificial samples were tested to analyze the differences of mechanical properties of artificial samples with the same physical properties under different consolidated stresses.The prediction model of mechanical properties of deep water shallow sediment is established.The constitutive model and rheological model of deep water shallow sediment were optimized and validated by using artificial sample through consolidation experiment,rebound experiment,direct shear test and rheological experiment.At the same time,taking advantage of repeatability of artificial sample,the single-factor research method was adopted and the influencing factor variables were controlled to establish the relationship between the the factors affecting the mechanical properties and mechanical properties of deep water shallow sediment.By sorting out a large number of exist research data and using the neural network method,a related model was established to predict various factors affecting the mechanical properties parameters of deep water shallow sediment by acoustic properties.And the purpose of predicting the mechanical properties was finally achieved by combining the aforementioned research.The model of sedimentary compaction model of deep water shallow sediment is established.(1)In the case of considering the rheological property,the one-dimensional rheological consolidation model of the deep water shallow sediment was established,and the reliability of the model in describing the long-term consolidation experiment of the deep water shallow sediment was verified by experimental data.(2)According to the one-dimensional rheological model of deep water shallow sediment,the three-dimensional rheological model was deduced and verified.Then,Visual Studio was used for the secondary development of Flac3 d software to develop the Flac3 d material model that can describe the rheological property of deep water shallow sediment.(3)Combining with the actual sedimentation process and sedimentary environment of deep water shallow sediment,Flac3 d software was used to establish the sedimentary compaction model of deep water shallow sediment considering the rheological property.The effects of rheological parameters,water depth,dry particle density,clay mineral content and sedimentation years on the sedimentary compaction of deep water shallow sediment were analyzed.On the basis of research on the physical and mechanical properties of deep water shallow sediment,the mechanical borehole model of deep water shallow sediment after drilling was established,and the borehole plastic zone area and super pore pressure caused by soil squeezing effect were taken into account to improve the wellbore stability analysis model of deep water shallow sediment.Based on the engineering examples of A block in the South China Ses and B block in West Africa,the stability analysis model of deep water shallow sediment is verified.The calculated results are in agreement with the actual situation,which indicates that the model can be used for the stability analysis of deep water shallow sediment.