| In this paper, the concept of shear bond strength at cement-interlayer interface (SBSCII) was defined firstly. SBSCII actually could be reflected by shear cement intensity of mud cake at cement-interlayer interface (CII). Then the composition and structure of mud cake was studied from the respect of soil mechanics. It concluded that the mud cake is cohesive soil essentially. On this basis, three-phase proportion index of the initial mud cake were studied. Fatherly, it concluded that the mud cake is saturated cohesive soil (mud). A new model of initial mud cake or sponge mud cake model was established based on the characteristics of mud cake and its formation process. The model is more specific compared to layered mud cake model and the network structure mud cake model. Based on the hypothesis of the model, some parameters of mud cake such as compressibility were studied according to its three-phase proportion index.According to Coulomb law, the influencing factors of SBSCII were investigated. The influencing mechanism of these factors can be divided to three aspects:the influence to the normal stress at failure surface, the influence to internal friction angle of mud cake, and the influence to cohesive force of mud cake. Among these influencing factors, the types of drilling fluid, the thickness of mud cake, the missing amount of interface and formation stress have notable influence to SBSCII.The morphology of mud cake and stress condition of CII was analyzed. It concluded that the mud cake was transformed from plastic state into semi-solid state, and at last to solid state gradually under a triaxial stress state eventually. However, the mud cake mainly bears radial stress and circumferential stress.The solid phase materials in mud cake, which mainly comes from drilling fluid, have important influence to SBSCII. The solid phase materials in mud cake can be divided to three types, they are clay minerals, weighting materials and drilling cuttings. The shear bond strength of remolded bentonite-iron ore powder soil and remolded bentonite-drilling cuttings soil were tested by direct shear apparatus so as to research the influence of weighting materials and drilling cuttings on shear bond strength of mud cake. The results show that under constant moisture content and compaction conditions, the internal friction angle and cohesive force of mud cake decrease linearly with the amount of iron ore powder (a kind of weighting materials) or drilling cuttings increase; the amount of clay minerals in mud cake is proportional to cohesive force of mud cake.The paper also explained the influence mechanism of mud cake thickness to SBSCII based on thick-wall cylinder theory. Firstly the radial displacement of interlayer borehole wall was calculated by Lame formulas. Calculation result showed that the displacement of interlayer borehole wall is in10-1mm level, which is the same level of compressibility of mud cake. Therefore, it’s possible that the mud cake can’t be compacted fully by radial displacement of interlayer borehole wall if the thickness of mud cake is too large. Whether the mud cake can be compacted fully and compacted extent of mud cake is closely related to the characteristics of mud cake. This is the influence essence of mud cake thickness to SBSCII. When the thickness of mud cake is small, or the radial displacement of interlayer borehole wall is larger than the compressibility of mud cake, the mud cake can be compacted fully. In this case, SSCⅡ is irrelevant to mud cake thickness, but related to material composition of mud cake; when the radial displacement of interlayer borehole wall is smaller than the compressibility of mud cake, the mud cake can’t be compacted fully. In this case, SSCⅡ also depends on shearing strength on failure surface in mud cake which is compacted to a certain extent. In addition, the influence of mechanical parameters of various parts of cement-interlayer system on radial displacement and stress were studied based on the flexibility analysis of composite elastomer. Results show that the casing and cement with low stiffness can effectively increase the radial displacement at CⅡ; cement with high stiffness can effectively increase the radial stress at CⅡ.Influence of interlayer interface defects on SBSCFI was studied. The formation reasons of interface defects are so many. It may be caused by dry micro-cracks, or cement dropping off from the borehole wall under strata stress and so on. However, no matter what is cause of the interface defects, the results is same and will lead to the gap between cement paste and interlayer strata. In that case, formation stress or fluid hydrostatic pressure in the casing can’t be transformed to failure face in mud cake effectively. Based on above theory, the interlayer interface defects is equivalent to missing of mud cake ring. A mathematical model was established between the SBSCⅡ and missing ration of mud cake. It shows that the SBSCⅡ is inversely proportional to the missing ratio of mud cake ring (λ). The verification results with the experimental simulation system show that the relative errors between the calculated values of mathematical model and experimental values are less than10%.Influence of hydraulic fracturing pressure and interlayer thickness on the interlayer interface channeling was analyzed. The destruction of the insulation layer is a tensile failure under the hydraulic fracturing process. When the pressure of fracturing fluid in the bottom hole can overcome the normal stress the CⅡ suffered and cohesive force of mud cake, the CII will be damaged and tensile cracks generate. When extension length of crack is greater than the interlayer thickness, the channeling will occur at CⅡ; If extension length of crack is less than the interlayer thickness, the channeling will not occur at CⅡ, SBSCII will decrease due to interlayer interface defects.Based on above findings, the paper established prediction models and prediction method of SBSCFI. Based on the prediction model, corresponding program was developed by Visual Basic language. |
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