Vol.7 No.4 2015

Research paper : Applicability of the technologies to the assessment of methane hydrate sediments (N. TENMA)−224−Synthesiology - English edition Vol.7 No.4 (2015) prepared to obtain the contact surface strength between the casing and cement. The steel rod is placed in the hollow sample made of cement, and data on contact surface strength between the materials is collected by conducting the push-out test in which the rod is pushed through. So far, experiments on the contact surface strengths of casing-sediment, casing-cement, and cement-sediment have been conducted. For example, for the contact surface strength of casing-cement, we derived an experimental equation using the effective confining pressure among others as parameters.[21][22] In the future, we plan to conduct further sensitivity analyses using experimental equations such as this, and to propose a method for the well design of MH development.At the contact surface, local deformations by particle crushing may occur, but it is difficult to understand the details of the local effect in the push-out test due to the settings of experimental conditions and experiment time. Therefore, investigation using the distinct element method (DEM) is carried out for evaluation by numerical analysis. Since the DEM is a method that tracks the motions of multiple particles, it is possible to quantitatively evaluate the micro-mechanical quantities that cannot be measured experimentally. Since it is known that the “roughness” of the contact surface is related to strength, we are attempting to systematize the unevenness of the well surfaces or sand particles at various scales of roughness using the DEM. By gaining a systematic understanding of the properties of the contact surface under various mechanical conditions by DEM, we hope to elucidate the mechanical behavior of the contact surface and its modeling.[23]3.3 Evaluation of wide-area deformationIn development for practical use of MH, it is thought that long-term production may not be possible because depressurization cannot be maintained in cases where there are discontinuities such as faults in areas surrounding the points of MH development, as faults may become flow-paths as shown in Fig. 3. We are therefore investigating a way of evaluating the effects of discontinuous surfaces. In MH development, there is a risk that deformation behavior such as consolidation before, during, and after production may change greatly when using the depressurization method. This prompted us to investigate sediment characteristics over the long-term. The following two research studies are being conducted for evaluation of wide-area deformation. First, for the purpose of establishing a method of selecting MH development areas, we created a numerical model based on the Eastern Nankai Trough, and investigated the effect of sediment layer heterogeneity and fault discontinuity by including the presence of faults in the numerical model. Specifically, by comparing and investigating the mechanical behavior when the depressurization method is applied in the numerical model with and without faults, it is possible to understand the effect of discontinuities, such as faults, on the mechanical behavior. Sensitivity analysis is then conducted to understand the conditions under which the sediment layer condition is optimal for the MH development area. This type of location is then selected as the MH development area. The parameters of sensitivity analysis considered so far include the distance between well and fault, the dip angle of the fault, and whether the fault is normal or a reverse fault.[24] The results of the sensitivity analysis so far confirm that the deformation behavior changes at the CementSand layerCasingSand layerCementCasingMH layerCementCasingUpper layerSeabed surfacePhotograph 2. Sample for measuring the contact surface strength between the casing and cement (part of the diagram has been modified)[21]Since the well is a combination of complex materials of casing, cement, and sedimentary layers, push-out tests were conducted by preparing various samples with combinations of different materials.


page 28