Abstract
In a project of methane gas extraction from a methane hydrate (MH) bearing sand formation in marine sediments, a production well is drilled into the sea floor from a marine platform. The depressurization method has been proposed for extracting methane gas. During the depressurization operation, sediment deformation and consolidation occurs because of MH dissociation and increases in the effective stress. This ingenerates friction on the production well, and shear failure may occur. These problems may cause gas leakage and depressurization impediment. Therefore, to ensure reliability of the well, it is very important to understand the frictional strength between the well and the sediments. The well consists of casing and cement, and it has three interface types: casing-cement, casing-sediment (sand or clay), and cement-sediment. In this study, we prepared simulated interface samples using steel rods (surface roughness: Rz = 2.7, 14.7, and 30.0 μm) and Class G cement. Moreover, a pushout test was carried out on these samples under different effective confining pressures. The test results showed that the maximum friction is not affected by the water-cement ratio of the cement, but it tended to increase depending on the effective confining pressure and the surface roughness. From these results, we expressed the maximum friction as a function of the effective confining pressure and the surface roughness.
