© 2018 Elsevier B.V. The unconsolidated nature of offshore methane hydrate-bearing formation poses challenges to sustainable methane gas production as the weak formation is susceptible to disturbance during wellbore construction. This could contribute to loss of well integrity which could manifest as sand production and error in the interpretation of downhole tests such as mini-frac tests. In this study, a simulation methodology of wellbore construction process is proposed. A finite element model adopting this methodology is developed in order to assess the effect of wellbore construction process on the integrity of the unconsolidated methane hydrate-bearing formation in the Nankai Trough, Japan. The main objectives are (i) to develop a modelling methodology of well construction process for numerical simulations, (ii) to assess the zone and magnitude of well construction-induced stress/strain disturbance in the formation and (iii) to evaluate relative impact of each well construction stage on the integrity of the formation. The results from this study show that the zone of horizontal stress disturbance from the geostatic state due to wellbore construction could extend to more than three times the radius of the wellbore. Following the wellbore construction, the deviator stress is concentrated in the hydrate reservoir sublayers with high hydrate saturation while plastic deviatoric strain has accumulated in the sublayers with low hydrate saturation. The results also show that modelling of cement shrinkage process is crucial in predicting the concentration of deviator stress in the high hydrate saturation layers.

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海上松散含甲烷水合物地层井筒施工模拟

© 2018 Elsevier BV 近海含甲烷水合物地层的松散性质给可持续的甲烷气生产带来了挑战，因为弱地层在井眼施工过程中容易受到干扰。这可能导致井完整性损失，这可能表现为出砂和井下测试（例如微型压裂测试）的解释错误。在这项研究中，提出了一种井眼施工过程的模拟方法。为了评估井眼施工过程对日本南海海槽未固结的含甲烷水合物地层完整性的影响，开发了采用这种方法的有限元模型。主要目标是 (i) 开发用于数值模拟的建井过程建模方法，(ii) 评估地层中建井引起的应力/应变扰动的区域和大小，以及 (iii) 评估每个建井阶段对地层完整性的相对影响。研究结果表明，由于井筒施工，地静止状态产生的水平应力扰动区可扩展至井筒半径的三倍以上。井筒施工后，偏应力集中在水合物饱和度高的水合物储层亚层，而塑性偏应变在水合物饱和度低的亚层积累。结果还表明，水泥收缩过程的建模对于预测高水合物饱和层中偏应力集中是至关重要的。