Analysis of the Contributing Factors to Wellbore Collapse in Shallow Water Flow Formations for Deepwater Drilling
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摘要:
为了定量评价和防治浅水流对深水钻井的危害，进行了浅水流地层井眼坍塌影响因素分析。基于流固耦合理论建立了浅水流地层井眼稳定理论模型，并分别根据Mohr-Coulomb 准则和最大拉应力准则判断砂体的剪切和拉伸破坏，分析了各因素对井周最大破坏半径及砂体临界超压的影响。计算分析发现：超压将导致浅水流地层发生破坏，随着超压增大，其最大破坏半径随之快速增至大；水平有效地应力与垂直有效地应力的比值（K0）越大，砂体埋深越大，浅水流地层越不容易发生破坏；砂体内摩擦角越大，砂体越稳定；井周最大破坏半径随着钻井液密度升高呈线性递减；浅水流地层存在临界超压，砂体埋深、强度和K0越大，临界超压越大。研究结果表明，浅水流地层井眼坍塌的风险较大，井周破坏程度与超压、地应力、砂体埋深、钻井液密度和内摩擦角等因素有关，当砂体中的超压超过临界值时，整个砂体都将处于不稳定状态，而提高钻井液密度有利于浅水流地层的稳定。
Abstract:In order to quantitatively evaluate and effectively prevent the damage of shallow water flow during deep water drilling, the contributing factors of wellbore collapse in shallow water flow formations during deep water drilling were analyzed. Based on the fluid-solid coupling theory, the theoretical model of wellbore stability in shallow water flow formation was established. Then they determined the shear and tensile damages of the sand body by the Mohr-Coulomb and maximum tensile stress criteria respectively, as well as analyzed the influence of each factor on maximum damage radius and sand body critical overpressure. The calculation and analysis found that the overpressure would lead to the damage of shallow water flow formation, the maximum damage radius increased rapidly with the increase of overpressure. The greater the ratio of effective horizontal geostress to effective vertical geostress (K0 ), and the greater the burial depth of sand body, and the less likely the shallow water flow formation is to be damaged; the larger the internal friction angle in the sand body, the more stable the sand body. Likewise, the smaller the internal friction angle, the more significant impact will be. The maximum damage radius around wellbore linearly decreases with the increase of drilling fluid density. There is a relations between critical overpressure found in the shallow water flow. The larger the burial depth, strength and K0 of sand body, the larger the critical overpressure. The results showed that the risk of wellbore collapse in shallow water flow formation was high. Further, the damage degree around wellbore was related to such factors as overpressure, geostress, sand body depth, drilling fluid density, internal friction angle, etc. When the critical overpressure is exceeded, the entire sand body will be in an unstable state, and the increase of drilling fluid density will be conducive to the wellbore stability in the shallow water flow formation.
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图 1 地层垂直剖面及相应的有限元实体模型
Figure 1. Stratigraphic vertical section and corresponding solid model
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图 2 不同超压下浅水流地层井周破坏情况
Figure 2. The damage around wellbore in shallow water flow formation under different overpressures
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图 3 不同超压下的井周最大破坏半径
Figure 3. The maximum damage radius around wellbore under different overpressures
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图 4 井周最大破坏半径随钻井液密度的变化
Figure 4. Variation of the maximum damage radius around wellbore with the density of drilling fluid
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图 5 不同砂体中部埋藏深度下的井周最大破坏半径
Figure 5. The maximum damage radius around wellbore at middle depth under the different sand body
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图 6 不同K0下浅水流地层的井周最大破坏半径随超压大小的变化
Figure 6. The variation of the maximum damage radius around wellbore in the shallow water flow formation with the overpressure under different K0
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图 7 不同内摩擦角下的浅水流地层井周最大破坏半径
Figure 7. The maximum damage radius around wellbore in the shallow water flow formation under different internal friction angles
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图 8 不同内摩擦角和K0时临界超压随砂体埋深的变化
Figure 8. Variation curves of critical overpressure with sand body burial depth under different internal friction angles and K0
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图 9 不同埋深和K0时临界超压随砂体内摩擦角的变化
Figure 9. Variation curves of critical overpressure with the internal friction angle of sand body under different burial depths and K0
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