天然气水合物地层PCS取样扰动有限元仿真分析

FEM simulation analysis of sampling disturbance with PCS drill bit in gas-hydrate-bearing stratum

针对天然气水合物地层钻进过程中,钻头扰动可能对井底和井周地层波速变化产生很大影响,从而导致难以通过钻探取心分析及测井获得天然气水合物地层准确信息的问题,借助有限元软件ANSYS/LS-DYNA,构建保压取心器(PCS)钻头钻进水合物地层的有限元模型,模拟PCS钻头钻进天然气水合物地层的动态过程,得到不同时刻的扰动形态,同时得到了钻压、转速分别与水合物地层扰动影响关系。研究结果表明:(1)当转速一定,钻压增大时,超前钻头附近井底地层及钻头附近井周地层扰动程度呈增大趋势;当钻压一定,转速增大时,超前钻头附近井底地层最大扰动程度基本不受转速改变的影响,而钻头附近井周地层最大扰动程度整体呈线性增大趋势。(2)钻压和转速一定时,钻进方向上井底地层距离超前钻头越近,其水合物地层扰动程度越强,反之则越弱,距离超前钻头0.09 m以内的井底地层扰动程度较大,在此范围之外的井底地层则扰动程度较小;同时径向上距离钻头体越近,其水合物地层扰动程度越强,反之则越弱;距离钻头体0.055 m以内的井周地层扰动程度较大,在此范围之外的井周地层则扰动程度较小。研究计算结果可为天然气水合物地层PCS取心分析及水合物测井提供参考。

In the process of drilling in gas hydrate formation, the drilling disturbance may have a profound influence on the wave velocity of the bottom and surrounding formation of the hole. So it is difficult to obtain accurate information of the gas hydrate formation by core drilling analysis and logging. Using finite element analysis software ANSYS/ LS-DYNA, the finite element model of the pressure core sampler （PCS） bit drilling in the gas hydrate formation was built, and the dynamic process of PCS bit drilling in the gas hydrate formation was simulated. The disturbance morphology at different time was obtained. The influence relationship of bit weight and rotary speed on the disturbance of gas hydrate formation also was acquired at the same time. The results show that, firstly, when rotary speed is constant, the disturbance degree of undrilled sub bottom hole near the cutting shoes and underground layer around the well near the PCS bit increases with the increase of the bit weight. Secondly, when bit weight and rotary speed are both constant, the shorter distance between the bottom formation of the hole and the cutting shoes in the drilling direction, the bigger disturbance degree will be obtained, and contrarily it would be smaller. The disturbance degree of bottom formation of the hole is relatively big when the distance to the cutting shoes is shorter than 0.09 m, while it is relatively small out of this distance. Meanwhile, the shorter distance between the surrounding formation of the hole and the PCS bit in the radial direction, the bigger disturbance degree will occur, and contrarily it could be smaller. The disturbance degree of surrounding formation of the hole is relatively big when the distance to the cutting shoes is shorter than 0.055 m, while it is relatively small out of this distance. The results can make a good reference for the PCS coring analysis and hydrate logging.