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井孔重力仪(BHGM)公认为求取地层体积密度的重要测井工具。当重力仪贯穿水平的、横向无限延伸的、均一厚度和常密度地层时,BHGM 得出的近似密度等于地层的体积密度。对于多数应用,当轻微偏离这些假定条件时,实际上给出研究地层的大范围体积密度。异常质量存在时,不可忽视的地层体积密度失真是可能存在的。在细长的井中得到的视密度异常,其偏离密度差与密度变化界面所对的角成正比。相对于井孔圆形对称的密度变化界面,层中心的视密度变化与所对角的正弦成正比。因为在水平界面以上和以下体积密度的变形是完全相同的(对于在界面上正交的井,在投限情况下),这个视密度陡变等于界面上的真密度变化。这个密度变化称“泊松跳跃”.为了由 BHGM 的视密度得到体积密度,这里提出两种方法(1)根据紧贴目标层外和里边两个站的数据,可近似地得到泊松跳跃和加上独立的密度资料(例如,伽玛—伽玛测井),(2)模型研究(以地震或以钻井为基础)加上 BHGM 确定的密度资料,可以求得目标层的视密度.即使采用大得多的站距,BHGM 也可以研究比一般所认为的更薄地层。
Well Bore Gravimeter (BHGM) is recognized as an important logging tool for determining the bulk density of a formation. The BHGM gives an approximate density equal to the bulk density of the formation when the gravimeter runs through the horizontal, infinitely extended, uniform, and constant density formations. For most applications, a slight departure from these hypothetical conditions actually gives a large range of bulk densities for studying the formation. In the presence of anomalous mass, the bulk density distortion that can not be neglected may exist. The apparent density anomalies obtained in slender wells are proportional to the angle at which the density changes due to the density difference. The apparent density change in the center of the layer is proportional to the sine of the diagonal, relative to the circularly symmetric density change interface of the wellbore. Because deformations above and below the horizontal and below bulk densities are identical (for a well orthogonal at the interface, in the case of a drop limit), this apparent density steepness equals the true density change at the interface. This density change is called “Poisson jump.” In order to obtain the bulk density from the apparent density of BHGM, two approaches are proposed here: (1) Poisson jumps can be approximated based on the data close to the two stations outside and inside the target layer The apparent densities of the target layer can be determined by adding independent density data (eg, gamma-gamma log), (2) model studies (based on earthquakes or drilling) plus the density data determined by BHGM, With much larger station distances, BHGM can also study thinner layers than is generally believed.