Velocities of compressional and shear waves in limestones
Velocities of compressional and shear waves in limestones
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DOI：
10.1046/j.1365-2478.2003.00349.x
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发表时间：
2003-01
影响因子：
2.6
通讯作者：
S. Assefa;C. Mccann;J. Sothcott
中科院分区：
文献类型：
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作者：
S. Assefa;C. Mccann;J. Sothcott
碳酸盐岩是重要的油气储集岩，具有复杂的结构和物性(孔隙度和渗透率)，主要由各种成岩作用(压实、溶解、沉淀、胶结作用等)形成。这些复杂性使得根据地震属性预测储集层特征(如孔隙度和渗透率)变得非常困难。为了探讨地震、岩石物性和地质性质之间的关系，采用脉冲回波法，在模拟现场压力条件下(静水有效压力50 Mpa)，分别在约0.85 MHz和0.7 MHz频率下进行了超声波纵波和横波速度测量。测量是在真空干燥和完全饱和的条件下进行的，地点是英格兰南部大橄榄石灰岩的橄榄石灰岩。有些岩石被石油完全浸透了。声学测量补充了孔隙率和渗透率测量，浸渍树脂的抛光薄片的岩石学和孔洞几何研究，X射线衍射分析和扫描电子显微镜研究亚微观结构和微孔。结果表明，纵波速度和横波速度(分别为Vp和Vs)随孔隙度的增加而减小，且Vp的减小速度约为Vs的两倍。石灰岩孔隙结构(如纵横比)的系统差异在速度与孔隙度关系上产生了很大的残差。结果表明，通过去除残差中与孔隙结构有关的变化，可以改善速度与孔隙度的关系。向孔隙中引入水使岩石的剪切模数降低了约2 Gpa，这表明在颗粒接触处存在流体/基质相互作用，从而降低了岩石的刚性。由于岩石-流体相互作用，预测的Biot-Gassmann速度值大于实际速度值。这在Biot-Gassmann速度模型和由于局部流动机制引起的速度弥散中没有考虑到。与Biot模型相比，Raymer和时间平均关系预测的速度高估了测量的速度。
Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (Vp and Vs, respectively) decrease with increasing porosity and that Vp decreases approximately twice as fast as Vs. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.