Fluid substitution in laminated sands
Fluid substitution in laminated sands
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DOI：
10.1190/1.1756839
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发表时间：
2004-05
期刊：
影响因子：
3.3
通讯作者：
C. Skelt
中科院分区：
文献类型：
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作者：
C. Skelt
岩石物理学家和地球物理学家通过“流体替代”来估计饱和条件下多孔岩石的密度和声速，例如完全烃类饱和或含有气体、凝析油和石油等流体的“气泡”。然而，在许多商业油藏中，Gassmann模型的实现有时会对层状砂-页岩层序的压缩声速产生不可信的结果。特别是，在图1所示的油基泥浆井的含气层段中，经常观察到流体效应随着页岩分数的计算而增加的反直觉现象。DTC FLUSUB （Gassmann fluid substitution）是将DTC WET替换为观测到的含水饱和度SW的结果。正如预期的那样（Brie et al., 1995），模拟的流体效应大于湿井和观测到的测井DTC OBS之间的差异。观察到的流体效应在孔隙度最高、最干净的砂岩中达到最大，但该模型预测在孔隙度较低的页岩砂中达到最大。图1所示。层合序列中Gassmann方程的击穿。通常的做法是指定一个页岩截止点，在此截止点之上计算的流体效应将被忽略。这是一种不令人满意的变通方法，因为截断点的选择是任意的，即使低于截断点，结果也是违反直觉的。电缆测井的分辨率和深度不匹配，以及观察到的Gassmann方程不适用于页岩，通常被认为是造成这些现象的原因，但这并不能解释图2中的建模结果。图2。显示Gassmann方程分解的模型。该模型是根据区域密度、速度和孔隙度趋势生成的。砂储层的定义是底部的突然过渡和顶部100英尺的线性过渡到页岩。砂的清洁部分并不完全没有粘土。已知研究区域的沙子中含有一些碎屑……
Petrophysicists and geophysicists do “fluid substitution” to estimate the density and sonic velocity of porous rocks at saturation conditions such as fully hydrocarbon-saturated or “fizz-bearing” with fluids such as gas, condensate, and oil. However, the Gassmann model implementation in many commercial packages sometimes yields implausible results for the compressional sonic velocity in laminated sand-shale sequences. In particular, the counterintuitive phenomenon of fluid effects increasing with computed shale fraction is often observed, as in the gas-bearing interval of a well drilled with oil-based mud shown in Figure 1. DTC FLUSUB (Gassmann fluid substitution) is the result of substituting DTC WET to the observed water saturation SW. As expected (Brie et al., 1995) the modeled fluid effect is larger than the difference between the wet and the observed log DTC OBS. The observed fluid effects reach a maximum in the cleanest sand with the highest porosity, but the model predicts a maximum in shaley sand with low porosity. Figure 1. Gassmann equation breakdown in laminated sequence. Normal practice is to specify a shale cutoff above which the computed fluid effects are ignored. This is an unsatisfactory workaround because the choice of cutoff is arbitrary, and results are counterintuitive even below the cutoff. Resolution and depth mismatches between wireline logs and the observation that the Gassmann equation does not apply in shale are frequently cited as reasons for these phenomena, but do not explain the modeled results in Figure 2. Figure 2. Model showing Gassmann equation breakdown. This model was generated from regional density, velocity, and porosity trends. A sand reservoir was defined by an abrupt transition at the base and a linear transition to shale over the top 100 ft. The clean part of the sand is not assumed totally free of clay. Sands in the study area are known to include some detrital …