Summary The objective of this study is to improve understanding of the geostatistics of vertical (bed-normal) permeability (kz) and its influence on reservoir performance during CO2 enhanced oil recovery (EOR) and storage. kz is scrutinized far less often than horizontal permeability (kx, ky) in most geological and reservoir modeling. However, our work indicates that it is equally important to understand kz characteristics to better evaluate their influence on CO2 EOR and storage performance prediction. We conducted this study on approximately 9,000 whole-core triaxial permeability (kx, ky, kz) measurements from 42 wells in a San Andres carbonate reservoir. We analyzed kz data, including heterogeneity, correlation, and sample sufficiency measures. We analyzed wells with the largest and smallest fractions of points with kz > kmax = max(kx, ky) to explore geological factors that coincided with large kz. We quantified these geological effects through conditional probabilities on potential permeability barriers (e.g., stylolites). Every well had at least some whole cores where kz > kmax. This is a statistically justifiable result; only where Prob(kz > kmax) is statistically different from 1/3 are core samples nonisotropic. In conventional core data interpretation, however, modelers usually assume kz is less than kmax. For the well with the smallest fraction (11%) of cores where kz >kmax, the cumulative distribution functions (CDFs) differ and coincide with the presence of stylolites. We found that kz is approximately twice as variable as kx in many wells. This makes kz more difficult to interpret because it was (and usually is) heavily undersampled. To understand the influence of kz heterogeneity on CO2 flow, we built a series of flow simulation models that captured these geostatistical characteristics of permeability, while considering kz realizations, flow regimes (e.g., buoyant flow), CO2 injection strategies, and reservoir heterogeneity. CO2 flow simulations showed that, for viscous flow, assuming variable kx similar to the reservoir along with a constant kz/kx = 0.1 yields a close (within 0.5%) cumulative oil production to the simulation case with both kx and kz as uncorrelated variables. However, for buoyant flow, oil production differs by 10% [at 2.0 hydrocarbon pore volume (HCPV) of CO2 injected] between the two cases. Such flows could occur for small CO2 injection rates and long injection times, in interwell regions, and/or with vertically permeable conduits. Our geostatistical characterization demonstrates the controls on kz in a carbonate reservoir and how to improve conventional interpretation practices. This study can help CO2 EOR and storage operators refine injection development programs, particularly for reservoirs where buoyant flow exists. More broadly, the findings potentially apply to other similar subsurface buoyancy-driven flow displacements, including hydrogen storage, geothermal production, and aquifer CO2 sequestration.

中文翻译：

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碳酸盐岩油气藏垂向渗透率及其对CO2提高采收率和封存的影响分析

总结 本研究的目的是提高对垂直（床层法向）渗透率 (kz) 地质统计学及其在 CO2 提高采收率 (EOR) 和封存过程中对油藏性能的影响的理解。在大多数地质和储层建模中，kz 的检查频率远低于水平渗透率 (kx, ky)。然而，我们的工作表明，了解 kz 特性以更好地评估其对 CO2 EOR 和存储性能预测的影响同样重要。我们对来自圣安德烈斯碳酸盐储层的 42 口井的大约 9,000 个全岩心三轴渗透率 (kx、ky、kz) 测量值进行了这项研究。我们分析了 kz 数据，包括异质性、相关性和样本充足性度量。我们分析了 kz > 点的最大和最小分数的井。kmax = max(kx, ky) 探索与大 kz 相吻合的地质因素。我们通过潜在渗透屏障（例如，针线石）的条件概率来量化这些地质影响。每口井至少有一些完整的岩心，其中 kz > 最大。这是一个统计上合理的结果；只有当 Prob(kz > kmax) 在统计上不同于 1/3 时，岩芯样本是非各向同性的。然而，在传统的核心数据解释中，建模者通常假设 kz 小于 kmax。对于具有最小比例（11%）的岩心（其中 kz > kmax）的井，累积分布函数（CDF）不同，并且与缝合线的存在一致。我们发现在许多井中 kz 的变量大约是 kx 的两倍。这使得 kz 更难以解释，因为它（并且通常是）严重欠采样。为了了解 kz 非均质性对 CO2 流动的影响，我们建立了一系列流动模拟模型，这些模型捕捉了渗透率的这些地质统计学特征，同时考虑了 kz 实现、流动状态（例如，浮力流动）、CO2 注入策略和储层非均质性。CO2 流动模拟表明，对于粘性流动，假设变量 kx 与储层相似以及常数 kz/kx = 0.1 会产生与 kx 和 kz 作为不相关变量的模拟情况接近（在 0.5% 以内）的累积石油产量。然而，对于浮力流动，石油产量相差 10% [在 2. 0 碳氢化合物孔隙体积 (HCPV) 注入的二氧化碳] 在两种情况之间。这种流动可能发生在小 CO2 注入速率和长注入时间、井间区域和/或垂直可渗透管道中。我们的地质统计学特征展示了对碳酸盐储层中 kz 的控制以及如何改进常规解释实践。这项研究可以帮助 CO2 EOR 和储存运营商改进注入开发计划，特别是对于存在浮流的油藏。更广泛地说，这些发现可能适用于其他类似的地下浮力驱动的流动位移，包括储氢、地热生产和含水层二氧化碳封存。和/或带有垂直可渗透的导管。我们的地质统计学特征展示了对碳酸盐储层中 kz 的控制以及如何改进常规解释实践。这项研究可以帮助 CO2 EOR 和储存运营商改进注入开发计划，特别是对于存在浮流的油藏。更广泛地说，这些发现可能适用于其他类似的地下浮力驱动的流动位移，包括储氢、地热生产和含水层二氧化碳封存。和/或带有垂直可渗透的导管。我们的地质统计学特征展示了对碳酸盐储层中 kz 的控制以及如何改进常规解释实践。这项研究可以帮助 CO2 EOR 和储存运营商改进注入开发计划，特别是对于存在浮流的油藏。更广泛地说，这些发现可能适用于其他类似的地下浮力驱动的流动位移，包括储氢、地热生产和含水层二氧化碳封存。特别是对于存在浮力的水库。更广泛地说，这些发现可能适用于其他类似的地下浮力驱动的流动位移，包括储氢、地热生产和含水层二氧化碳封存。特别是对于存在浮力的水库。更广泛地说，这些发现可能适用于其他类似的地下浮力驱动的流动位移，包括储氢、地热生产和含水层二氧化碳封存。