The electrical conductivity of CO2-bearing pore waters at elevated pressure and temperature: a laboratory study and its implications in CO2 storage monitoring and leakage detection
The electrical conductivity of CO2-bearing pore waters at elevated pressure and temperature: a laboratory study and its implications in CO2 storage monitoring and leakage detection
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DOI：
10.1093/gji/ggv331
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发表时间：
2015-11
影响因子：
2.8
通讯作者：
J. Börner;V. Herdegen;J. Repke;K. Spitzer
中科院分区：
文献类型：
--
作者：
J. Börner;V. Herdegen;J. Repke;K. Spitzer
岩石电导率是二氧化碳(CO2)注入和运移过程的敏感指标。为了可靠地平衡孔隙空间中的游离二氧化碳与岩石物理模型(如阿奇定律)或用于检测二氧化碳的迁移，对与二氧化碳相互作用过程中的孔隙水传导性的详细了解是必要的，但目前还无法获得。与通常的假设相反，不能假设孔隙水的电导率是恒定的，因为二氧化碳是一种活性气体，它大量溶解在孔隙水中，并由于碳酸的解离提供了额外的电荷载体。因此，我们开展了系统的实验室实验，以量化和分析CO2在热力学平衡时引起的咸水孔隙水电导率的变化。电导率是在压力高达30兆帕、温度高达80°C的孔隙水样品上测量的。参数范围涵盖所涉及的二氧化碳的气态、液态和超临界状态。研究了从0.006到57.27g L−-1氯化钠的孔隙水盐度，以及选择性的其他离子物种。同时，用湿化学法测定了盐溶液中的CO2浓度。出现了两种状态的行为：对于较小的盐度，我们观察到电孔隙水电导率增加了高达3倍以上，这强烈地依赖于溶液盐度(低盐度状态)。这是一种预期的行为，因为来自碳酸解离的额外离子对溶液的导电性有积极的贡献。然而，当考虑到盐度增加时，这种影响就完全减弱了。对于高盐度溶液，增加的相互阻碍会导致所有离子的迁移率降低，这可能会导致电导率显著降低15 / %，尽管添加了二氧化碳(高盐度区域)。我们给出了二氧化碳效应的压力、温度、盐度和离子种类依赖的数据集。此外，利用考虑物种相互作用的电孔隙水电导率的半解析公式对观测数据进行了分析和预测。为了在实际勘探和监测中应用我们的结果，我们还提供了一个纯经验公式来计算平衡状态下CO2对孔隙水电导率的影响，该公式只需要输入压力、温度和盐度信息。
The electrical rock conductivity is a sensitive indicator for carbon dioxide (CO2) injection and migration processes. For a reliable balancing of the free CO2in pore space with petrophysical models such as Archie's law or for the detection of migrating CO2, detailed knowledge of the pore water conductivity during interaction with CO2is essential but not available yet. Contrary to common assumptions, pore water conductivity cannot be assumed constant since CO2is a reactive gas that dissolves into the pore water in large amounts and provides additional charge carriers due to the dissociation of carbonic acid. We consequently carried out systematic laboratory experiments to quantify and analyse the changes in saline pore water conductivity caused by CO2at thermodynamic equilibrium. Electrical conductivity is measured on pore water samples for pressures up to 30 MPa and temperatures up to 80 °C. The parameter range covers the gaseous, liquid and supercritical state of the CO2involved. Pore water salinities from 0.006 up to 57.27 g L−1sodium chloride were investigated as well as selective other ion species. At the same time, the CO2concentration in the salt solution was determined by a wet-chemical procedure. A two-regime behaviour appears: for small salinities, we observe an increase of up to more than factor 3 in the electrical pore water conductivity, which strongly depends on the solution salinity (low-salinity regime). This is an expected behaviour, since the additional ions originating from the dissociation of carbonic acid positively contribute to the solution conductivity. However, when increasing salinities are considered this effect is completely diminished. For highly saline solutions, the increased mutual impeding causes the mobility of all ions to decrease, which may result in a significant reduction of conductivity by up to 15 per cent despite the added CO2(high-salinity regime). We present the data set covering the pressure, temperature, salinity and ion species dependence of the CO2effect. Furthermore, the observations are analysed and predicted with a semi-analytical formulation for the electrical pore water conductivity taking into account the species’ interactions. For the applicability of our results in practice of exploration and monitoring, we additionally provide a purely empirical formulation to compute the impact of CO2on pore water conductivity at equilibrium which only requires the input of pressure, temperature and salinity information.