A reactive transport model for Marcellus shale weathering
A reactive transport model for Marcellus shale weathering
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DOI：
10.1016/j.gca.2017.08.011
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发表时间：
2017-11
影响因子：
5
通讯作者：
Peyman Heidari;Li Li-Li;Lixin Jin;J. Williams;S. Brantley
中科院分区：
文献类型：
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作者：
Peyman Heidari;Li Li-Li;Lixin Jin;J. Williams;S. Brantley
页岩层占全球陆地面积的25%，贡献了美国天然气使用量的很大比例。最高产的页岩气地层之一是马塞卢斯页岩，这是一种富含有机质和黄铁矿的黑色页岩。作为了解马塞卢斯页岩如何与地表或深层地下的水相互作用的第一步，我们开发了一个反应运移模型，以模拟受土壤和水化学数据约束的常温常压条件下的页岩风化。这项模拟是在冰川消融后1万多年的时间里进行的，假设基岩风化和土壤发生在最后一次冰盛期之后开始。结果表明，前1000年的风化是由黄铁矿溶解引起的，导致低pH，绿泥石溶解增强，氢氧化铁沉淀。黄铁矿枯竭后，绿泥石溶解缓慢，主要是由于CO2和有机酸的存在，形成了作为次生矿物的蛭石。敏感性分析表明，对风化最重要的控制因素包括反应气体(CO2和O2)的存在、比表面积和渗入大气降水的流速。土壤化学和矿物学数据不包括反应气体是无法复制的。例如，如果土柱中没有持续存在O2，即使在10,000年后，黄铁矿仍然留在土壤中；同样，如果土壤气体中没有CO2，绿泥石仍然丰富，孔隙度仍然很小。只有当模拟的活性矿物的比表面积比粉状矿物的表面积值小1-3个数量级时，才能成功地模拟现场观测。较小的表面积可能与某些流体由于表面涂层而无法接触到矿物表面相一致。此外，某些矿物表面可能只与平衡的孔隙流体相互作用。水分渗透率的增加通过去除溶解产物和保持远离平衡的条件来增强风化作用。这些观测结果表明，反应表面积的有效性、水和气体的运移是影响浅层马塞卢斯页岩风化速率的最重要因素。这项风化研究证明了反应输运模型对复杂地下过程的实用性。这样的模拟可以扩展到理解注入流体与更高温度、压力和盐度条件下的马塞卢斯页岩气藏之间的相互作用。
Shale formations account for 25% of the land surface globally and contribute a large proportion of the natural gas used in the United States. One of the most productive shale-gas formations is the Marcellus, a black shale that is rich in organic matter and pyrite. As a first step toward understanding how Marcellus shale interacts with water in the surface or deep subsurface, we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions, constrained by soil and water chemistry data. The simulation was carried out for 10,000 years since deglaciation, assuming bedrock weathering and soil genesis began after the last glacial maximum. Results indicate weathering was initiated by pyrite dissolution for the first 1000 years, leading to low pH and enhanced dissolution of chlorite and precipitation of iron hydroxides. After pyrite depletion, chlorite dissolved slowly, primarily facilitated by the presence of CO2and organic acids, forming vermiculite as a secondary mineral.A sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases (CO2and O2), specific surface area, and flow velocity of infiltrating meteoric water. The soil chemistry and mineralogy data could not be reproduced without including the reactive gases. For example, pyrite remained in the soil even after 10,000 years if O2was not continuously present in the soil column; likewise, chlorite remained abundant and porosity remained small if CO2was not present in the soil gas. The field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1–3 orders of magnitude smaller than surface area values measured for powdered minerals. Small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings. In addition, some mineral surface is likely interacting only with equilibrated pore fluids. An increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far-from-equilibrium conditions. We conclude from these observations that availability of reactive surface area and transport of H2O and gases are the most important factors affecting rates of Marcellus shale weathering of the in the shallow subsurface. This weathering study documents the utility of reactive transport modeling for complex subsurface processes. Such modelling could be extended to understand interactions between injected fluids and Marcellus shale gas reservoirs at higher temperature, pressure, and salinity conditions.