Underground storage of CO2 in geological structures is very often accompanied by chemical interactions between the storage rock formation, existing fluids (e.g. brine) and injected CO2. Depending on the mineralogy and initial petrophysical properties of the rock formation, such reactions may also alter the petrophysical properties of the rock through dissolution, precipitation, fines migration and compaction mechanisms. In fact, carbonate formations are often highly reactive with carbonated brine and the extent of any reaction often depends on the precise rock composition as well as the accessible surface area with the fluid; a higher surface area will typically increase the reaction rate for heterogeneous systems between solids and liquids. Furthermore, fracturing and weakening of oil-bearing chalk reservoirs are approaches that have been implemented to improve oil recovery from various fields worldwide. In this paper, we present the results of an experimental study on a heterogeneous chalk sample (calcite concentration > 98.9 wt%) which has been cut in half (to form an inlet and outlet sample) subjected to carbonated brine flooding under in situ reservoir conditions. The results show a significant increase in the post-flood permeability of the inlet plug and a slight decrease in the outlet plug. The increase in permeability of the inlet sample is supported by X-ray CT and SEM images which reveal significant mineral dissolution and establishment of preferential flow paths (or wormholes). On the other hand, dissolution is not observed in the outlet sample. This suggests that the fluid has reached equilibrium (i.e. achieved solute saturation) with the rock samples after traversing the first sample (i.e. there is no further mineral dissolution). This is strong evidence for the existence a dissolution front that forms during the core flooding process. With continued flooding of CO2-saturated brine, this front eventually traverses the whole sample and the dissolution becomes more substantial along the entire length of the core. As a result of the dissolution process, there is some degree of fines migration (induced by the dissolution in the first samples) into the outlet sample which has negative impacted its permeability. This change in permeability could also be caused by later precipitation of minerals from the brine, but this is likely a minor effect as the pore pressure/temperature conditions (for example, causing a pH change) do not vary significantly along the length of the samples. NMR T2 distribution analysis shows reductions in the porosity and pore sizes are observed in both inlet and outlet plugs of the composite sample and these changes are likely due to a combination of compaction (caused by dissolution-induced weakening) and mineral dissolution/precipitation.

中文翻译：

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CO2饱和盐水对白垩岩储层注入性和完整性的影响

地质结构中二氧化碳的地下储存经常伴随着储存岩层、现有流体（例如盐水）和注入的二氧化碳之间的化学相互作用。根据岩层的矿物学和初始岩石物理特性，此类反应还可能通过溶解、沉淀、细粒迁移和压实机制改变岩石的岩石物理特性。事实上，碳酸盐岩地层通常与碳酸盐水高度反应，任何反应的程度通常取决于精确的岩石成分以及流体的可及表面积；较高的表面积通常会增加固体和液体之间异质系统的反应速率。此外，含油白垩岩储层的压裂和弱化是已实施的方法，以提高世界各地不同油田的石油采收率。在本文中，我们介绍了在原位油藏条件下进行碳酸化盐水驱替的非均质白垩样品（方解石浓度 > 98.9 wt%）的实验研究结果，该样品已被切成两半（以形成入口和出口样品） . 结果表明，入口塞的洪水后渗透率显着增加，出口塞的渗透率略有下降。X 射线 CT 和 SEM 图像支持入口样品渗透率的增加，这些图像揭示了显着的矿物溶解和优先流动路径（或虫洞）的建立。另一方面，在出口样品中未观察到溶解。这表明流体在穿过第一个样品后（即没有进一步的矿物溶解）与岩石样品达到平衡（即达到溶质饱和）。这是在岩心驱替过程中形成溶解前沿的有力证据。随着 CO2 饱和盐水的持续驱替，该前沿最终穿过整个样品，并且沿整个岩心长度的溶解变得更加明显。由于溶解过程，有一定程度的细粒迁移（由第一个样品中的溶解引起）进入出口样品，这对其渗透率产生了负面影响。渗透率的这种变化也可能是由盐水中矿物质的后期沉淀引起的，但这可能是一个很小的影响，因为孔隙压力/温度条件（例如，导致 pH 值变化）不会沿样品长度发生显着变化。NMR T2 分布分析显示，在复合样品的入口和出口塞中观察到孔隙率和孔径减小，这些变化可能是由于压实（由溶解引起的弱化引起）和矿物溶解/沉淀的组合。