Migration of CO2 through North Sea Geological Carbon Storage Sites: Impact of Faults, Geological Heterogeneities and Dissolution

在深层地质构造中储存二氧化碳是减少人为向大气排放二氧化碳的主要技术手段之一。这一过程需要从源头捕获二氧化碳（如燃煤电厂），将二氧化碳运送到注入地点，并将液化二氧化碳泵入一公里深的多孔储层，这些储层通常最初在盐水中饱和或先前含有石油或天然气。最初，浮力强的二氧化碳倾向于通过多孔储层上升，直到它被不透水的层所困，就像石油或天然气被困了几千年一样。随后，通过将二氧化碳溶解到水中（碳酸水比非碳酸水密度更大，会下沉），或者通过毛细管力将二氧化碳保持在孔隙空间的小范围内，可以更安全地捕获浮力二氧化碳。因此，这些捕集过程减少了浮力二氧化碳通过覆盖层迁移的任何风险。限制实际地质覆盖层的溶解速率和毛管圈闭是量化和降低泄漏风险策略的关键组成部分。英国在地理位置上很适合实施海上二氧化碳储存，北海有许多潜在的储层。通过量化断层和非均质地层的捕获率，以及量化地震探测覆盖层中任何二氧化碳的能力，该建议将提高我们对覆盖层泄漏风险的理解。二氧化碳的粘性比水小，会沿着渗透性更强的地层移动。沉积层在所有尺度上都表现出很大的渗透率变化，这将大大增加二氧化碳在地层水中的溶解速率。该分析的范围和所产生的技术虽然一般，但适用于Goldeneye油田，这是二氧化碳储存的目标，也是政府CCS商业化竞争的候选项目。我们的方法是对覆盖层内相关的地质非均质性进行地质表征，并绘制该地区断层内流体流动的结构和倾向。钻芯提供岩石样本（5 × 20厘米），然后可以在实验室进行研究。我们将直接成像，在典型的覆盖层条件下，流体流动、溶解和毛细管捕获的速率，在岩石内单个孔隙的尺度上（微米）和在岩心的长度上（厘米）。流体的地球化学分析将使我们能够在岩心驱油实验中测量原位溶解和沉淀速率。为了确定如何在油藏和覆盖层的尺度上应用流动速率和圈闭速率，必须根据1-100厘米尺度的地质非均质性和沿断层的流动来解释结果。为了评估非均质性对捕获速率的影响，我们将构建沿主要分层地层或沿横切断层的简化流动模型，以及实验室模拟实验，我们可以在光学上评估捕获速率，从而为我们的预测提供坚实的基准。最后，在更大的尺度上，我们将在现有的二氧化碳实验（如北海的Sleipner）中对烟囱结构的流动进行成像，从而对我们在储层覆盖层中通过地震解决泄漏途径的能力进行定量估计。我们的提案将开发工具，用于描述北海覆盖层的地质特征，提供地质复杂覆盖层和断层复合体中捕获率的定量估计，并展示地震解析流体流动路径的能力。迄今为止，已经在相对安全的储存地点证明了二氧化碳的地质储存。这项工作将通过量化地质更复杂的储存地点与泄漏相关的潜在风险，极大地扩大地质二氧化碳储存的潜力。

The storage of CO2 in deep geological formations is one of the chief technological means of reducing anthropogenic emissions of CO2 to the atmosphere. The process requires capturing CO2 at source (e.g. coal-fired power plants), transporting CO2 to the injection site, and pumping liquefied CO2 into kilometre deep, porous reservoirs that are typically initially saturated in saline water or previously contained oil or gas. Initially, buoyant CO2 tends to rise through the porous reservoir until it is trapped by an impermeable horizon, in the same way that oil or gas has been trapped over millennia. Subsequently, buoyant CO2 may be more securely trapped by dissolving CO2 into water (carbonated water is more dense than non-carbonated water and will sink), or by capillary forces acting to hold the CO2 in the small confines of the pore space. Any risk of buoyant CO2 migrating through the overburden is therefore reduced by these trapping processes. Constraining the rates of dissolution and capillary trapping in realistic geological overburden is a key component of strategies to quantify and reduce the risks of leakage. The UK is geologically well placed to implement offshore CO2 storage, with many potential reservoirs in the North Sea.This proposal will improve our understanding of the risks of leakage through the overburden by quantifying trapping rates in faults and heterogeneous strata typical of the overburden of North Sea reservoirs, and by quantifying our ability to seismically detect any CO2 in the overburden. CO2 is less viscous than water and will finger along more permeable layers. Sedimentary strata exhibit large variations in permeability on all scales that will substantially increase the rates at which CO2 dissolves in the formation waters.The analysis, while general in scope and resultant techniques, is applied to the Goldeneye field, a target for CO2 storage and a candidate for the Government's CCS commercialisation competition. Our approach is to geologically characterise the relevant geological heterogeneity within the overburden, and to map the structure and propensity for fluid flow within faults in that locality. Drill core provides samples of rock (5x20 cm) that can then be interrogated in the laboratory. We will directly image, at conditions typical of the overburden, the rates of fluid flow, dissolution, and capillary trapping both at the scale of individual pores within the rock (microns) and over the length of the core (centimetres). Geochemical analysis of the fluids will allow us to measure in situ dissolution and precipitation rates in our core flooding experiments. In order to determine how rates of flow and trapping may be applied at the scale of the reservoir and overburden the results must be interpreted in light of flow through 1-100 centimetre scale geological heterogeneities and along faults. To assess the impact of heterogeneities on the rates of trapping we will construct simplified models of flow along predominantly layered strata, or along cross-cutting faults, along with laboratory analogue experiments in which we can optically assess trapping rates and thereby provide a firm benchmark for our predictions. Finally, at larger scales, we will image flow up chimney structures in existing CO2 experiments (eg Sleipner in the North Sea) and thus provide quantitative estimates of our ability to seismically resolve leakage pathways in the storage overburden. Our proposal will develop tools needed to geologically characterise the North Sea overburden, provide quantitative estimates of trapping rates in geologically complex overburden and fault complexes, and demonstrate the ability to seismically resolve fluid flow pathways. To date geological CO2 storage has been demonstrated at relatively safe storage sites. This work would greatly expand the potential for geological CO2 storage by quantifying the potential risks associated with leakage in more geologically complex storage sites.