Depleted oil and gas fields constitute potentially important storage sites for CO2 in the subsurface, but large-scale injection of supercritical (sc) CO2 in chalk has not yet been attempted. One of the risks is the adverse effect of the substantial amount of remaining oil in the chalk reservoirs on scCO2 injection. In order to counter an undesired effect on injectivity, a fundamental understanding of the spatial distribution and quantity of the movable, semi-movable, and non-movable oil, and solid bitumen/asphaltenes fractions of the remaining oil is critical. In this study a combination of organic geochemistry (gas chromatography of the saturated fraction and programmed pyrolysis), and reflected light microscopy was applied to evaluate and measure the spatial distribution, volume, and saturation of different oil fractions in a well-defined reservoir interval of a waterflooded Maastrichtian chalk reservoir in the Danish Central Graben, North Sea. A total of 127 samples from a slightly deviated vertical well and two ∼5 km-long horizontal wells from the Halfdan and Dan fields were analyzed. An original uneven distribution of oil saturation and composition or different production efficiency of different levels in the reservoir may account for variations in the total oil and oil fraction saturations. Gas chromatography shows that the solvent extractable oil is quite similar in composition, characterized by a dominance of polar compounds and a high content of asphaltenes. Extended slow heating (ESH) pyrolysis reveals that most of the remaining oil saturation consists of semi-movable oil and total non-movable oil (non-movable oil plus solid bitumen/asphaltenes). Reduced oil gravity values (API) are related to evaporation loss of the lightest hydrocarbon fraction during core storage and increase of the relative proportion of the heavier oil fractions by waterflooding during production. Microscopy disclosed three forms of oil: i) Patchy distributed lighter, movable oil showing a bluish fluorescence, ii) Brownish staining with a dark orange to brownish fluorescence, and iii) Dark brown non-fluorescing oil and black solid bitumen/asphaltenes occurring in microfossils and along deformation bands and stylolites, constituting the heavy non-movable oil fractions. There is a general correlation between bulk rock porosity and the total non-movable oil saturation. It thus appears that the heavy non-movable oil fractions preferentially occur in association with low-permeability heterogeneities within high-permeability stratigraphic intervals. These intervals appear to favor accumulation of non-movable oil and solid bitumen/asphaltenes and may carry a higher risk for impeding scCO2 flow.

中文翻译：

---

丹麦北海枯竭的马斯特里希特白垩油藏中剩余可移动和不可移动石油馏分的空间分布：对 CO2 封存的影响


枯竭的油气田构成了地下 CO2 的潜在重要储存地点，但尚未尝试在白垩中大规模注入超临界 （sc） CO2。风险之一是白垩油藏中大量剩余石油对 scCO2 注入的不利影响。为了抵消对注入率的不良影响，从根本上了解可移动、半可移动和不可移动的油以及剩余油的固体沥青/沥青质馏分的空间分布和数量至关重要。在这项研究中，有机地球化学（饱和馏分的气相色谱法和程序热解）和反射光显微镜相结合，以评估和测量丹麦中部北海马斯特里希特白垩储层明确定义的储层间隔中不同油馏分的空间分布、体积和饱和度。共分析了来自一口略微偏差的垂直井和两口 ∼5 公里长的水平井的 127 个样品，这些样品来自 Halfdan 和 Dan 油田。油饱和度和成分的原始分布不均匀或储层中不同水平的生产效率不同可能是总油和油馏分饱和度变化的原因。气相色谱表明，溶剂可萃取油的成分非常相似，其特点是极性化合物占主导地位，沥青质含量高。延长慢热 （ESH） 热解显示，大部分剩余的油饱和度由半可移动油和总不可移动油（不可移动油加上固体沥青/沥青质）组成。 降低的石油比重值 （API） 与堆芯储存过程中最轻的碳氢化合物馏分的蒸发损失以及生产过程中水驱导致较重油馏分的相对比例增加有关。显微镜揭示了三种形式的油：i） 斑片状分布的较轻的可移动油，显示蓝色荧光，ii） 棕色染色，带有深橙色至棕色荧光，以及 iii） 深棕色非荧光油和黑色固体沥青/沥青质出现在微化石中，沿着变形带和幡石，构成重的不可移动油馏分。块状岩石孔隙度与总不可移动油饱和度之间存在一般相关性。因此，重质不可移动油馏分似乎优先与高渗透性地层间隔内的低渗透性非均质性相关。这些间隔似乎有利于不可移动的石油和固体沥青/沥青质的积累，并且可能具有更高的阻碍 scCO2 流动的风险。