超深层油气相态转化过程及其控制因素:来自原油可视化热模拟实验的启示

Phase Transformation Mechanisms and Controlling Factors of the Ultra-Deep Oil and Gas:Insights From Visual Thermal Simulation of Crude Oil

超深层油气相态转化过程及主控因素缺乏系统的实验研究,也缺少对转化过程的可视化直观呈现。为研究超深层油气相态转化过程及主控因素,以SHB7井正常原油为研究对象,开展了原油相态转化过程的在线及离线可视化观测实验。得出如下结果:(1)在热模拟实验温度达到原油发生裂解的起始温度之前,原油的红绿熵值(Q650/500)在升、降温过程中发生可逆变化,说明原油荧光不仅受油成分、密度的影响,还受到温度本身的影响。(2)在实验温度达到原油发生裂解的起始温度之后,原油的Q650/500发生不可逆的变化,指示油的成分受热裂解的影响发生了不可逆变化,并且随着模拟温度的增高原油中液态烃的含量呈减少的变化趋势,而沥青的含量呈增加的变化趋势,表明温度是控制原油热裂解及相态转化的关键因素。(3)对比0.1℃/min、0.7℃/min、5℃/min升温速率下原油的荧光演化,热解后残余油发生荧光(成分)分异的温度点依次升高,表明升温速率越小越有利于原油的热裂解,因此长期缓慢升温的地质条件下不利于液态烃的保存。(4)对比不同油充填度的样品实验结果,可知原油经历过Ro=1.89%的热演化后,压力增加促进了原油的热演化。所以,压力不一定抑制液态油裂解转化,反而有可能促进热裂解转化。综合原油热裂解的可视化分析结果,将原油热裂解过程划分三个阶段:第一阶段的Ro范围是0.80%~1.24%,重质饱和烃优先裂解,原油中的饱芳比下降,液态烃荧光颜色发生红移;第二阶段的Ro的范围是1.24%~1.55%,该阶段大量的芳烃缩合成固体-半固体沥青并在降温后附在管壁,导致残余油的饱和烃相对含量略微升高,液态烃荧光蓝移。第三阶段的Ro大于1.90%,芳烃继续缩合形成固体-半固体沥青,毛细管中的烃类在室温下出现明显荧光分异现象,其中发蓝色荧光的烃类为由非极性饱和烃为主要成分的轻质油,而发橙黄色荧光的烃类为由极性沥青质为主要成分的重质油。这种荧光分异现象或可为同一微域下不同荧光颜色的油包裹体共生提供解释。塔里木盆地顺北油气田的油气分布规律验证了实验结果中温度和压力对油裂解的控制作用。

There is a lack of systematic experimental studies on the phase transformation processes of ultra-deep oil and gas and their controlling factors,as well as a lack of visual presentation of these processes.In this study,we used crude oil of the SHB7 well as a case study and conducted both online and offline visual observation experiments.The conclusions are summarized as follows:(1)Before the temperature of the thermal simulation experiment reaches the crude oil cracking point,the red-green quotient value(Q650/500)of the crude oil changes reversibly during the heating and cooling processes.This indicates that crude oil fluorescence is influenced not only by its composition and density,but also by temperature.(2)After the temperature reaches the cracking point,the Q650/500 of the crude oil changes irreversibly with temperature.This suggests that the oil’s composition has undergone irreversible alterations due to heating and cracking.The observed trend of decreasing liquid oil and the increasing solid or semi-solid asphaltene content indicates that temperature plays a crucial role in controlling crude oil cracking and phase transformation.(3)Comparing the fluorescence evolution of crude oil at the heating rates of 0.1,0.7,and 5℃/min,the temperature required for the separation of residual oil components increases with the heating rate.This suggests that a lower heating rate is more conducive to oil cracking,implying that long-term and slow heating under geological conditions is unfavorable for the preservation of liquid hydrocarbons.(4)Comparing the experimental results of different samples with various oil filling ratios,we conclude that an increase in pressure promotes the thermal evolution of crude oil after reaching a thermal maturity of Ro(1.89%).This suggests that pressure does not necessarily inhibit the cracking of liquid oil but may accelerate the process.(5)Based on the results of the visualization experiment,the process can be divided into three stages.In the first stage,with an Ro range of 0.80%-1.24%,heavy saturated hydrocarbons crack preferentially,leading to a redshift in the fluorescence color of the liquid hydrocarbons.In the second stage,with an Ro range of 1.24%-1.55%,numerous aromatic hydrocarbons condense into solid or semi-solid asphaltenes that adhere to the tube wall after cooling.This results in a slight increase in the ratio of saturated to aromatic hydrocarbons in the residual oil and a blue shift in the fluorescence of the liquid hydrocarbons.In the third stage,with an Ro greater than 1.90%,aromatic hydrocarbons continue to condense,forming solid or semi-solid asphaltene.The hydrocarbons in capillary capsules exhibit distinct fluorescence differentiation at room temperature.The blue fluorescent hydrocarbons represent light oil,primarily composed of non-polar saturated hydrocarbons,while the orange fluorescent hydrocarbons indicate heavy oil,predominantly containing polar asphaltenes.This phenomenon may explain the coexistence of fluid inclusions with different fluorescence colors within the same microdomain.