挥发作用对C_5—C_8轻烃指标的影响:模拟实验证据

Effects of natural evaporation on C_5-C_8 light hydrocarbon indicators:evidence from experimental results in the labratory

C5—C8轻烃指标在石油勘探开发中扮演着重要角色,然而其易挥发的自然属性导致C5—C8轻烃指标在实际应用过程中存在诸多不确定性。本研究利用塔中地区奥陶系海相凝析油在实验室内进行了原油挥发模拟实验,基于实验结果遴选和构建了受挥发作用影响较小的系列轻烃指标,具有较强的实用性。模拟实验结果显示挥发作用对C5—C8轻烃指标的影响程度受轻烃分子量和分子构型共同控制。nC5—nC8正构烷烃挥发量随正构烷烃碳数增加而逐步降低,不同分子构型C7化合物挥发量由高到低依次为:C7-支链烷烃>nC7>甲基环己烷>甲苯,二甲基环戊烷挥发量则大体相当。正(异)构烷烃与环烷烃比值和挥发速率相近的相同分子构型轻烃比值受挥发作用影响相对较小。原油挥发过程中,残留液态原油内C6—C8轻烃单体稳定碳同位素组成随挥发量增加逐步富集13 C,nC6—nC8正构烷烃稳定碳同位素组成变化程度随碳数增加逐渐降低。当C6—C8轻烃挥发量小于70%时,C6—C8轻烃稳定碳同位素组成变化不大,当其挥发量大于70%时,正构烷烃、异构烷烃和芳烃稳定碳同位素组成均产生1.0‰以上的较大变动,环烷烃稳定碳同位素组成则变化不大。受挥发作用影响较小的轻烃指标即正(异)构烷烃与环烷烃比值和环烷烃稳定碳同位素组成在塔中地区原油成熟度标定和原油类型划分方面取得了较为理想的应用效果,展现了良好的实用性和应用前景。

Light hydrocarbon indices of C5-C8 play an important role in petroleum exploration and exploitation, while a large number of uncertainties occur in their applications due to the volatile nature of crude oils. A simulation experiment on natural evaporation of a condensate sampled from Ordovician marine formations in the central area of the Tarim Basin was conducted in the laboratory. On the basis of experimental results, a series of light hydrocarbon indices were screened and refined in order to make them more resistant to natural evaporation and more functional in applications. The results indicated that impacts of natural evaporation on C5-C8 light hydrocarbon indices largely depend on both molecular weights and chemical structures of individual compounds. Evaporation amounts of C5-C8 normal alkanes decrease progressively with increase of carbon number, and those of C7 compounds with different molecular configurations follow an descending order of isoheptane, n-heptane, methylcyclohexane and toluene, while dimethylcyclopentanes show an eguivalent amount of natural evaporation. In general, natural evaporation has a minor impact on the ratios of normal alkanes to cycloalkanes and isoalkanes to cycloalkanes, and so does on ratios of light hydrocarbons that have similar natural evaporation rates and identical molecular configurations. During oil evaporation, stable carbon isotopic compositions of individual C6-C8 compounds within liquid residues are progressively enriched in 13 C with increasing evaporation, while the varying extent of carbon isotope ratios for C6-C8 normal alkanes decreases gradually with increase of carbon number. When an evaporative loss is less than 70 %, δ^13C values of C6-C8 normal alkanes show no much variations （〈1.0‰） ; and when an evaporative loss exceeds over 70%, δ^13C values of all nor mal alkanes, isoalkanes and light aromatics vary obviously （ 〉1.0‰）, with the exception of cycloalkanes whose δ^13C value still chan-ges a little （~ 1.0‰）, this indicated that δ^13C value of cycloalkanes can be used with confidence. Thus, some light hydrocarbon indices less affected by natural evaporation, such as ratios of normal alkanes to eycloalkanes and isoalkanes to eyeloalkanes, and δ^13C value of cycloalkanes, were applied successfully to maturity determination and type classification of crude oils in the central area of the Tarim Ba- sin, demonstrating a good practicability and bright application future.