Abstract:

The natural gas is the main product in Kuqa Depression, while there is a small amount of crude oil. Gas charged into reservoir later than oil, and the oil pools formed in early stage tended to be transformed by natural gas injected in later stage. In order to evaluate gas injection intensity quantitatively，it is very important that the crude oil composition changes are calculated, which can provide direct evidence. For crude oils undergo no gaswashing, the logarithm of molar concentration of nalkanes have a linear relationship with the corresponding carbon number, while gaswashing can make oil components changed. During this process, some hydrocarbon components dissolve in dry gas, and migrate forward with the natural gas. The light nalkanes are prone to dissolve in gas, thus the light fraction of nalkanes reduces significantly in original reservoirs. Based on this theoretical foundation, the quantitative calculation model of nalkanes loss was established. The results showed that the crude oil in Kela2 Structural Belt had the highest level of nalkanes loss, with an average of 70%. The crude oil in Dabei Structural Belt had different degrees of nalkanes loss, resulted from the fault blocks in this region. Gaswashing made nalkanes reduced in crude oil, while other compounds dissolved little in natural gas, such as diamondoids and polycyclic aromatic hydrocarbons, and could be concentrated and enriched, leading to their relative contents increased greatly in original reservoirs. The content of light aromatic hydrocarbons would also increase, resulting in aromaticity increasing and paraffinity reducing. Based on the simulation experiment, we preliminarily estimated the degree of gas washing, and it was showed that the crude oil in Kela2 Structural Belt underwent the strongest gaswashing, 25 times stronger than in other belts.

Key words: gas washing, oil composition, diamondoids, Kela2 natural gas reservoir, evaporation fractionation