油層流体を対象にしたアスファルテンの相挙動に関する実験的解析

―アスファルテンの析出・再溶解現象と分子量特性について―

抄録

In order to evaluate asphaltene deposition behavior near wellbore region in the reservoir as well as inside the tubing in Minami-Kuwayama field, Japan, a single phase bottom hole sample was devoted to laboratory experimental analysis such as the Asphaltene Deposition Envelope (ADE), molecular weight profile at different pressure/temperature conditions and re-dissolution tests.
The attenuation of laser light transmission due to asphaltene precipitation was used to identify the onset pressure of asphaltene precipitation. Once coming across the asphaltene onset pressures, asphaltene deposition volumes significantly increased as the pressures decreased, which was quantitatively evaluated by SARA analysis. Consequently, the ADE was generated based on these experimental results. The asphaltene deposition behavior was estimated by comparing the ADE with measured pressure/temperature profile near wellbore region in the reservoir and inside the tubing.
The asphaltenes were separated from the deposits inside the tubing and bottom hole sample using Heptanes. The composition and molecular weight of asphaltenes were obtained using LDI-TOFMS (Laser Desorption Ionization Time of Flight Mass Spectrometer). According to the analysis, the average molecular weight of asphaltenes in the deposits inside the tubing and that in bottom hole sample were 1,400 and 1,290, respectively.
The asphaltene in single phase bottom hole sample precipitated by pressure reduction was re-pressurized to test the re-dissolution. The laser light transmission and SARA analysis showed that it was reversible. When the asphaltenes were re-dissolved in the oil sample, the concentration of asphaltenes in the oil increased. In the re-dissolution process, the concentration of low molecular weight component in the asphaltenes tended to increase, and the average molecular weight of asphaltenes tended to decrease. This indicated that high molecular weight component of asphalten were more easily dissolved at early stage, whereas low molecular weight component of asphalten were more hardly and slowly dissolved.