Search

The organic acid is important in forming secondary pores and enhancing total porosity of the clastic rock. But there is little study to investigate the effect of organic acid on the porosity distribution of clastic rock from deep reservoirs. The changes in porosity, permeability, and porosity distribution of the dissolved medium-grained lithic arkose, from Yanchang Formation in Ordos Basin, were examined by nuclear magnetic resonance (NMR). The results show that porosity and permeability of rock increase with the time of exposure of the rock to the organic acid solution and it increases rapidly in the early period, then slightly increases to a saturation level. Besides, organic acid mainly changes the frequency and the shapes of T2 distribution in NMR analysis, and this study suggests that organic acid has more effect on rocks with larger pore throat than those with smaller pore throat.

The thermal stability of organic acids has a close relationship with the formation mechanism of secondary pores in deep reservoirs. Experiments were conducted to investigate the thermal stability of oxalic acid under high pressure and the influence of K-feldspar on oxalic acid decomposition. The experiment temperatures were set in the range of 130-330°C and each experiment was performed for 72h under 60MPa. Results show that both temperature and K-feldspar have significant influence on the decomposition of oxalic acid. The oxalic acid decomposed slowly at temperatures less than 180°C, and most of the oxalic acid decomposed at temperatures between 180°C and 230°C. Besides, the decarboxylation reaction proceeded more slowly in the presence of K-feldspar than the mineral-free experiments, which is most likely attributed to the increasing pH caused by dissolution. In addition, because the decomposition rate of oxalic acid was low, the K-feldspar dissolution was not affected at temperatures lower than 230°C.