Classification and evaluation of tight sandstone reservoirs based on diagenetic facies: a case study on Chang 6 reservoir in the center-west Ordos Basin

Tight sandstone reservoirs dominated by are developed in the Chang 6 oil layer group of the Yanchang Formation in the central-western part of the Ordos Basin. Featuring the lacustrine delta facies, Chang 6 formation in the center-west area of Ordos Basin shows an increasing petroleum reserve expectation. Its exploitation practice, however, has many problems caused by tight sandstone reservoir features. According to diagenetic and pore analysis, the diagenetic facies in the study area are grouped into four types: chlorite-film-intergranular-pore, feldspar-dissolution, clay-cemented-micropore, carbonate-cemented-tightness for their obvious differences in mineral feature and pore evolution. By introducing the comprehensive classification parameter synthesized from 9 other parameters, the reservoir quality is divided up into four levels: I(Feci > 1), II(3 ≤ Feci ≤ 7), III(-2 ≤ Feci ≤ 3), IV(Feci ≤ -2). The reservoir quality division matches well with the diagenetic facies group. To decide the diagenetic type and reservoir quality division in all wells, the logging data are utilized with the Fisher discriminant method, which has obtained a good performance. The method enables the reservoir quality analysis expanding to all wells from samples, which is helpful for exploitation of the study area.

SIMULATION EXPERIMENT ON THE EFFECT OF HYDRODYNAMICS ON ALTERATION OF ORTHOCLASE UNDER RESERVOIR CONDITIONS

The dissolution of feldspar is the main mechanism for the formation of secondary pores in deep sandstone reservoirs, which is controlled by pore fluid properties and fluid dynamics. The simulation experiment analyzed the effect of different fluid/rock ratios (hydrodynamics) on the orthoclase dissolution kinetics under reservoir diagenesis conditions. The experimental temperature was set at 100°C; the solutions were divided into acetic acid solutions and hydrochloric acid solutions, with pH values at 2 and 4; the reaction time was between 30 days and 80 days. The experimental results show that the fluid/rock ratio is an important factor controlling the feldspar dissolution. The higher the fluid/rock ratio, the stronger the fluid’s ability to dissolve at orthoclase, and the more the release of Al and Si. With the increase of fluid/rock ratio, it is possible to observe more noticeable dissolution characteristics from the feldspar morphology. The lower the fluid/rock ratio, the higher the concentration of Al and Si ions released by orthoclase dissolution, and the easier it is to cause the precipitation of clay minerals, which is not conducive to the improvement of reservoir physical properties. The fluid/rock ratio is an important controlling factor on the formation and distribution of authigenic minerals such as kaolinite. The research results are of great implications in the feldspar dissolution to form secondary pores in the burial diagenesis process. In the sandstone-shale contacts or sandstone reservoirs with good initial physical properties, the strong fluid flow is conducive to the secondary pores formation. The experimental findings are helpful for us to understand better the process and mechanism of feldspar alteration and secondary pores formation.

Experimental study of the hydrothermal reactivity of oxalic acid under high pressure

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.