A dissolution porosity increase model for sandstone reservoir in the Yanchang Formation in central and southern Ordos Basin—model building and model applications

doi:10.13745/j.esf.sf.2022.2.73

Abstract

Abstract:

Dissolution as a constructive diagenetic process plays an important role in expanding reservoir storage capacity and improving reservoir quality of sandstone reservoirs. However, in the existing studies, the influence of dissolution on sandstone porosity is mostly characterized from a qualitative perspective, whilst the amount of dissolution porosity increase in sandstone reservoirs and the effect of dissolution on reservoir quality improvement are not quantitatively characterized. In this paper, based on the mud log and wireline logging data, combined with sequence stratigraphy, burial history and thermal history data, the porosity characteristics and genesis of sandstone of the Yanchang Formation in central and southern Ordos Basin are analyzed by using the basic principle of chemical reaction dynamics, and a dissolution porosity increase model for sandstone is established. Using this model porosity evolution of the Yanchang sandstone is investigated via simulation. The main results are: (1) The present porosity profile for the Yanchang sandstone can be divided into two parts: “normal trend” section and “dissolution porosity increase” section; and, according to the initial horizon and morphological characteristics of secondary pore formation, the sandstone porosity profile can be divided into four types: bimodal I and II, and unimodal I and II. (2) The present Yanchang sandstone mainly experiences two types of porosity changes: porosity decrease in shallow or deep strata by mechanical compaction or by compaction and cementation, and porosity increase by organic acid-induced dissolution of feldspar and other minerals. (3) The dissolution porosity increase model calculates dissolution porosity increase as follows: ① Before the sandstone acidizing condition is reached in the strata, the amount of dissolution porosity increase is zero. ② As the strata condition moves inside the window of acidification, the accumulative dissolution porosity increase is a function of the present secondary porosity increase and time. ③ As the strata continue to be deeply buried post acidification, dissolution porosity does not change. That is, the model defines a piecewise function to calculate dissolution porosity increase. (4) Porosity evolution modeling allows quantitative characterization of dissolution porosity increase in the Yanchang sandstone. It reveals secondary porosity increases of 5.6%, 3% and 6.5%, respectively, in three sandstone reservoirs in the study area, namely, the Chang-8 sandstone of well Honghe-1, Zhenjing area in the south; the Chang-6 sandstone of well Dan-40, Ansai area in the east; and the Chang-4+5 sandstone of well Feng-12, Jiyuan area in the north. The model established in this paper can be used to quantitatively evaluate the amount of secondary porosity increase due to dissolution within the window of acidification. And for the first time quantitative simulation of sandstone dissolution and dissolution porosity increase in sandstone is demonstrated, which is of great significance for quantifying the effect of dissolution on improving reservoir quality of sandstone reservoirs.

Key words: Ordos Basin, Yanchang Formation, sandstone, dissolution porosity increasing model, porosity, quantitative characterization

CLC Number:

TE122.2

LIU Zhen, ZHU Maolin, PAN Gaofeng, XIA Lu, LU Chaojin, LIU Mingjie, LIU Jingjing, HOU Yingjie. A dissolution porosity increase model for sandstone reservoir in the Yanchang Formation in central and southern Ordos Basin—model building and model applications[J]. Earth Science Frontiers, 2023, 30(2): 96-108.

Figures/Tables 10

Fig.1 Structural units of the Ordos Basin and location of the study area

Fig.2 Stratigraphic column of the Yanchang Formation in the study area. Modified after [39⇓-41].

Fig.3 Four types of porosity depth profiles for sandstone of the study area

Fig.4 Characteristics of secondary pores in sandstone strata of the Yanchang Formation in the study area

Fig.5 Effect of diagenesis on pore size in clastic reservoir

Fig.6 Diagenetic sequence of Chang-8 member of the Yanchang Formation in the study area

Fig.7 Thermal and burial history curves for sandstone-bearing strata of the study area identifying the cementation starting position (red dot)

Fig.8 Modeling of porosity increase in Chang-8 sandstone of well Honghe-1, Zhenjing, southern study area

Fig.9 Modeling of porosity increase in tChang-6 sandstone of well Dan-40, Ansai, eastern study area

Fig.10 Modeling of porosity increasing in Chang-4+5 sandstone of well Feng-12, Jiyuan, northern study area

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