Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (2): 96-108.DOI: 10.13745/j.esf.sf.2022.2.73

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

LIU Zhen1(), ZHU Maolin1, PAN Gaofeng1, XIA Lu2, LU Chaojin1, LIU Mingjie3, LIU Jingjing4, HOU Yingjie5   

  1. 1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China
    2. Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China
    3. Sichuan Natural Gas Geology Key Laboratory, Southwest Petroleum University, Chengdu 610500, China
    4. Petroleum Exploration & Production Research Institute, SINOPEC, Beijing 102206, China
    5. No.5 Gas Production Plant of Changqing Oilfield Branch of PetroChina, Xi’an 710018, China
  • Received:2021-06-16 Revised:2022-01-10 Online:2023-03-25 Published:2023-01-05

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

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