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摘要:
提升致密背景下相对优质储层预测的能力，是当今油气勘探开发理论亟待破解的瓶颈和难题。致密油气储层的非均质性强，黏土矿物含量高且是吸附油接触最多的矿物之一。有机酸对黏土矿物的溶蚀影响，是实现致密油高效开采的关键。本文选择鄂尔多斯盆地三叠系延长组为研究对象，通过有机酸与砂岩的溶蚀模拟实验，对实验产物进行pH值、阳离子检测、孔隙度和渗透率测试以及扫描电镜观察。探讨了时间、温度和不同有机酸类型对黏土矿物的溶蚀影响。实验结果显示：①随时间的增长（1~9d），孔隙度增幅呈先增长后降低的趋势，渗透率的增幅呈持续增长趋势；温度升高（80~95℃）对有机酸溶蚀致密砂岩中的黏土矿物具有促进作用；②不同类型的有机酸对黏土矿物具有选择性溶蚀作用。酒石酸溶蚀大量黏土矿物、碎屑长石以及少量方解石胶结物；乙酸则相反，主要溶蚀方解石；甲酸、乙酸和丙酸配比的合酸以及甲酸、乙酸、丙酸和酒石酸配比的合酸溶液，均优先溶蚀绿泥石化、泥化的长石和方解石，直至方解石完全溶解；③不同类型的有机酸对储层物性的改造能力不同，甲酸对孔隙度的改善不明显，乙酸和丙酸对孔隙度改善明显，合酸对孔隙度的影响是单一酸改善的综合反映。综合分析，有机酸流体与致密砂岩的溶蚀反应机理主要为两种：①有机酸流体提供氢离子，溶蚀致密砂岩中的易溶矿物；②有机酸直接与致密砂岩矿物发生络合反应，影响络合物稳定性的因素主要是有机酸种类和pH值。乙酸、甲酸、丙酸和酒石酸等不同类型的有机酸对致密砂岩中黏土矿物的选择性溶蚀，对储层物性影响程度不一致。
Abstract:BACKGROUND Improving the prediction ability of relatively high-quality reservoirs under tight backgrounds is a bottleneck and a challenge in current oil and gas exploration and development theories. For tight reservoirs, exploring the dissolution mechanism of organic acid fluids on the reservoir is particularly important. Previous researchers have conducted a large number of water rock reaction simulation experiments on the dissolution of organic acids leading to the formation of secondary pores. It is proposed that the dissolution effect of acidic fluids is the main factor for increasing porosity in tight reservoirs, and it is also a key way to find “sweet spots” in tight reservoirs[3]. Based on previous research on dissolution pores in sandstone, scholars have focused on studying the chemical mechanism of organic acids in the dissolution of carbonate and feldspar minerals. Some studies have shown that the dissolution of calcite requires a lower pH[19] and the acidity of binary acid is very strong, which can greatly improve the solubility of aluminosilicate minerals[20-22]. However, the heterogeneity of tight sandstone reservoirs is strong, and its complex mineral components and pore structure characteristics differ greatly; it has a higher content of clay minerals, and during the diagenesis process, clay minerals often precipitate on the rigid particle surface of the pore inner wall as authigenic minerals. The main mineral in contact with crude oil is of clay composition in dense sandstone reservoirs[23-25]. Chlorite, kaolinite, illite and other minerals are common and important clay mineral types. As important factors affecting reservoir exploration and development, their organic acid dissolution effects on clay minerals need to be further studied.
OBJECTIVES (1) In order to explore the main influencing factors of organic acids on the dissolution of clay minerals in tight sandstone, by analyzing the influence of time, temperature and different types of organic acids on the dissolution of clay minerals. (2) To reveal the dissolution reaction mechanism between organic acid fluids and tight sandstone, providing a theoretical basis for improving the prediction ability of relatively high-quality reservoirs under tight backgrounds.
METHODS (1) The Triassic Yanchang Formation in the Ordos Basin was selected as the research object, and the ratio of reaction fluid to sandstone dissolution simulation experiment was conducted according to the type and content of organic acid in the thermal evolution fluid of Source rock. (2) After the reaction, the column rock sample was rinsed multiple times with distilled water, placed in a drying oven, dried for 24h, and then taken out for testing. The porosity and permeability of the column rock sample after the reaction were tested on the PoroPDP-200 overlying pressure pore permeability meter before and after the experiment, and the intensity of dissolution was quantitatively calibrated. (3) Small samples of 5-8cm for argon ion polishing were selected, and observed under the Quanta450FEG field emission environment scanning electron microscope (Lanzhou Oil and Gas Resources Research Center, Chinese Academy of Sciences). Then, through thin section identification and scanning electron microscope observation of the petrology characteristics of the samples, the cement and pore characteristics of the samples before and after the experiment were compared. (4) Using Optima 8000 inductively coupled plasma-optical emission spectrometer (PerkinElmer Company, USA) to detect cations, the practical range of the measured standard curve was 0.1-20mg/L, and samples that were not within the test range were diluted. The standard curve solution contains a total of 8 ions: K, Ca, Na, Mg, Al, Si, P and Mn.
RESULTS (1) With the increase of time (1-9 days), the increase of porosity dissolution increases first and then decreases, reaching its peak at 6 days; the increase of penetration rate shows a continuous growth trend. The increase in temperature can also promote the dissolution of sandstone by organic acids (Fig.3, Fig.4). (2) Different types of organic acids have selective dissolution of clay minerals. Tartaric acid mainly dissolves clay minerals, detrital feldspar and a small amount of calcite cement; on the contrary, acetic acid mainly dissolves calcite. The sequence of propionic acid dissolution is from calcite to feldspar detrital particles, from dissolution cement matrix to argillized feldspar; the mixed acid solution of formic acid, acetic acid and propionic acid and mixed acid solution of formic acid, acetic acid, propionic acid and tartaric acid preferentially dissolve chlorinated and argillized feldspar and calcite until calcite is completely dissolved (Fig.4, Fig.5). (3) The improvement of pores by formic acid is not significant among different types of organic acids. Propionic acid greatly improves porosity. The influence of combined acids on porosity is a comprehensive reflection of the influence of single acids (Fig.6).
CONCLUSIONS Formic acid has little effect on porosity, whereas acetic acid and propionic acid have obvious effect on porosity. The combined effects of formic acid, acetic acid, propionic acid, and tartaric acid on porosity and permeability are a comprehensive reflection of the improvement of a single acid. The selective dissolution of clay minerals in tight sandstone by different types of organic acids has different effects on the physical properties of the reservoir, providing a scientific basis for improving the prediction ability of relatively high-quality reservoirs under tight background.
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Key words:
- clay minerals /
- dense sandstone /
- organic acid /
- corrosion simulation experiment
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图 5 两组实验不同有机酸类型溶蚀砂岩析出金属离子含量分析（a和c是两组实验的所有金属离子含量分析。由于Ca离子析出量过多，b和d是对应剔除Ca离子的其他离子含量分析）
Figure 5.
图 6 酒石酸溶蚀过后，仍存在很多方解石（a、b），溶出的多为绿泥石组分（c）；形成孔喉；钠长石溶蚀现象明显（d）。乙酸溶蚀过后，长石溶蚀现象明显（e、f、g、h）；溶蚀后形成粒间孔（g）。合酸溶蚀后局部可见少量未溶方解石（i）；见钾长石表面的黏土矿物溶蚀（j）；长石溶蚀附近可见新生矿物颗粒附着（k、l）
Figure 6.
表 1 主要有机酸类型占总有机酸的比例
Table 1. Data comparison of major organic acids.
样品 流体压力
（MPa）实验温度
（℃）主要有机酸类型占总有机酸的比例 实验系统 乙酸（%） 丙酸（%） 甲酸（%） 柠檬酸（%） 酒石酸（%） 富马酸（%） 1 16.9 250 84.81 9.92 1.55 0.00 0.00 0.00 封闭[27] 2 22.1 300 54.09 9.43 1.14 2.32 29.56 0.39 3 32.5 350 45.09 7.54 1.16 4.30 38.89 0.15 4 37.7 370 32.77 5.15 0.68 0.83 57.51 0.00 5 42.9 400 87.09 10.29 0 0.00 0.00 0.00 6 52.2 450 34.12 2.71 0.90 60.47 0.00 0.00 1 15 150 1.04 0 0 0.00 97.67 0.00 半封闭[28] 2 20 200 1.31 0 0 0.72 96.26 1.13 3 25 250 4.11 0 0 1.44 86.63 1.38 4 30 300 4.91 2.96 0.46 4.13 72.00 11.06 
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表 2 样品实验条件及实验后的pH值、孔隙度和总面孔率统计数据
Table 2. Sample experimental conditions and statistical data of pH value, porosity, and total porosity after the experiment.
实验样品编号 岩性 溶液配比 温度点（℃） 恒温时间（d） 实验后pH值 实验后孔隙度（%） 总面孔率（%） C7-1-1 砂岩 甲乙丙酒 96 3d 3.74 8.034 2 C7-1-2 砂岩 甲乙丙酒 96 6d 4.26 8.575 5 C7-1-3 砂岩 甲乙丙酒 96 9d 4.21 8.509 5 C6-1-1 砂岩 甲乙丙酒 96 3d 3.51 11.745 15 C6-1-2 砂岩 甲乙丙酒 96 6d 3.97 13.343 16 C6-1-3 砂岩 甲乙丙酒 96 9d 3.7 12.805 10 注：甲乙丙酒为甲酸、乙酸、丙酸、酒石酸的体 积比为 1∶47∶7.4∶84。
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表 3 不同时间的溶蚀实验前后样品的pH值、总面孔率和物性变化
Table 3. Changes in pH value, total porosity, and physical properties of samples before and after experiments at different time.
实验样品编号 实验时间 实验后pH 原孔隙度（%） 实验后孔隙度（%） 原渗透率（md） 实验后渗透率（md） 总面孔率（%） C6-2-1 1d 3.01 8.233 8.465 0.0138 0.0141 5 C6-2-2 2d 3.4 7.296 7.487 0.0101 0.0181 7 C6-2-3 3d 3.52 7.488 7.756 0.0111 0.0266 9 C6-2-4 4d 3.52 7.905 8.42 0.0101 0.0268 5 C6-2-5 5d 3.67 6.974 7.361 0.0103 0.0283 10 C6-2-6 6d 3.72 8.032 8.43 0.0125 0.0396 14 C6-2-7 7d 3.8 7.407 7.741 0.0119 0.0384 13
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表 4 不同类型有机酸溶蚀实验后样品的pH值、孔隙度和总面孔率统计数据
Table 4. Statistical data on pH value, porosity, and total porosity of samples in different types of organic acid dissolution experiments.
实验样品编号 实验条件 原始pH值 实验后的pH值 实验后孔隙度（%） 总面孔率（%） C6-1-0 去离子水，25℃，9d - - 9.794 6 C6-1-5 酒石酸，95℃，9d 2.5 3.26 10.448 6 C6-1-6 乙酸，95℃，9d 2.5 3.6 10.29 20 C6-1-7 甲乙丙酸a，95℃，9d 2.5 3.81 12.0623 20 C6-1-8 甲乙丙酒酸b，95℃，9d 2.5 3.7 12.8085 10 注：a为甲酸、乙酸、丙酸的体积比为1∶47∶7.4； b为甲酸、乙酸、丙酸、酒石酸的体积比为1∶47∶7.4∶84。下同。
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表 5 不同类型有机酸溶蚀实验前后样品的pH值、总面孔率和物性变化
Table 5. Changes in pH value, total porosity, and physical properties of samples before and after different types of organic acid dissolution experiments.
实验样品编号 溶液配比 实验前pH值 实验后pH值 原始孔隙度（%） 实验后孔隙度（%） 原始渗透率（md） 实验后渗透率（md） 总面孔率（%） C6-2-0 去离子水 - 7.94 7.401 7.472 0.0141 0.0184 5 C6-2-8 酒石酸 2.5 3.47 7.348 7.541 0.0133 0.0261 8 C6-2-9 乙酸 2.5 3.58 7.425 7.921 0.013 0.0372 8 C6-2-10 甲酸 2.5 3.55 7.872 8.363 0.0126 0.0285 6 C6-2-11 丙酸 2.5 3.65 7.764 8.751 0.0131 0.0301 10 C6-2-12 甲乙丙酸 2.5 3.67 8.252 9.164 0.0122 0.0311 8 C6-2-13 甲乙丙酒酸 2.5 3.72 8.032 8.43 0.0125 0.0396 14
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