有机酸对致密砂岩中黏土矿物的选择性溶蚀研究

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。

Selective Dissolution of Clay Minerals in Tight Sandstone by Organic Acids

BACKGROUNDImproving 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.