鲕粒白云岩内部溶蚀及孔隙非均质性演化研究

在地表和埋藏条件下溶蚀作用形成的次生孔隙是碳酸盐岩油气储层重要的储集空间。为探究孔隙的溶蚀改造机理及其控制因素,通过柱塞样,开展了0.2%的乙酸环境中鲕粒白云岩的溶蚀实验,实验温度和压力范围分别为40~160 ℃和10~50 MPa。实验采取连续取样获得溶液中Ca²⁺与Mg²⁺离子含量,通过CT 成像技术获取溶蚀反应前后孔隙图像并运用分形与多重分形方法定量分析孔隙在二维和三维空间上的非均质性。研究表明,溶蚀过程中,鲕粒白云岩溶蚀液中Ca²⁺+Mg²⁺]浓度在实验开始时最低,之后稳步上升并在120 ℃、40 MPa时达到最大值,而后缓慢下降,溶蚀窗范围出现在70 ℃(20 MPa)~120 ℃(40 MPa)之间。二维和三维微观孔隙结构在空间上的分布具有显著的多重分形特征,溶蚀作用使得孔隙的不规则性减少、孔隙的奇异范围缩减。这些结果对于认识近地表和深埋藏条件下溶蚀作用对碳酸盐岩储层孔隙的改造强度及孔隙的动态演化规律具有重要的理论及实际意义。

Internal Dissolution and Pore Structural Evolution of Oolitic Dolomite

Secondary pores formed by dissolution under surface and burial conditions are very important sites for carbonate reservoirs. To explore the pore erosion mechanism and its controlling factors, the dissolution experiments of oolitic dolomite in 0.2% acetic acid environment were carried out with one oolitic column sample. The experimental temperature and pressure were of 40-160 ℃ and 10-50 MPa, respectively. Continuous sampling was conducted to measure the Ca²⁺ and Mg²⁺ contents in the solution. Pore images before and after the dissolution reactions were obtained by CT imaging. Fractal and multifractal methods were used to quantify the pore heterogeneity in two-dimensional (2D) and three-dimensional (3D) space. The results show that the Ca²⁺+ Mg²⁺] concentration of oolitic dolomite was the lowest at the beginning of the dissolution experiment, and then increased steadily and reached the maximum at 120 ℃ and 40 MPa, and subsequently decreased slowly with the dissolution window from 70 ℃ to 120 ℃. The 2D/3D spatial distribution of micro-pore structure has multifractal characteristics. This implies that the dissolution reduced the irregularity and singularity of the pores. These results are of great theoretical and practical significance to understand the dissolution effect on the pore structures, and the pore dynamic evolution in carbonate reservoirs under near-surface or deep-burial conditions.