低渗透复杂润湿性储层电阻率实验及导电机理研究

Study on resistivity experiments and conductive mechanism in low-permeability reservoirs with complex wettability

低渗透岩性油气藏发育的黏土膜吸附原油造成了储层亲油,电阻率异常高,高阻油水层、水层的存在给储层流体性质识别带来了很大挑战.为了明确不同润湿性储层的电阻率响应特征以及微观导电机理,本文选取了鄂尔多斯盆地西部三叠系延长组长8段的岩心,模拟了油驱水、老化和水驱油过程,并测量了岩心薄片洗油后的接触角.实验结果表明,洗油后异常高阻岩心已表现为不完全亲水,然而,其测量的胶结指数m与正常电阻率岩心相差很小.油驱水至束缚水时,正常电阻率岩心的电阻增大率I_r与含水饱和度Sw的关系在双对数坐标下基本表现为直线的关系,而异常高阻岩心则表现为明显的凸曲线特征.且老化过程前后,异常高阻岩心的电阻率基本不变.结合对异常高阻岩心不同状态下的核磁共振T_2谱的分析,表明在油驱水过程中,岩石的润湿性已经向亲油方向发生转变,老化过程对润湿性的改变影响很小.水驱油至残余油时,异常高阻岩心的I_r-S_w曲线表现为近似直线特征,反映出水驱油过程中岩石的导电结构并未发生改变.基于实验结果的分析与讨论,明确了一种适用于低渗透复杂润湿性储层的成藏模式及其导电机理,说明了高阻水层主要是亲油润湿性条件下的连续导电路径遭到破坏造成的.

As clay membrane developed in low-permeability lithologic reservoirs absorbs oil, reservoirs become oil-wet, which results in abnormally high resistivity oil-water layers and water layers. Such layers bring great challenges to identification of oil and water layers. In order to make clear of the resistivity response characteristics and conductive mechanism in reservoirs with different wettability, cores were selected in Chang 8 formation, Yanchang group, Upper Triassic, the western Ordos Basin of China. Experiments were tested on these samples to simulate the process of oil displacing water, aging and water displacing oil. What＇s more, contact angles of thin sections after washing oil were also tested. The experimental results show that abnormally high resistivity cores are not completely water-wet after washing oil. However, the formation factors of abnormally high resistivity cores and normal resistivity cores have little difference. In the process of oil displacing water, the relationship between normal resistivity core resistivity index and water saturation is linear in log-log plot, while that of abnormally high resistivity cores is convex. The resistivity of abnormally high resistivity cores remains unchanged during aging process. Combining the analysis of Nuclear Magnetic Resonance （NMR） T2 spectra under different conditions, it can be inferred that wettability of abnormally high resistivity cores has become less water wet after oil displacing water. What＇s more, the aging process has little effect on the wettability of abnormally high resistivity cores. In the process of water displacing oil, the relationship between abnormally high resistivity core resistivity index and water saturation is almost linear, which shows that conductive structure of rock is not changed. Based on the analysis and discussion of these experimental results, an accumulation model and the corresponding conductive mechanism are proposed for low-permeability reservoirs with complex wettability. It shows thai abnormally high resistivity water layers are caused by the damage of continuous conductive path under oil-wet condition. The researches and experiments are of great importance for understanding the conductive mechanism, oil layers identification and the evaluation of water flooded formation in low-permeability reservoirs with complex wettability.