基于核磁共振技术的页岩油润湿性及其对原油动用特征的影响

Rock wettability and its influence on crude oil producing characteristics based on NMR technology

为了识别和评价页岩油储层岩石润湿性,深化其对储层原油动用程度的认识,以鄂尔多斯盆地西233地区页岩油储层岩样为研究对象,开展完全饱和水及束缚水状态下的核磁共振T_(1)-T_(2)二维图谱测试和基于核磁共振技术的自吸法润湿性实验,建立基于核磁共振T_(1)-T_(2)二维图谱评价润湿性的基本方法,进而选择平行岩样开展基于核磁共振技术的岩心水驱油实验,分析评价润湿性对水驱油过程中原油动用特征的影响。研究结果表明,油驱水后(即束缚水状态),水相与孔喉壁面的作用力明显减小,其在孔喉空间中表现出自由流体状态下的体弛豫特征,因此岩样的润湿性表现为油湿,这与基于核磁共振技术的自吸法润湿性实验评价结果一致。由此可见,通过对比不同状态下的核磁共振T_(1)-T_(2)二维图谱能够评价岩样的润湿性;岩样在水驱油过程中主要动用的是中-大孔喉中的原油,小孔喉中原油的动用效果较差。当增加注入水毛细管数的同时减弱储层岩石的油湿特性,小孔喉的动用效果可明显提升。因此注水开发过程中应综合考虑注入水毛细管数和储层岩石的润湿性,进而提高页岩储层的水驱油效率。

In order to identify and evaluate the rock wettability of shale oil reservoirs and deepen the understanding of the crude oil producing in the reservoirs,this paper takes rock samples from the shale oil reservoirs in the West 233 area of Ordos Basin as the research object and carries out two-dimensional nuclear magnetic resonance(NMR)T_(1)-T_(2)map measurement as well as spontaneous imbibition wettability experiment based on NMR technology in the state of fully saturated water and irreducible water. In addition,the paper establishes the basic method for evaluating wettability based on the two-dimensional NMR T_(1)-T_(2)maps and selects parallel rock samples for core water flooding experiment based on NMR technology,so as to analyze and evaluate the influence of wettability on the crude oil production characteristics during the water flooding. The research results show that after the oil flooding water(i.e.,in the irreducible water state),the force between the water and the pore throat wall is significantly reduced,and the water exhibits the characteristics of bulk relaxation in the state of free fluid in the pore throat space. As a result,the wettability of the rock samples is oil-wet,which is consistent with the results of the spontaneous imbibition wettability experiment based on NMR technology. Therefore,it can be seen that the wettability of the rock samples can be evaluated by comparing the two-dimensional NMR T_(1)-T_(2)maps in different states. During the water flooding,the crude oil is mainly produced in the medium and large pore throats of the rock samples but poorly produced in small pore throats. When the capillary number of injected water is increased,and the oil-wet characteristics of the rock in reservoirs are weakened,the crude oil producing in the small pore throats is significantly improved.Therefore,the capillary number of injected water and wettability of the rock should be comprehensively considered during water flooding development,so as to improve the water flooding efficiency of shale reservoirs.