应用核磁共振技术评价水驱开发效果

 通常的室内岩心分析不能给出岩心水驱油过程中油的动用特征及剩余油的分布情况,借助核磁共振T₂谱技术能够直观地观察到岩心经水驱后不同孔隙中油的动用规律及剩余油分布特征.对3类不同渗透率级别的岩心样品进行水驱油实验,并借助核磁共振T₂谱技术对其渗流机制进行分析,评价水驱后的采出程度.结果表明,对于低渗透、特低渗透的岩心,驱替过程中,大、中孔隙中的油优先被驱替出来,见水时水驱采出程度已较高,见水后可通过增大驱替压力进一步提高采出率,故此类油藏适合注水开发且效果较好.但驱替压力过高也会改变孔隙中剩余油的分布,将部分油挤进更小的孔隙中,很难再被开采出来.驱替结束后所有尺度的孔隙中都有残余油分布,即使大孔隙,T₂谱显示也有部分剩余油存在.对于超低渗透岩心,水驱过程中压力较高且见水时水驱采出程度很低,故此类油藏注水开发效果较差,需通过酸化、压裂等技术改善储层物性,或寻找其他途径有效开发.

Water-drive Mechanism with Nuclear Magnetic Resonance Technology

The conventional core analysis conducted interiorly usually could not give the oil flowing performance in the process of water-flooding and the distribution of residual oil at the end of water-flooding. However, with Nuclear Magnetic Resonance (NMR) T₂ relaxation spectrum, the oil movement in porous media and the distribution of remaining oil could be directly observed. Some core samples which had different permeability levels were chosen to conduct displacement experiments. The results show that for the cores with low permeability, the oil in the big pores will be displaced easily and the breakthrough recovery is high. Increasing displacement pressure is also able to further enhance the final oil recovery. Therefore, in the development process of these reservoirs, injecting water will be reasonable and realistic and is able to obtain better result. However, exorbitant displacement pressure might change the distribution of residual oil, that is, some parts of oil will be driven into the smaller pores and will become harder to be developed in the future. When the experiments have been completed, some of oils are still found in all sizes of the pores, including the big ones. For the cores with ultra-low permeability, breakthrough recovery is very low compared with the cores with low permeability. Therefore, water-flooding displacement is no longer an efficient development mode for these reservoirs. Oil field needs some other techniques, such as acidifying treatment, fracturing work, etc to assist the development of the cores with ultra-low permeability.