Discussion on the Method of Evaluating Oil and Water Layers by Using Ratio of Deep Resistivity and Flush Zone Resistivity

The depth-shallow resistivity ratio method for the same measuring point is an important method to identify oil and water reservoirs, But its application effect in conventional logging data is not good. By studying the invasion characteristics of resistivity curves of reservoirs, the ratio of deep resistivity and flush zone resistivity at the same measuring point is used to evaluate oil and water layers with a good effect. Further research shows that the ratio of deep resistivity to flushing zone resistivity can reflect the oil-bearing property of reservoir. The fact that the ratio of deep resistivity and flush zone resistivity of water layers in the same well is basically a constant supports this view, which can be deduced from Archie's formula. That is, the ratio of deep resistivity to flushing zone resistivity is Rw/Rmf, which is the ratio of formation water resistivity to mud filtrate resistivity. On this basis, a method of evaluating oil and water reservoir by using the ratio of deep resistivity and flush zone resistivity is proposed. One is logging crossplot evaluation method, the other is apparent water saturation calculation and evaluation method. These two methods are not affected by the changes of lithology and resistivity of the reservoir, and do not need data such as porosity, rock-electro parameters and formation water resistivity, which can quickly identify oil and water layers. As the ratio between the deep resistivity and the resistivity of the flush zone is not related to the resistivity of the reservoir, it can be used to evaluate the low resistivity oil reservoir. The results show that it is effective to identify low resistivity reservoirs and it is a reliable method to evaluate low resistivity reservoirs.