Initial water injection model and simulation results

Figure 3.23 Projection of anomalous bodies of Model 3.3.2 on the surface.

anomalous potentials do appear in three regions surrounded by dotted lines in Figure 3.24. The fact is that oil distribution and water distribution own distinct resistivity respectively and consequently their capabilities of attracting electrical currents are different from each other. Figure 3.24 reveals contrasting results, the area of oil projection exhibits negative anomalies, and on the contrary, the area of water projection shows a positive anomaly. In order to make the abnormal phenomena more obvious, the operator of the second derivative is employed to enhance resolution.

Figure 3.25 is the processed result by the operator of the second derivative. From Figure 3.25, we can see that the resolution is improved a lot by the process, however in the meantime it brings some false anomalies. To address the false anomalies, we perform an integrated interpretation by overlapping two kinds of anomalies, and the area covered by two types’ anomalies at the same time indicates the real information.

Figure 3.26 shows the processed result using the method of integrated interpretation.

Figure 3.24 Surface electric potentials of Model 3.3.1 (unit: μV).

Figure 3.25 The second derivative of surface potentials of Model 3.3.1 (unit: nV/m²) in Y-direction.

Figure 3.26 The result of integrated interpretation. The false information is filtered from Figure 3.25 by overlapping original electric potential (Figure 3.24).

With the purpose of studying the vertical distribution of anomalies, Slice 1 and Slice 2 are cut from the Model 3.3.1, and their relationships are described in Figure 3.21. The modeling result of Slice 1 is exhibited in Figure 3.27, and it indicates that anomalous electric potentials do appear in the corresponding areas. Compared with the consequence of Slice 1, the result of Slice 2 is not ideal enough. However, the general outline of anomalies can still be ascertained in Figure 3.28. In addition, it is middle of Figure 3.28 are considered to be caused by the source of current. However the anomalies can be judged as anomalous bodies or the source effects, which can be determined by the extremums of surface electric potential. From the consequences of two slices we can summarize that the negative potentials (as shown in Figure 3.27) are corresponding to oil distribution, while the positive potentials (as shown in Figure 3.28) is responsive to water distribution. As a matter of fact, this information is quite beneficial for us to identify oil and water distributions and also useful for further inversion study.

Figure 3.27 Modeling result of Slice 1 (unit: V).

Figure 3.28 Modeling result of Slice 2 (unit: V).

In spite of that, the vertical boundaries of anomalous bodies still cannot be decided from above results. Hence the cases of cross-well electric potential will be taken into consideration. As already expressed in Figure 3.20, Well-1, Well-2 and Well-3 are noted as receivers to be further studied for determining the thickness and positions of the layers at vertical direction. Figure 3.29 clarifies the positioning relationship among Well-1, Well-S and the oil layer. With the purpose of obtaining a high vertical resolution, the operator of the first derivative is employed to process the electric potentials of Well-1, and the results are shown in Figure 3.30. In consequence, the result provides precisely enough vertical resolutions which can help us recognize the oil layer from the formation. Considering the symmetry between Well-1 and Well-2, we can deduce that Well-2 and Well-1 will have the same results.

Figure 3.29 Diagram of cross-well

existing between Well-1 and Well-S.

Figure 3.30 The first derivative of electric potentials of Well-1.

Figure 3.31 helps to describe the situation where the well passes through the water layer. In a similar way, the electric potential has been processed by the operator of the first derivative. In this case, the consequence can assist us to determine the vertical position, and moreover the lower and upper bounds of water layer can also be confirmed, as explained in Figure 3.32.

Figure 3.31 Diagram of cross-well

existing between Well-3 and Well-S.

Figure 3.32 The first derivative of electric potentials of Well-3.

Based on the above results of Model 3.3.1, the distribution of water and oil at the horizontal direction can be located by combined information between surface electric potential and its second derivative as shown in Figure 3.24 and Figure 3.25. In addition, the depth of oil and water layers can be gained with the assistance of useful information offered by cross-well electric potential technique as shown in Figure 3.30 and Figure 3.32. Hence the spatial distribution of oil and water can be ascertained by combining the beneficial information provided by borehole-surface electric potential and cross-well electric potential, as demonstrated in Figure 3.33.

Figure 3.33 Spatial distribution of oil and water in the early period of oilfield development.

According to above analysis, we can conclude the following viewpoints. It is manifest that we can determine the horizontal location of underground distributions of oil and water through analyzing the surface electric potentials which can be acquired by borehole-surface electric potential technique. Furthermore, the application of mathematical algorithm such as the operator of the second derivative is beneficial for detecting boundaries of oil and water. Moreover, with the help of electric potentials obtained by the cross-well method, vertical depth can be identified. Therefore, it is possible for the whole spatial position of distribution of oil and water to be decided by combining borehole-surface electric potential and cross-well electric potential together.

The superimposition between electric potential and its second derivative can remove the noise brought during the period of improving high resolution. In the meanwhile, we can obtain an obvious result. In the vertical direction, the boundaries of oil or water layers can be displayed clearly.