Measurements of Streaming Potential for Downhole Monitoring in Intelligent Wells
Figure 1: Vertical Cross-section Through a 3D Reservoir Model Showing Water Saturation (A and B) and Streaming Potential (C)
A -500 -300 -200
Distance from production well (m) -100
0 100
520
-600 B -500
560
C
-600 5
4 3
The legend shows the distance of the waterfront from the well. As the waterfront approaches, the streaming potential increases at the well, with the peak potential located at the centre of the reservoir layer. This increase is caused by the approaching waterfront, as shown in Figure 1.
2 1 0 -300 231 days -200 -100
Distance from production well (m) 347 days
463 days 0 100
5
602 days
A: Water saturation after 231 days; B: Water saturation after 463 days as water (white) encroaches on a vertical oil (black) production well located at 0m on the horizontal axis. C: Streaming potential versus distance from the production well along a 1D horizontal profile through the centre of the model, at four different time-points. The peak of the potential curve is located at the position of the advancing waterfront, but the envelope of the curve encompasses the production well when the front is several tens to hundreds of metres away. This is why streaming potential measurements can detect advancing waterfronts while they are still some distance from the well.
In this model, the maximum potential measured at the well begins to change sharply while the waterfront is approximately 100m away from the well (see Figure 3). This suggests that waterfronts may be detected while they are some distance away. The length-scale over which the electrical signal decays with distance from the front depends on the electrical conductivity of the reservoir and confining layers, while the magnitude of the peak signal depends principally on the pressure drawdown into the well and the value of the streaming potential coupling coefficient. This is a key petrophysical property, the magnitude of which depends largely on the salinity of the injected and formation brine. In the examples shown, the brine has the same salinity as seawater. Streaming potential signals will be largest in low-permeability reservoirs produced at high rate, saturated with brine of low salinity.
Imaging an Encroaching Waterfront
Although it is instructive to investigate the behaviour of a flat waterfront in a homogenous model, in real production scenarios the waterfront will not be flat because of reservoir heterogeneity or uneven drawdown along the well. The effect of heterogeneity can be simulated by introducing a high-permeability layer in the lower section of the reservoir model. The encroaching water exploits this layer, preferentially flowing through it towards the production well (see Figure 4). The streaming potential field is affected by the presence of the layer. This is
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
4 3 2 1 0 300 400 500 600 Time (days)
A significant increase in potential that is larger than the estimated noise level of the downhole electrodes (<0.1mV) is observed when the waterfront is still 100m away.
partly because it changes the movement of water through the reservoir and so the current source associated with the waterfront, where the divergence of the streaming current is non-zero, is located in a different place. It is also partly due to saturation changes at the boundaries of the layer that act as current sources, because the divergence of the streaming current is also non-zero at these boundaries. In the absence of permeability heterogeneity, the streaming potential measured at the well has a maximum at the centre of the reservoir and decreases towards its top and base (see Figure 4). When the lower section of the reservoir has higher permeability, the streaming-potential field is different. When the waterfront is far from the well, the maximum potential is associated with the intersection between the front and upper boundary of the high-permeability layer (see Figure 5). When the waterfront has reached the well, the maximum potential is located
45
700 800 900 200 150 80
Figure 3: Maximum Streaming Potential Measured at the Well versus Time and Distance of the Waterfront from the Well
Distance (m) 0
580 600 0 0.5 1.0
100m – 463 days 20m – 579 days
1.5 2.0 Streaming potential (mV)
55m – 520 days 10m – 620 days
2.5 3.0 540
Figure 2: Streaming Potential Measured Along the Length of the Production Well Shown in Figure 1
500
Streaming potential (mV)
Depth (m)
Depth (m)
Streaming potential (mV)
Depth (m)
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