Velocity contrasts

Figure 29 shows the resulting changes in velocity. In the center of the pattern, at the steam injector, the velocity decreases by up to 800 m/s after 19 months of steam injection. Assuming an interval velocity of 2.0 km/s for the reservoir, this velocity contrast works out to be -40%, which is exactly what was predicted by the steamflood model and core data! To the west of the injector, the velocity initially increases by about 400 m/s, which is about +20%. Again, this is exactly what was predicted for the pressure front in the absence of heat. At 13 and 19 months (monitors 4 and 5), the pressure front loses some of its velocity contrast, probably due to heating from the expanding steam zone. On the east side, a 100-200 m/s velocity increase (+5-10%) is consistent with an annulus of hot water. Beyond that, the reservoir changes very little until the fifth survey where a velocity decrease of 200 m/s (-10%) seems to indicate heat has just arrived.

 

res0t
Figure 21 Traveltime picks before steam injection

 

res1t
Figure 22 Traveltime picks after 2 months of steam

 

res2t
Figure 23 Traveltime picks after 5 months of steam

 

res3t
Figure 24 Traveltime picks after 9 months of steam

 

res4t
Figure 25 Traveltime picks after 13 months of steam

 

res5t
Figure 26 Traveltime picks after 19 months of steam

 

tcurves-ann
Figure 27 Traveltime picks along the base of the P formation from all six time-lapse seismic images.

 

dtcurves-ann
Figure 28 Traveltime changes along the base of the P formation from all six time-lapse seismic images.

 

dvcurves-ann
Figure 29 Velocity changes along the base of the P formation from all six time-lapse seismic images.