Publications

A crosswell seismic survey was conducted to characterize a steamed reservoir in the Midway Sunset oilfield in California. The field part of this work was successfully completed in June, 1989. A second experiment, a comparative study of the Chevron research-prototype, borehole vibrator and the commercially available borehole airgun was also done along with the tomography survey.The Production Research Department at Chevron is undertaking an extensive review and evaluation of the effects of injecting foam as a means to control the movement of steam in thermal enhanced oil recovery and thereby improve the sweep efficiency of the steam. During this process it is critical to establish the location of the steam during different phases of the experiment in order to evaluate the effectiveness of the foam injection. Presently the steam movements are determined by temperature monitoring wells. It would be very valuable to determine the steam front location at a given time without being dependent on the location of the observation wells relative to the steam front. We tried crosswell tomography here because it has the potential to determine the steam-liquid interface. The critical parameter, the velocity contrast between liquid and gas-saturated, poorly-consolidated sediments, is well documented.Four tomographic P-wave velocity cross sections were generated in the 68BW steam pattern using an 80 cubicinch, downhole airgun as the seismic source. Three of the sections intersect the steam zone while the fourth does not. The results show low velocities in the sections crossing the steam front, which is consistent with existing models of the steam distribution. While there is an indication that the low velocity zone starts to move upward in the cross section furthest from the steam injector, indicating steam override, there is much less vertical spreading of the steam than what is commonly observed in other areas. This can be explained by the thin layer structure of shales and diatomite observed in cores taken from this steam pattern.The crosswell images also indicate that the steam zone is further west than expected, a result that was confirmed by a temperature survey. The practical result is that crosswell surveys are being considered as replacements for more expensive temperature monitoring wells, at least at Midway Sunset.In the second experiment, a comparison between the vibrator and airgun, it was found that the P-wave signal strength of the vibrator was more than 50 times that of the airgun. The vibrator data also contained reflected Pand Shear-wave energy from. various velocity boundaries within the cross sections. These reflections can add important new information to the tomograms.