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Gas blowout after abandonment
Production logging tools can provide important information for assessing a gas blowout after abandonment.
A well was drilled through two gas zones on the way to test a deeper oil zone. The well was abandoned and the wellhead cut at the seafloor.
Six months after abandonment, there was a gas blowout to the surface, causing the sea to churn. A relief well was drilled in order to flood the blowing zone. A magnetometer survey (Fig. 4a) shows the distance of the relief well from the original wellbore. The separation of the two wellbores "corkscrews" between approximately 5 and 25 ft, an effect caused by changes in the direction of the original wellbore. The two gas-zone locations are marked on the figure.
A noise log (Fig. 4b) was run in the relief well to identify which of the two gas zones was the source of the gas blowout. The noise log is applicable for this purpose because of its ability to "listen" to the sound in the original wellbore.
Analysis of data
The dead-well noise levels in the interval of the relief well at the location of Gas Zone (2) shows it is not the source of the blowout. The noise-log response is elevated above the dead-well level in an interval of the relief well corresponding to the location of Gas Zone (1) showing that this gas zone is the source of the blowout. Above this location, the noise log exhibits several peak responses, two of which are larger than the response at the source of the gas blowout. The location of the largest corresponds to the location of least separation above Zone (1) in Fig. 4a. The other spikes are likely at locations of tight spots in the flow path.
It is also possible to estimate the distance between a blowing well and a relief well a point source of noise in the blowing well. This is done from the rate at which the noise level attenuates with distance along the relief wellbore from the peak noise location, and is possible because the noise level is inversely proportional to distance from the source. For example, the 200-Hz noise level at the location of zone (1) in Fig. 4b is 17 millivolts. By 3,500 ft, a distance of 140 ft in the relief well, this level has dropped to 2.0 millivolts. The separation between the wells at zone (1) is therefore estimated by the expression:
This value compares to the 13–14 distance indicated by the magnetometer survey. Likewise, the separation at the large peak at 3,150 ft is estimated from the decay at a value of 6.5 ft, which is the same distance as that shown on the magnetometer survey.
In this example, only two production logs were used. In considering this point, it is important to remember that the logging objective is quite limited: to determine which of the two known gas zones is the source of the blowout. Given that the logging is in the relief well, the noise log is better suited to this purpose than any other production log.
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