Production logs to assess gas kick
Use of several types of production logs in combination can provide important information, often quite cost effectively, for diagnosing a gas kick encountered during drilling. An example is discussed below.
Gas kick while drilling
In this development well, 10 3/4-in. pipe is set to 3,500 ft, and 7 5/8-in. casing is set to slightly below 14,000 ft. Behind the 7 5/8-in. casing, the top of the cement is just below 6,000 ft. During the coring of a gas sand at 15,000 ft, a pressure kick occurred, gas pressure was then lost at the surface, and mud was added periodically to keep the drillpipe full.
Assessing the issue
On the day after the gas kick, noise and temperature logs were recorded during the same run inside the drillpipe with the well static. These logs were run to identify the flow path of a likely underground blowout. The temperature log is basic to understanding a well’s behavior, and the noise log is particularly responsive to the movement of gas through liquid.
At 15,000 ft, the noise log (Fig. 1a) exhibits a nearly "dead-well" noise level, indicating no activity (i.e., no fluid movement). Above 15,000 ft, however, the noise log departs from dead-well response. This departure shows that the gas, which enters the wellbore just above 15,000 ft, moves uphole. Above 14,000 ft, the noise log decreases very rapidly to dead-well response, showing that the gas moving uphole enters a formation at 14,000 ft, approximately 100 ft above the 7 5/8-in. casing shoe. The log thus shows a crossflow of gas (single-phase) from approximately 14,800 to 14,000 ft. The absence of gas pressure at the surface on the annulus between the drillpipe and the 7.625-in. casing means that the annulus is plugged by a bridge at some unknown depth above 14,000 ft.
A temperature log is quite sensitive to liquid flow and will reveal details regarding the flow path of any mud returns, which should move uphole in the annulus between the drillpipe and the 7 5/8-in. casing, owing to the periodic addition of mud to the drillpipe at the surface. Each time mud is added at the surface, some hotter mud is pushed upward in the annulus. For a given length of wellbore, the volume of hotter mud in the annulus greatly exceeds the volume of cooler mud in the drillpipe. During the time between additions, the two volumes equate in temperature to a value warmer than static. The periodic addition of mud at the surface should therefore produce what appears to be a "production" profile on a temperature survey. The temperature log in Fig. 1b does indeed show such a profile below Depth C. From this depth downward to 14,000 ft, the temperature is warmer than static, indicating displacement of annular mud upward to Depth C. This depth happens to be the location of a zone that was fractured and used during drilling for disposal of sour mud from the mud pit. The log therefore shows the movement of mud from deep in the well upward and into the disposal zone at Depth C. Above Depth C, the temperature trace lies to the cool side of static, reflecting the addition of cool mud to the drillpipe without displacement of warmer mud upward in the annulus.
For this flow path to exist, the 7.625-in. casing must also have failed at some depth. The temperature trace shows the location of this failure at Depth A just below 5,000 ft. Above this depth, the displaced mud moves behind the casing and loses heat to the formation at an increased rate. The corresponding mud temperature, although still warmer than static, is not as warm as the mud still in the annulus below Depth A. A mud path is therefore established as shown by the dashed line on the wellbore schematic on the left side of Fig. 1b. The only remaining task is the location of the depth of the failure in the drillpipe.
To investigate the mechanical integrity of the drillpipe, a radioactive tracer log was run, with the tracer tool stationary in the drillpipe at various depths while mud was pumped into the drillpipe continuously. The tracer traveled down the drillpipe with the mud flow, and the time lapse between its ejection and its subsequent detection by a detector below the ejector was measured. Time-lapse measurements made at various depths indicated a significant mud leak from the drillpipe in the interval between 12,800 and 12,900 ft. As further confirmation of the mud leak, tracer was ejected at 12,800 ft, and the tracer tool was quickly repositioned to 12,500 ft. While the logging tool was at 12,500 ft, the detector responded to the ejected tracer as it moved upward in the drillpipe-casing annulus.
Correcting the problem
As a result of this survey, one can further conclude that the annulus is plugged to gas flow at some depth below the leaking joint of drillpipe at approximately 12,800 ft. The location of this bridge was determined from a free-point survey, allowing a joint of split drillpipe to be replaced. The underground gas crossflow was then "killed" with mud.
Once the underground blowout was eliminated, the drillpipe was removed and the 7 5/8-in. casing string was examined with a casing collar locator to complete the investigation of mechanical integrity. The collar-locator log showed a 4-ft vertical separation in the casing string at Depth A because of a casing joint that was unscrewed from the collar above.
In this example, a careful analysis of a suite of inexpensive production logs yields definitive information of a diagnostic nature. Please note that the first suggestion of mechanical integrity problems resulted from a thorough examination of the temperature profile. Because the temperature profile was analyzed during the logging, the tracer and collar-locator logs were run while at the wellsite, avoiding the additional setup, pressure, and depth charges that would occur if the service company were to return for the tracer and collar-locator logs at a later date.
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