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Friday Haggler

A reminder of last Friday’s Haggler

The Haggler Generator was shut down for essential maintenance, now it’s back up and running!

On a bit trip in a high angle 12-1/4” hole section the BHA was lubricated out of hole (pumping with low flow and no rotation) to mitigate swab as the drill team were aware that they had a narrow margin between collapse and losses. The trip out was trouble-free, however the trip back in hole with the new bit was not. The BHA hung up at several points and cavings were seen over the shakers when attempting to clean up. The team are slightly baffled as the MW should be adequate to maintain a stable wellbore, the OBM appears well maintained, there were no signs of cavings while drilling and the team are confident that the hole was not swabbed below stability MW on the trip out. What could be causing the problem? See a shale sample below that has been exposed to the drilling fluid at downhole pressure and temperature in a lab test, some cavings pictures and drilling fluids parameters:

The Answer

Long Answer:

The reducing WPS and OWR indicate that formation fluid is entering the wellbore due to a high osmotic gradient.

Figure 1: Time Based Drilling Fluid Property Charts

This is driven by the high drilling fluid activity, which is significantly underbalance to the formation fluid activity, resulting in dehydration cracking in the troublesome shale interval. See shale sample tests below.

Figure 2: Water Phase Salinity Effect on Dehydration Fractures

The assumption is that these dehydration fractures make the shale more susceptible to borehole instability, due to the MW no longer providing adequate radial pressure to prevent compressive shear failure in the weakened formation and potentially due to fluid invasion into the fractures reducing the near wellbore overbalance required to maintain stability. This explains the apparent time dependent instability; more trouble on the trip in hole compared to when pulling out of hole and first cavings in returns observed when back on bottom after the round trip. This baffled the drill team, as time dependent instability is not normally expected when drilling with a well maintained SBM.

Figure 3: Cavings Observed in Shaker Returns

The cavings shown are predominantly blocky in nature, evidence of existing planes of weakness in the formation, however it is difficult to draw a conclusion from one picture of a sample of cavings, as it is not necessarily representative of all the cavings and there is always a high degree of uncertainty about what remains downhole or what has not been identified on the shakers. Although we can conclude that at least some intervals of shale are unstable.

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