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At Sea Report from the Bismarck Sea: The hole is done and we are moving along
December 22nd, 2000
The saga of Hole 1188F came to an end when we reached a depth of 387 mbsf. Drilling conditions became more difficult as the core recovery was decreasing therefore we decided to stop drilling operations and begin to deploy the logging tools. It was time for us to do some work.
We deployed four tool strings in Hole 1188F and the results were interesting to say the least. As I mentioned, drilling operations achieved a total depth (TD) of 386.7 mbsf in Hole 1188F and all wireline tool deployment reached a logging TD of 356 mbsf. An obstruction was encountered 30 m above the hole's TD and all attempts to get past this were unsuccessful. Prior to the beginning of logging operations, a borehole restriction was encountered at 362 mbsf during a wiper trip, and although this spot is 6 m deeper than the logging TD, the hole problems were probably related.
The hole size was also unexpected as we were drilling with a 7 1/4 inch bit but the caliper measurements that we obtained with our logging tools (these measurements are made with either one or two mechanical arms that open up and gauge the size of the borehole) were giving reading in excess of 17 inches. These results were basically telling us that the drilling process was washing out the formation as water was being pumped into the hole. This has severe consequences in the log data quality because the farther the tools are from the borehole walls, the worse the data are going to be. However, there was not much that we could do about it so we continued.
One interesting result was that the temperature at the bottom of the hole was increasing every time we deployed a tool string. The temperature increased from approximately 65º C to 100º C in less than 24 hours indicating that there was a significant amount of thermal rebound in the system. One interesting aspect was that the temperatures were relatively cold in the top of the hole, but they increased rapidly in the bottommost 100 m.
After finishing logging operations in Hole 1188F we moved to a previously drilled hole (Hole 1188B) to make a temperature measurement and see how hot the conditions were in this hole. This hole had been drilled 26 days earlier to a depth of 72 mbsf. We wanted to see how the temperatures had equilibrated after a long period of time. Shortly after re-entering Hole 1188B, the bottom of the drill pipe encountered a restriction at 3-mbsf and we could not advance any further. Nevertheless, we decided to lower the temperature probe and the tool penetrated to a logging TD of 7 mbsf. The tool recorded a maximum temperature of 4.8º C during a 15-minute stationary measurement, which was consistent with the shallow temperatures in the nearby Hole 1188F. We also deployed a water sampling tool to collect fluid samples for chemistry and microbiology work. After all these tool deployments, 26 hours had gone by and we were ready for bed.
More next week.
Gerry
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