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Logging times are calculated by the Logging Staff Scientist prior to her initial contact with the Co-Chief Scientists and are revised, if necessary, before the pre-cruise meeting. At the meeting, the Logging Staff Scientist will discuss the logging plan and logging time estimates with the Co-Chiefs, TAMU Staff Scientist and other cruise participants as part of the leg planning process. The logging time estimates, together with time estimates for drilling, transit and other operations, are commonly published in the leg scientific prospectus, which is distributed among the leg participants and is also available on the prospectus page of the Science Operator web site. During the cruise, the Logging Staff Scientist will continuously monitor and modify logging time estimates. Drilling and logging plans are commonly altered during the course of the leg in order to accommodate changes in the original plan due to unforeseen hole conditions or the dynamics of the scientific drilling process. The purpose of this document is to provide shipboard scientists with the basic information the Logging Staff Scientist incorporates in the course of preparing logging time estimates.
Logging time depends on several variables, such as water depth, length of logged interval, logging speed, and type and number of tool strings used. Logging speed is an important variable because it may affect logging data quality. Slower logging speeds usually result in better counting statistics for nuclear tools (such as the NGT, AACT and GST). Faster logging speeds on the other hand usually lead to less tool sticking that adversely affects all logs. Table 2.1, shown immediately below, provides some guidelines regarding the logging speeds commonly used for logging on the JOIDES Resolution. High-resolution logs (neutron porosity and bulk density data sampled every 2 and 1 inches respectively) can only be obtained by logging at the lower logging speeds (typically 900 ft/hr or less). The currently available logging winch onboard the JOIDES Resolution (as of January 1995) is capable of stable minimum logging speeds of 600 ft/hr. However, even at speeds of a few hundred feet per hour, the effect of only partially compensated ship's heave achieved by the WHC system may lead to the tool string undergoing significant oscillatory motion in the hole during logging.
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TABLE 2.1 -- Typical Logging Speeds
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Log Quality:
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Fair
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Good
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Excellent
(Hi-Res)
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Tool
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m/hr
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ft/hr
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m/hr
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ft/hr
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m/hr
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ft/hr
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Triple
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549
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1800
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335
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1100
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274
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900
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GLT
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183
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600
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137
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450
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91
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300
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FMS/Sonic
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549
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1800
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488
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1600
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274
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900
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GHMT
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549
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1800
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488
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1600
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274
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900
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A spreadsheet (logtime.xls) has been created to facilitate standardized estimation of logging times for all loggers and logging proponents. The spreadsheet provides the means for calculating times for both standard and specialty tools. Some borehole experiments with specialty tools, such as vertical seismic profiling, case inspection logs, "CORKS" and others, may require consultation with the scientists involved in order to accurately estimate the times.
Invariably, real-time decisions have to be made in response to changes in conditions uphole and downhole; thus, knowing how the condition changes affect the estimates is important. In considering the time required for operations it is important to note that the Operations Superintendent assumes that hole preparation time, including mud circulation, wiper trip, bit release and pulling pipe to logging depth, is not included in the logger's estimate. In this spreadsheet, note that Logging Time is counted as the time between the positioning of the base of the bottom-hole assembly at the logging depth and the time when tools and wireline are rigged down and the pipe can begin to be pulled out of the hole.
The spreadsheet has been continually updated. It allows for easy calculation of the logging times for an individual hole, as well as for input of different logging speeds, retrieval speeds and the addition of repeat logging runs. The input variables are outlined in Table 2.2. The user simply inputs the values for water depth, pipe depth, hole depth, tool strings run and their respective logging speeds, whether the conical side entry sub (CSES) is used or not, whether the bit is released or not (bit is released only when RCB is used in drilling) and the tool retrieve speed in pipe, and the total time is automatically calculated. (See Table 2.2, immediately below.) All the variable inputs to the spreadsheet are separated from the detailed outline of logging operations. This detailed outline can be tailored for or changed according to the specifics of an individual leg, or from experience of actual logging times as a leg progresses.
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TABLE 2.2 -- Variable Input
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HOLE: SCS-8
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Logs
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Tool
Deployed?
(Yes/No)
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Speed
(m/hr)
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Repeat?
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Rep.
Speed
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Rep.
Interval
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Triple Combo
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Yes
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488
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Yes
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488
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100
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Variables
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(in meters)
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FMS/Sonic
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Yes
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250
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Yes
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250
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250
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Water Depth:
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1800
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GHMT
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Yes
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500
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Yes
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500
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100
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Pipe Depth:
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150
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BHTV (UBI)
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No
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250
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Yes
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250
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100
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Hole Depth:
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400
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GLT
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Yes
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183
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Yes
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183
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100
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Open Hole Int. Logged:
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250
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LDEO TLT
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Yes
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180
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No
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N/A
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N/A
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Pipe Length:
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1950
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RSMAS
Hi Temp Tool
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Yes
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180
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No
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N/A
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N/A
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Station Interval
(meters)
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Time/Station
(min)
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WST
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Yes
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50
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20
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Can CSES be used?
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OK to use CSES
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with SES (hrs)
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without SES (hrs)
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Total Time:
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55.5
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47.3
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Prior to Leg 112, the major uncertainty in logging time estimates involved delays associated with bridges. A bridge is a constricted-hole interval that the logging tool may not be able to get past when it is on its way down through open hole. Nearly all ODP bridges are found in sedimentary sequences and are caused by clay swelling after drilling. With the routine use of sepiolite muds, the clay swelling problems have significantly diminished. Bridges can also form in heavily fractured formations, but these types of situations have been much rarer.
Deep basalt holes rarely exhibit bridging. Bridging is very difficult to predict before a leg begins. Even after drilling and before logging, the likelihood of bridges cannot always be estimated reliably. The drilling engineers and operations superintendent usually have a good "feel" for hole conditions prior to logging from their observations during the wiper trip.
If a bridge is encountered that stops the logging tool, there are two options: 1) simply log the interval above the bridge and cancel plans to log beneath the bridge; or 2) pull the logging tool out of the hole and up onto the ship, set pipe through the bridge, then lower the logging tool again. Nearly always, the much heavier drill pipe can punch through bridges that had stopped the lighter logging tool. This second option requires about 3-4 hours for each bridge.
To prevent lost time or lost logs associated with bridges, the JOIDES Resolution has the capability of using a conical sidewall entry sub (CSES) during logging. When inserted into the drill string, the CSES allows for the addition or removal of drill pipe while a logging tool is downhole. The CSES strategy is to lower pipe to near the bottom of the hole, lower the logging tool into open hole just beneath the pipe, then log up while simultaneously pulling pipe at the same speed. In this way open hole logs are obtained by minimizing the time between pipe removal and logging so substantial bridges cannot develop. Even though the use of the CSES is not ordinarily planned for, the Logging Scientist will estimate logging which incorporates its use. If the CSES is planned for but not needed, logging operations will take 4-12 hours less than planned at a site.
- The spreadsheet calculations assume that the entire interval below the pipe depth is logged for all toolstring combinations.
- Logging rates as specified are constant for the duration of logging.
- Time estimation is not required for the rig up and rig down of the TAP tool. Actual rig time required for this tool is negligible (approximately 5 min).
- "Fixed" time estimates for operations such as retrieval speeds (while tool is in drill pipe), tool rig-up and rig-down may vary from leg to leg.
The logging times provided in the spreadsheet do not include three contingencies that commonly occur:
- Time required to punch through bridges or change to the sidewall entry sub if one starts logging without the CSES.
- For reentry holes in which it is not permissible to drop the bit at the bottom of the hole, time to pull the drillstring, take off the bit, and reenter the hole.
- Time beyond 1 hour to drop the bit, due to problems with the bit release tool or cable breakdown, which occurs at about 10% of sites and requires 1-3 hours extra to deploy a backup tool or cut off faulty cable.
"Safety" margins have not been included but can be by increasing the fixed time necessary for certain operations (e.g., item 3 above), or by decreasing logging speeds.
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