Logging Operations

We may divide cased-hole logging operations into two groups: those in which the tools are run through tubing and those in which they are run in casing. Throughtubing tools include those designed to evaluate flow conditions downhole, along with certain nuclear tools. Most other tools are larger in diameter and are used before placing tubing into the well (such as for cement-bond surveys), or after pulling the existing string.

Logging Operation

1. Logging truck
2. Mast truck—The mast hydraulically folds up and down for easy transport to and from location. The mast may be part of the logging truck, especially when logging pumping wells.
3. Wellhead with valves on top
4. Lubricator, or riser pipe—The logging tool is placed in the lubricator; the pressure in the lubricator is equalized to that of the wellhead pressure, the wellhead valves are opened, and the logging tool is lowered into the well. At the completion of logging, the tool is raised into the lubricator; the wellhead valves are closed, and pressure is bled down before the tool is removed. Note that a number of riser pipe sections may be connected to accommodate longer tool strings.[post_ad]
5. Cable—This is usually a single-conductor armored cable (monocable). This cable is wound onto the winch of the logging truck for storage.
6. Pressure bleed-off hose to relieve pressure from the lubricator after the job
7. Grease line to maintain the grease seal
8. Grease pump and reservoir for the grease seal
9. Grease seal—Grease is injected into the small annulus between the cable and the seal tubes to effect a pressure seal around the cable.
10. Instrument truck—This unit may or may not be required, depending on the instruments run.
11. Prssure bleed-off hose—This is where pressure is released from the lubricator.
12. Upper and lower sheave wheels—Note the lower sheave wheel chained to the wellhead.
13. Flare line—Gas may be flared, or produced into the flowlines. Liquids may be produced into stock tanks or flowlines.

On through-tubing surveys, it is sometimes preferable (subject to safety considerations) to run the logging tool down through the tubing with the well flowing. This ensures that the interval being evaluated is stable in terms of production and fluid saturation.

Certain small-diameter logging tools may be run in rod-pumped wells. This requires a special wellhead that has an access for a wireline tool to be run in the tubing/casing annulus. The tubing anchor must be removed so that the tool can easily fit into the completed interval. The pump should be placed 50 to 100 ft (15.2 to 30.5 m) above the top producing interval, and the well should be stabilized prior to logging.

Depth control in a cased hole is achieved by running a gamma ray and collar locator, typically at the time of perforating. The cased-hole gamma ray log, which responds to formations’ natural radioactivities is similar to an openhole gamma ray log. Correlation of these two logs enables easy location of the collars with respect to down-hole zones. A short joint of pipe at or near the zone of interest is very helpful in accurate depth control.

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