Logging System
General Description
Modern Logging Tools
The actual running of a log involves the tool on the end of the logging cable, the cable itself, and the controlling and recording apparatus on the ground surface. Before discussing downhole tools, however, the common elements of all logs will be presented. Figure 1 illustrates the basic components of any logging system. A sensor, incorporated in a downhole measurement instrument called a sonde, together with its associated electronics, is suspended in the hole by a multiconductor cable. The sensor is separated from virgin formation by a portion of the mud column, by mud cake, and, more often than not, by an invaded zone in the surrounding rock. The signals from the sensor are conditioned by the electronics for transmission up the cable to the control panel, which in turn conditions the signals for the recorder. As the cable is raised or lowered, it
activates a depth-measuring device-a sheave wheel, for example-which in turn activates a recording device-either an optical camera (making a film) or a tape deck (making a digital recording on magnetic tape). The film (or tape) is reproduced to provide a hard copy of the recorded data.
In general, well-logging jargon distinguishes between a logging survey, a logging tool, and a log, as well as a curve. There is frequently some confusion about these terms when logging matters are discussed. A logging survey is provided by a logging service company for a client. During the course of the survey, the logger may employ several different logging tools, and record several different logs, on each of which are presented several different curves. The logging tools, in turn, consist of multiple sensors. Figure 2 illustrates these terms and their interrelationship.
Typical Logging Setup
Rigging Up to Run a Log
Figure 1 shows a typical setup for a logging job. A logging truck is anchored about 100 to 200 ft from the well. Two sheave wheels are mounted in the derrick, with one suspended from the crown block and the other chained down near the rotary table. The logging cable from the truck winch is then passed over the sheave wheels, attached to the logging tool string, and lowered into the hole. A more detailed diagram of this hookup is shown in Figure 2 .
Two mechanical details regarding this method of rigging up are worth noting. Between the top sheave wheel and the elevators a tension device measures strain on the logging cable and displays it in the logging truck ( Figure 3 ). The tension on the elevators is twice that on the cable. The elevators should be securely locked and the traveling block braked and chained.
The tie-down chain for the lower sheave is also of great importance. If it breaks or comes untied, the snap will probably break the cable and catapult the sheave wheel several hundred feet ( Figure 4 ).
Logging Unit
Logging Trucks
Logging service companies offer a variety of logging units, each of which has the following components:
logging cable
winch to raise and lower the cable in the well
self-contained 120-volt AC generator
set of surface control panels
set of downhole tools (sondes and cartridges)
recording mechanism (tape and/or film)
Figure 1 shows a cutaway of a typical logging truck. Land units are mounted on a specially adapted chassis reinforced to bear the load of a full winch of cable (up to 30,000 ft long). The instrument and recorder cabs are usually cramped, noisy, too hot or too cold, and sometimes filled with ammonia fumes from an ozalid copier.
Offshore units are mounted on skids and bolted (or welded) to the deck of the drilling barge, vessel, or platform.
Other units can be disassembled into many small fragments and flown into remote jungles suspended under helicopters. However, all logging units are basically similar, and require good mechanical maintenance to avoid problems during logging operations.
Logging Cables
Modern logging cables are of two types: monoconductor and multiconductor. Monoconductor cables, with a diameter of 1/4 in., are used for completion services, such as shooting perforating guns, or setting wireline packers and plugs, and for production logging surveys, such as flowmeters and temperature logs in producing wells. Multiconductor cables, with a diameter of about 1/2 in, are used by most logging service companies for recording openhole surveys. The multiconductor cables contain 6 or 7 individual insulated conductors in the core.
The outer sheath is composed of two counterwound layers of steel wire. Such a cable has a breaking strength of between 14,000 and 18,000 lb and weighs between 300 and 400 lb per 1000 ft. It is quite "elastic" and has a stretch coefficient of around I x l0-6 ft/1b.
The "Head" and the "Weakpoint"
The cable ends at the logging "head." The head anchors the cable and attaches to the logging tool by a threaded ring. Thus, the head provides both the electrical connection between the individual cable conductors and the various pins in the top of the tool and the mechanical connection. Built into the head is a "weakpoint," a short length of aircraft cable designed to break at a given tension (usually about 6000 lb, but deep-hole weakpoints are designed to break at lower tension, e.g., 3500 lb). The weak-point provides a means to free the cable from the tool when it becomes irrevocably stuck in the wellbore. Several examples follow.
Computerized Logging Units
Available Systems
Major service companies now offer logging services from computerized logging units. The advantages of using these units are many and their use is encouraged.
Features of Computerized Units
In contrast to conventional logging units, computer-based units offer the following features:
All logs are directly recorded on digital magnetic tape or onto a hard disk.
Computer control of the data gathering allows logs to be recorded either logging up or down with all curves mutually on depth.
Calibrations are performed under programmed control more quickly and accurately than in conventional units.
Logs can be played back from the data tapes on many different scales (both depth and response scales).
Wellsite computation of raw data ranges from completion aids (hole volume integration for cement volumes) to dipmeter computations and complete log analysis.
Figure 1 is a schematic of a computerized logging system. The logging engineer accesses the system by keyboard. At his command, the computer loads programs to perform such functions as calibration, logging, computation, and playback.
Calibration Methods and Tolerances
Conventional logging units require human operation of both sensitivity and zero offset control. Figure 2 depicts a typical conventional calibration system.
The variable offset resistor is adjusted when the logging sensor is at the low end of its range of measurement (for example, the caliper tool in a 6-in. ring), and the variable gain resistor is adjusted when the sensor is at the high end (e.g., the caliper tool in a 12-in. ring).
The computer units eliminate the need for human intervention, other than to place the tool to be calibrated in the correct environment (e.g., putting the 6-in. ring over the caliper arms). The data-gathering system accepts the raw uncalibrated readings of the tool and computes a calibration equation to transform raw uncalibrated data into calibrated data. Figure 3 illustrates this concept.
The important things to check include the agreement between all three numbers with the specified tolerances listed in Figure 4 . Note that these sets of numbers refer to Schlumberger logs. Other service company tools use different numbers. Booklets explaining calibration techniques by each logging service company can be obtained from their sales personnel.
The tolerance table of Figure 4 shows that the near count rates are allowed a variation of ±22 cps and the far count rates a variation of ±14 cps. Thus, the wellsite calibration in this case can be considered good.
Exercise 1.
Read the difference in count rates between the Before and After survey calibrations from Figure 1 for both the near and far count rates.
a. Are they within allowable tolerances?
b. What effect will the drift have in terms of changes in the logged parameter, bulk density (B)?
Solution 1:
a.
| Before | After | Change | Tolerance |
FFDC | 335 | 337 | +2 | ±14 |
NFDC | 526 | 529 | +3 | ±22 |
b.
-2.118 kg/m3, or
-2.118 l0-3 gm/cc
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