Production Logging of Multiphase Flow in Horizontal Wells

Logging Equipment and Techniques Used in Horizontal Wells

This section provides an overview of the equipment and techniques used to convey logging tools to total depth in high-angle wells.

Logging Equipment and Techniques used in Horizontal Wells

Special equipment and techniques have been developed to run logging tools through horizontal or highly deviated boreholes. There are essentially three techniques currently available:

• pump down technique,
• coiled tubing technique, and
• tractor technique

Each of the above techniques can be used to run logging tools in either of two modes:

• with a real-time surface readout of the wireline log, or
• with a memory cartridge.

The real-time surface readout enables the operator to monitor the log at the surface as the sensors pass through the wellbore. Real-time monitoring is desirable because unexpected anomalies often occur, and real-time detection allows the operator to examine such anomalies more closely before pulling the tool out of the hole.

With a memory cartridge, the logging tools are pre-programmed to begin recording downhole at a specified time. Logging passes are made as required for any production logging job. However, the data is not available for analysis until the tool returns to the surface when the data is retrieved from the tool’s memory. Unexpected anomalies or tool failures will not be discovered until after the job is completed. On the other hand, memory equipment is considerably less expensive than surface readout equipment.

Pump-Down Technique

Pump-down equipment typically employs a special pump-down sub that is attached to the bottom of the logging tool. This assembly may employ swab cups to better enable the tool to be pumped down to the end of tubing or drillpipe. In some cases, the annulus between the tool and tubing is sufficiently small to pump the tool down into the hole without using swab cups or a pump-down sub; however, this technique is not commonly used in production logging. The tool must be pulled back into the tubing across the interval of interest. Spinners and other non-nuclear sensors may not provide adequate flow information along the horizontal section of the well. This technique is, however, useful for pulsed neutron formation evaluation carried out through stuck drillpipe, or where tubing has been run to TD.

An example of this application is shown in Figure 1 – Pumpdown logging technique with oxygen activation water flow measurements.

Tubing was run to TD, and a pulsed neutron oxygen activation tool was pumped into the hole to evaluate water injection profiles through horizontal sections of the well. While water was being injected, stationary water flow measurements were made at six locations to detect flow outside of the tubing. The well crosses five major fracture systems, and measurements were taken between fracture systems or near the ends of the horizontal section. The velocities measured are shown in feet per minute on the figure. This test indicates that the velocity of the annular fluid remained constant across the first three fracture systems encountered. Only the last two fractures are taking injected water.

Coiled Tubing

Currently, the coiled tubing unit is the most common method used to run production logging tools in horizontal wells. A coiled tubing operation is shown in Figure 2 – Logging job performed with coiled tubing. This technique uses continuously spooled steel tubing.

The logging tool is mechanically connected to the end of the coiled tubing, and the tool is moved across the horizontal interval as tubing is pushed through the injector head into the well. Measurements may be taken in both the “down” and “up” directions. Certain types of equipment utilize a special kind of coiled tubing that contains an internal electric wireline cable. This cable runs between the tool head and the surface unit to provide log data at the surface in real-time. Coiled tubing units that do not feature the internal electrical wireline cable are used to run production logging tools with memory cartridges. While less expensive than units that feature the internal cable, they are subject to the same shortcomings mentioned earlier in this section.

Downhole Tractors

The newest technique for logging horizontal wells utilizes a downhole tractor. Though tractors exist in a number of configurations, a typical example is shown in Figure 3 – Downhole Tractor for production logging.

This design employs folding arms equipped with drive wheels to propel it through the wellbore. When the arms are extended, the wheels engage the casing wall. The wheels are powered to push or pull the tool downhole. Most tractors are only capable of pushing the tool to TD, and only propel themselves in one direction (though a few now feature reverse capability). This limitation does not usually pose a problem, however, because the tractor is retrieved as the surface wireline unit pulls the tool out of the well. The logs are typically run when the tool is pulled back by the wireline unit. Most wireline equipment cannot record logs while the tractor is powered. While the tractor must be powered from the surface, the logs may be recorded with memory equipment.