Well Logging Tools & Techniques (Production Logs)

Production Logs

Production Logs

Production Logs fit into three categories: profile logging, fluid identification, and temperature logging.

Profile Logging Profile logging may be used to monitor injection rates in injection wells, to monitor production rates in producing wells, or to detect casing, tubing, and/or packer leaks, and channeling behind pipe in poorly cemented zones.

Although some tools can handle both environments, there are some methods applicable only to injection profiling.

In general, profiles may be obtained without disturbing dynamic well behavior by using the proper pressure control equipment and operating techniques; i.e., logs can, and should, be run through tubing without having to kill the well or pull the tubing.

Before attempting to obtain a profile log, plan the operation in advance with the logging service company, paying particular attention to:

• expected flow rate
  

casing and tubing size, type, and weight

expected wellhead pressure

type of Christmas tree connections

tubing restrictions

corrosive or poisonous production fluids

completion records

openhole logs

Profiling tools available for measurement of fluid flow rates fall into three major categories:

• continuous flowmeters
  

packer or restrictor type flowmeters

radioactive tracers (velocity and tracer modes)

Figure 1 , Figure 2 , and Figure 3 illustrate the three types of flowmeter--the packer, the continuous, and the fullbore; Figure 4 illustrates a radioactive tracer tool.

 

        

        

The accuracy of fluid flow rate measurements depends on:

• the number of commingled phases
  

the well deviation

the type of tool and the way it is run

hole diameter variations

production/injection rate variations

Greater confidence in results can be expected when there is only one phase flowing (oil or water or gas), when the well is vertical, and when the appropriate tool is used for the particular well conditions. A lesser degree of confidence can be placed in results in deviated wells, conditions producing froth or slug flow, in wells that are "heading," and where the design limitations of the tools are exceeded (e.g., continuous flow-meters in low flow rate wells). For safety reasons, radioactive tracer surveys should only be run in injection wells.

    

Figure 5 shows a production profile made from a flowmeter survey.

Figure 6 shows a radioactive tracer survey made in a "time-lapse" mode. Note the final destination of the released tracer material.

Fluid Identification

Production logging tools that can differentiate between oil, gas, and water in a producing well allow diagnosis of a number of completion problems, better understanding of reservoir performance, and monitoring of secondary and tertiary recovery projects.

In particular, they help to pinpoint gas, oil, and water entries into, and exits from, the production string, as well as to determine, in combination with flow measurements, how much of which fluid is produced from which horizon.

Many tools are available to distinguish one type of fluid from another. Their functions are measurement of fluid density, measurement of fluid dielectric constant, recovery of a fluid sample at well flowing pressure, and measurement of frequency spectrum of noise generated by fluid flow.

Two commonly used devices are:

• the gradiomanometer ( Figure 1 ), which measures the pressure difference in the wellbore between two pressure sensors a fixed distance apart
  

  the fluid density log ( Figure 2 ), which measures the absorption of gamma rays by the fluid between a gamma ray source and a detector
  

 

The hydro log ( Figure 3 ) measures the dielectric constant of the fluid flowing in the wellbore. Because of the large difference between the dielectric constant of oil and water, the holdup of the flowing mixture may be estimated.

 

Figure 4 illustrates a downhole fluid sampler. This instrument may be used to retrieve a sample of fluid from the well. It is useful for collecting oil, water, and gas samples for PVT analysis and pinpointing fluid levels in a well.

Turbulent fluid movement generates noise. Both the amplitude and frequency of this noise vary with the quantity and type of fluid and the medium through which the fluid is flowing. Measurements of these characteristic sounds can be interpreted to indicate the type of fluid flow and its location. In the case of gas, it is possible to calculate the approximate rate of flow.

 

 

 

Temperature Logging

Temperature logs may be used to monitor fluid flow in production or injection wells; they have the added advantage of detecting fluid flow outside the completion string in tubing/casing annulus or casing/formation annulus. They are particularly useful for finding gas entries to, or exits from, the wellbore; channels in poorly cemented sections; lost circulation zones in openhole; and the cement top in a recently cemented well.

Three types of temperature measurements are commonly available: a conventional temperature survey, a differential temperature survey, and a radial differential temperature survey.

Figure 1 illustrates a conventional thermometer and associated temperature survey.

 

 

Figure 2 illustrates the radial differential thermometer and its associated survey, in connection with which the operator should

• choose an appropriate scale so that there are no excessive scale changes over the zone of interest
  

  log going down where possible so that the presence of the tool and cable in the wellbore does not influence the measurement being made
  

remember that temperature-measuring devices are normally quite sensitive to temperature changes, but not very accurate in absolute terms

 

Figure 3 illustrates a temperature log showing oil production through a perforated interval.