Two distinct objectives of cement placement monitoring exist, detecting the top of cement, and determining the quality of the cementation (zonal isolation). For top of cement detection, the temperature log detects the heat of hydration (setting), the unfocused gamma ray density log responds to behind pipe density, and the acoustic cement top (bond) log measures the damping by cement of induced pipe ringing. Cementation quality or zonal isolation is addressed by a number of acoustic tools (at least according to the manufacturers), by the temperature tool which reveals temperature anomalies due to flow, by the noise tool which measures sound from turbulent flow, and in certain cases by the RATS. Neutron activation logs can create their own tracer in water behind casing.|
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The third category utilizes specialized tools, including mechanical caliper tools and the electromagnetic casing inspection tools. Mechanical calipers can assess internal casing corrosion and measure the shape of the casing. Electromagnetic inspection tools respond to changes in metal thickness, either inside or outside the casing containing the tool, and are of the eddy current type or the flux leakage type, or a combination of the two. These electromagnetic tools make indirect measurements that can be related to metal loss through calibration.
The last two categories, monitoring fluid contacts in formations and selecting recompletion zones involve various cased hole nuclear logs. Gamma ray logs, neutron logs, and various pulsed neutron logs are commonly used.
Most production logging tools, and cased hole tools in general, are equipped with a magnetic casing collar locator (CCL) as part of the tool string. Pressure control becomes an important safety consideration when logging wells with positive wellhead pressure. Some production logging tools are made as small as .75 or .875 inch diameter to facilitate logging in the annular space between tubing and casing. There are many other production logging tools including fluid samplers, pressure sondes, and many specialty tools.
Again turning to Dr, McKinley, we are given three critical warnings: (1) Log quality control is of the utmost importance; procedure, tool calibration, and depth control all require close attention. (2) Production logging tools should be run in complementary suites, so that the logs can be compared (crossplotted). Seldom does a single log sufficiently identify a problem to prescribe remedial action. (3) Production logs should be interpreted in a consistent fashion, first identifying features that are normal or expected. The abnormal elements can then be examined to determine which parts are relevant to the problem under scrutiny. It is irrelevant features that so often confound the novice.
This brief effort draws heavily on the work of Dr. R.M. "Mac" McKinley, now retired from Exxon Production Research. Dr. Mac has helped us immeasurably, and his contributions have helped everyone in the industry, whether they know it or not!
The foregoing is an oversimplified discussion of production logging; most theoretical background and interpretative information has been omitted.