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Injection profiles are run to determine the health of injector units, monitor fluid dispersion, find and measure out–of–zone losses and provide a comprehensive picture of the overall downhole activity on an injection well.
Listed below are explanations of the common surveys used in the injection profile package.
RADIOACTIVE TRACER LOG
The radioactive tracer log determines injection flow profiles by monitoring the reduction in tracer material as it moves down the well. A slug of radioactive tracer is added to the injection fluid. As the slug moves down the well, several gamma ray logs are recorded at well-defined time intervals.
The position of the slug is seen as a large gamma ray peak whose size is proportional to the flow rate. A reduction in the size of the peak indicates a loss of fluid into the formation. Fluid velocity can be calculated from the time interval and the distance the peak has moved using time-slug analysis.
Radioactive tracer logs are used to determine injection flow profiles and detect channels or leaks. Tracer loss measurements are used mainly to give a general idea of fluid flow.
A temperature log presents a record of the temperature gradient in a well. The temperature log is interpreted by looking for anomalies or departures from the reference gradient. Most anomalies are related to the entry of fluids into the borehole or fluid exit into the formation.
Since the temperature is affected by material outside the casing, a temperature log is sensitive not only to the borehole but also the formation and the casing formation annulus. Temperature logs have many applications. The most common include identifying zones producing or taking fluid, evaluating a cement or hydraulic fracture treatment and locating lost circulation zones and casing leaks.
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A caliper log shows the measured diameter of the borehole along its depth. Since wellbores are usually irregular, it is important to have a tool that measures diameter at several different locations simultaneously. This tool is known as a multi-finger caliper. Drilling engineers use caliper measurement as a qualitative indication of both the condition of the wellbore and the degree to which the mud system has maintained hole stability.
This information is crucial to all types of production logging. The actual internal diameter of a wellbore must be known in order for an accurate fluid rate to be calculated.
GAMMA RAY LOG
A gamma ray log identifies total natural radioactivity, measured in American Petroleum Institute (API) units. The measurement can be made in both open hole and through casing. Shales and clays are responsible for most natural radioactivity, so the gamma ray log is often a good indicator for this type of rock. The log is also used for correlation between wells, for depth correlation between open and cased hole, and for depth correlation between logging runs.
Leaks present in any pipe string that you can pump into can be tested with this same process. These logs are often required by state regulations on disposal wells to ensure all injected fluid is leaving the well in the permitted zones and is not channeling up to higher intervals. Trial runs before pumping cement can also be performed to ensure abandonment procedures are met and cement locations are approved by state authorities.
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