Tracer Technology Finds
Expanding Applications, T.R. Bandy, Petroleum Engineer International, June
1989.
ABSTRACT
Tracer Technology Finds
Expanding Applications
Tracers are becoming a
commonly used tool to study the production, injection, and processing of oil
field fluids. Additionally, tracers evaluate the placement mechanics of well
completion fluids and slurries. Other related fields, such as geothermal
energy, hydrology, and underground storage-disposal, have also applied
tracers to aid in the understanding and subsequent optimization of their
specific operations. Not unexpectedly, the phenomenal advances in electronic
instrumentation and computer science have brought about an evolution in the
detection of tracers and analysis of tracer tests.
The Random House Dictionary
defines a tracer as "a substance, usually radioactive, traced through a
biological, chemical, or physical system to study they system". Indeed,
tracers of every conceivable form have been formulated to satisfy the
requirements of this definition. Thus tracers of all three physical states
(solid, liquid, gaseous) and a myriad of chemical compositions of each are
available. Most oil field tracer applications require downhole detection via
wireline conveyed instruments; thus, the use of gamma-ray emitting
radioactive isotopes is quite common. In other applications such as
interwell tracer testing, the collection of produced fluid samples and
subsequent direct analysis require the use of many different types of
tracers. Generally, tracers can be categorized as follows:
- Gamma-ray emitting
radioactive tracers (can be detected downhole).
- Particle emitting
radioactive tracers (cannot be detected downhole).
- Chemical tracers (both
organic and inorganic).
- Optical tracers (dyes and
flourescents).
When selecting various
tracers for specific applications, certain criteria must be considered; the
most important factor is the accuracy with which the tracer will follow the
material being traced. Partitioning of the tracer into a phase other than
the one of interest has resulted in many invalid tracer tests. Also, the
amount of tracer used must be sufficient to account for the following:
- Naturally occuring
concentrations of the tracer species.
- Adsorption onto tubulars
or formation during transport.
- Molecular diffusion, fluid
dispersion, and dilution.
- Chemical and biological
degradation.
- Radioactive decay
(half-life).
- Interference of other
matter with detection technique.
Additionally, in downhole
detection of gamma-ray emitting tracers, the distance between the tracer and
detector and the shielding values of the materials separating them must be
considered. Radiation intensity follows the inverse square law with respect
to distance; thus, if the distance between a gamma-ray emitting tracer and
the detector is increased from 2 to 4 ft, the gamma-ray intensity will be
only one-fourth the original value. Furthermore, dense materials (such as
steel pipe) can greatly diminish radioactive tracer detectability.
Because of these two factors,
downhole detection of gamma-ray emitting tracers has undergone considerable
improvement, beginning with techniques for discerning relative placement of
tracers inside the well bore versus in the formation2 and differentiation
techniques for multiple tracers. Two works published within the last year
describe an analytical spectrum unfolding technique, and a relative distance
measurement technique which ultimately should lead to true radial
quantification of such near-well bore treatments as primary cementing and
gravel packing.
In downhole well logging, the
industry has used gamma-ray detectors for many years to measure naturally
occurring radiation followed by processing of the spectral data into
potassium, uranium, and thorium equivalents. These natural gamma-ray
spectroscopy instruments, historically housed in large diameter tools
(3-5/8-in. OD) have recently been augmented with smaller diameter
(1-11/16-in OD) tools so that through-tubing operations now can be
conducted.
Additionally, calibration of
these spectroscopy tools for use in differentiating multiple gamma-ray
emitting tracers, and their placement relative to the well bore (inside
versus outside) have been conducted in the laboratory. All these recent
efforts have resulted in numerous field-proven services, readily available
throughout the industry.