Gamma Ray Tracers Help
Evaluate Acid Diversion, J.L. Taylor, III, et al, Petroleum Engineer
International, February 1990.
ABSTRACT
Gamma Ray Tracers Help
Evaluate Acid Diversion
The use of multiple gamma ray
tracers has helped evaluate acid diversion in several North Sea completions.
The use of multiple radioactive tracers and subsequent logging with advanced
gamma spectroscopy techniques offers a cost-effective and convenient method
for direct measurement of vital stimulation parameters such as diverter
effectiveness.
Productive intervals in the
Norwegian sector of the North Sea tend to be quite thick: Danian pay ranges
up to 550 ft, and Maestrichtian up to 500 ft. Average porosity can reach
48%, and matrix permeability varies from less than 0.1 to 5 md. Well
productivity seems dependent on the degree of natural fracturing, and
pressure transient testing derived permeability estimates can be as much as
75 times the matrix permeabilities obtained from core measurements.
Perforations are placed in
10-to 20-ft clusters spaced 40 to 80 ft apart, with a shot density of 2
shots/ft throughout each cluster. The acid stimulation treatments are then
pumped in multiple stages, with each stage consisting of a viscous pad,
acid, overflush, and diverter (ball sealers are most often employed).
The tracer studies outlined
in this article were conducted on six stimulation treatments to determine if
the diverter techniques employed result in relatively even treatment of all
pay, and to evidence the creation of multiple hydraulic fractures. All
evidence suggests adequate diversion usually occurs and new fractures are
propagated on each stage.
The specific tracer technique
used involved the placement of a different discernible gamma emitting tracer
in each stimulation stage to determine its relative placement and thus infer
the effectiveness of the diverter stages. Three tracers, Antimony (124Sb),
Iridium (192Ir), and Scandium (46Sc) were added to each stage to
differentiate the placement of up to three stages or groups of stages.
Following each treatment, a PrismŽ log was run to identify tracer
placement. A detailed description of the materials used and the tagging and
logging techniques were discussed in earlier articles.
The tracers were prepared as
ceramic particle encapsulations, with a mean particle size of 0.5 mm. This
proprietary preparation exhibits a tracer washoff of less than 0.01% in 28%
HC1 at 100xC, and has a specific activity of approximately 0.89 mCi/gm (32.8
MBq/gm) or 0.0014 mCi/particle (0.0527 MBq/particle). The use of these
tracers in particulate form was preferred to using soluble forms to minimize
environmental concerns of returning radioactive residue to the surface with
the flowback of the spent acid. The tracers and the equipment used to inject
them into the stimulation process were transported to the well platforms
from the UK aboard the service company's vessel performing the treatment.
Generally, about 20 mCi (740 MBQ) of each tracer was injected continuously
throughout each acid stage. Specific licensing to perform the radioactive
tracer studies was required from Norway's National Institute of Radiation
Hygiene.
The wells were logged using a
1.6875-in. (4.2863-cm) OD Prism tool, which contains a 1-in. by 6-in.
scintillation crystal. The logging speed was 500 ft/hour (152.4 m/hour). At
each 3-in. (7.62 cm) interval, the entire 256-channel gamma ray spectrum was
encoded and transmitted to the surface and recorded on magnetic tape. This
data was subsequently processed using the proprietary software on a
microcomputer at a log analysis center in Stavanger, Norway. The software
mathematically unfolds the gamma ray spectrum to determine tracer yields and
indicate the location of individual isotopes along the well bore.
Furthermore, the program determines the lateral tracer placement (inside or
outside the casing) by using a photopeak to downscatter ratio.
The results of the six tracer
studies are presented in tabular form in Table 1. The Prism logs from wells
A, B, and C are presented as Figs. 1,2, and 3, respectively.
In summary the following
conclusions are made:
- Tracer materials of the
type and packaging used are effectively placed in the formation and do
not flow back into the well. In consequence, reliable Prism data may be
obtained in one pass after cleanup flow of the well.
- Where the cement bond log
indicates effective mechanical isolation of perforated zones in the
treated interval and the number of perforations is low, good diversion
occurs.
- Breakdown of both single
and multiple zones on individual stages were observed.
- Limited fracture heights
and formation of multiple fractures occurs.
- Tracer material positioned
during the early treatment stages is partially stripped away during the
later stages. This is particularly apparent when the number of
perforations is low and flow velocities will, in consequence, be high.
- The logging technique and
analysis allows us to determine the placement of isotopes in the
presence of radioactive scale.