Tracers Can Improve
Hydraulic Fracturing, J.W. Chisholm, Petroleum Engineer International, July
1989.
ABSTRACT
Tracers Can Improve
Hydraulic Fracturing
Because the success of well
stimulation treatments often dictates the economic justification of
petroleum field development, much effort has been devoted to the measurement
of various parameters associated with this critical and costly operation.
specifically, the prediction, measurement, and optimization of induced
hydraulic fracture geometry is an endeavor which has resulted in a major
industry-wide research effort. in the past 10 years, extraordinary advances
have been made and the evolution of well stimulation technology is still
proceeding at an incredible rate.
Many methods of actually
measuring or inferring fracture geometry during or after a frac treatment
have been developed and tested; however, few are considered sufficiently
pratical, convenient, and cost-effective to be performed routinely. Analysis
of pressure data from frac treatments and prefrac injection tests can lead
to quantification of certain fracture parameters such as closure stress,
fluid efficiency, and leakoff coefficient; however, computation of most of
these properties requires knowledge of the vertical fracture height.
Of all the available vertical
fracture height measurement techniques, post-treatment tracer and
temperature surveys are by far the most common because they are convenient
and relatively inexpensive to conduct. Temperature surveys can provide
quantitative vertical fracture height determinations; however, they are
plagued by the following problems:
- Cross flow and
pressure-induced fluid redistribution following the treatment can result
in temperature surveys that are difficult to interpret.
- In wells where the
formation temperature differs only slightly from the surface ambient
temperature, these surveys are not possible.
- If significant amounts of
proppant remain in the well bore and must be circulated out before
logging, the circulation process may distort the temperature anomalies
created by the frac treatment, or the temperature anomalies created by
the treatment may completely dissipate by the time the temperature
survey can be conducted.
Because of these problems
particularly the last, frac treatments are frequently tagged with
radioactive tracers. The major objections to using gamma emitting tracers
have been that:
- Only single tracer
operations were pratical, unless tedious multiple logging runs using
tracers with greatly differing half-lives were conducted.
- A conventional gamma ray
log cannot differentiate tracer material actually placed in the
formation from residual tracer left in the well bore; thus, the
determination of actual vertical fracture height is often obscured.
- The depth of detection
from the well bore is limited to less than a meter unless excessive
concentrations of radioactive tracer are employed.