American Oil and Gas Reporter - June 2017 - 21

After-Closure Analysis
Offers Speedy, Accurate Way
To Evaluate Permeability
Recently in Houston, I had the opportunity to attend a
Society of Petroleum Engineers workshop about diagnostic
fracture injection tests. The purpose of the course was to look at
best practices, analysis methods and theories. The group also
looked at how to identify common issues on both the operational
and analytical sides as well as how to incorporate the data into
an existing workflow.
Data collection and utilization have expanded rapidly in
recent years, however sometimes incorporating that data into
existing engineering workflows can be confusing. Although injection testing has been in use for years, analysis of this data can
mean the difference between a good well and a "barn burner."
It may be immediately obvious, but effective reservoir permeability is a-if not the-key parameter in reservoir characterization
and stimulation design. Accurate identification and analysis of
reservoir permeability is not only important to understanding
overall reservoir performance, but conventional methods such
as a pressure buildup test can be time-consuming and costly.
The industry has several terms for diagnostic fracture injection
testing, such as an after-closure analysis, post-closure analysis,
mini-falloff test or injection test analysis, but all work in much
the same way and offer similar results.
When factors such as time and cost come into play, the
temptation to cut corners during conventional testing and
analysis becomes very real and, ultimately, can reduce the
well's overall performance. With this in mind, an operator
looking at a well's stimulation design may stray from conventional
methods of measuring permeability, or at least be skeptical of
overly quick test results.
In the tradition of the oil and gas industry's ability to create
more effective, equally accurate processes to save time and
money, several years ago Ken Nolte pioneered a process called
after-closure analysis (ACA) to help identify the flow regimes
and, ultimately, calculate reservoir transmissibility with results
comparable to conventional methods such as pressure buildup
and drill stem tests.
An ACA involves injecting fluid into the formation at low
rates and accurately recording the formation response during
the pressure decline. Once these data are analyzed, it becomes
fairly easy to calculate the effective formation permeability.
This process can be completed within days while providing the
same accuracy as a pressure buildup test, which in certain
cases, requires nearly a year to accurately complete.
An injection test causes a disturbance within the reservoir or
a fracture, which creates an impulse behavior. Monitoring and
analyzing the pressure decline makes it possible to estimate effective permeability with consistent accuracy. ACAs have
multiple uses, but typically are effective in low permeability
zones, and can be used for reservoir characterization and fracture
treatment design.
The ACA process begins with an injection test that pumps
fluid into a formation until it creates a fracture, after which the
well is shut in. Because the fluid does not contain proppant, the

"An ACA involves injecting fluid into the
formation at low rates and accurately
recording the formation response
during the pressure decline.

"

fracture is allowed to close and pressure is monitored for two
key intervals: the time it takes for the fracture to close, and the
time it takes to return to reservoir pressure.
As one may imagine, a good understanding of initial reservoir
pressure is key to a successful ACA. A well-performed ACA
also must inject into an undisturbed reservoir. Additionally, it
is important to accurately measure the volume of fluid being
pumped. Finally, the most critical factor in completing an
accurate ACA entails waiting long enough after the well is shut
in to see pseudo-radial flow.
This interval is proportionate to the square of the fracture
half-length for a given effective reservoir permeability. Using
expected fracture half-lengths and the time to reach pseudoradial flow, one can calculate the in situ effective permeability,
informing engineers' decisions for stimulating that particular
reservoir.
At the end of the day, it's all about completing productive,
optimized wells within a reasonable time frame. Without a
solid understanding of the characteristics of the reservoir and
its permeability, the stimulation engineer faces a greater challenge
making the best choices for a given well. However, a wealth of
tools and techniques are at his fingertips to accomplish this
goal. The skilled engineer will know which tool to use under a
particular set of circumstances. ACA is one of many tools that,
if performed correctly, can have a dramatic effect on overall
well performance.
r

JEREMY VISCOMI is the MidContinent regional lead for
the Petroleum Technology
Transfer Council. He has more
than a decade of experience
in developing and organizing
technical conferences and special events, primarily in the
oil and gas industry.
JUNE 2017 21



American Oil and Gas Reporter - June 2017

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