American Oil and Gas Reporter - October 2022 - 29

ACAs Provide Companies
Timely, Affordable Data
To Inform Stimulation Plans
During my time in the oil field, I have seen several " feast or
famine " industry cycles; however, the rebound from this latest
downturn has been a bit more restrained and conservative. In
fact, according to Baker Hughes, rig counts during 2021 were
down compared with similar rebounds.
In addition, companies are taking a more conservative approach
as they deal with headaches associated with labor markets,
limited sand and steel supplies, and unresolved constraints
affecting capital and other factors. Add in emerging environmental,
social and governance awareness alongside new environmental
regulations, and suddenly optimizing wells and stimulation treatments
to maximize investment is showing signs of life again
after a long period of " pump and pray " operations. More and
more, I hear about reservoir characterization and injection tests,
specifically as they relate to hydraulic fracture stimulation.
Most engineers working in the upstream oil field understand
that effective reservoir permeability is a-if not the-critical parameter
in reservoir characterization and stimulation design.
Accurately identifying and analyzing reservoir permeability are
crucial to determining overall reservoir performance. However,
measuring permeability can be time-consuming and costly with
conventional methods such as a pressure buildup test.
When these factors come into play, the very real temptation
arises to cut corners when performing traditional testing and
analysis, and ultimately can reduce a well's overall performance.
In the last several years, we have seen many companies
stimulate wells with an " any frac is a good frac " mentality. As
such, many companies have stimulated on the cheap and still
get solid initial results. That said, as pressure mounts to address
ESG challenges while still developing a profitable well, it is
more important than ever to maximize each well to responsibly
produce the reservoir in ways that minimize the volume of resources
that are left behind.
Several years ago, I wrote about a process called afterclosure
analysis (ACA). Initially, ACA was developed and pioneered
by frac legend Ken Nolte to help identify flow regimes
and, ultimately, calculate reservoir transmissibility, offering
results comparable with 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. After data analysis, it becomes relatively
easy to calculate the effective formation permeability. This
process can be completed within days while offering the same
accuracy as a pressure buildup test, which in some instances,
can take almost a year to accurately complete.
A 2019 Journal of Petroleum Technology article shows that
this type of analysis can be a low-cost method of obtaining a
substantial field-wide dataset during development. This dataset
can be used to speedily inform development choices that generally
may require a working production facility. One of the authors of
that paper, Prue Smith, a senior technical specialist with BP,
asserts that the injection causes a disturbance within the reservoir
or a fracture, which creates an impulse behavior. The pressure
decline is monitored and analyzed to accurately estimate effective
permeability. ACAs have multiple uses, but typically are effective
in low permeability zones, and can be used for reservoir characterization
and fracture treatment design. The industry has several
variations of and terms for an ACA, such as diagnostic fracture
injection test, post-closure analysis, minifalloff test or injection
test analysis, but all yield similar results.
The ACA process begins with an injection test that pumps
fluid into a formation until a fracture is created. Once the
fracture appears, the well is shut in. Because the fluid does not
contain proppant, the fracture is allowed to close and pressure
is monitored. While monitoring this pressure decline, the analyst
looks for two key intervals: how long it takes for the fracture to
close and how long it takes to return to reservoir pressure.
As one may imagine, understanding initial reservoir pressure
is key to an ACA's success. It is also important for an ACA to
inject into an undisturbed reservoir. Additionally, the volume of
the fluid being pumped must be measured accurately. Finally,
the most critical factor in completing an accurate ACA is
waiting long enough after the well is shut in to see the pseudoradial
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 pseudo
radial flow, one can calculate the in situ effective permeability,
allowing engineers to make the best decisions for stimulating
that particular reservoir.
As it was almost a decade ago, companies generally strive to
complete productive, optimized wells that maximize production
within a reasonable time frame. With investor pressures mounting,
stimulation optimization will be crucial for well development.
Without a solid understanding of the characteristics of the
reservoir and reservoir permeability, the stimulation engineer
may struggle to make the best choices for a given well. However,
engineers seeking to produce each reservoir responsibly may
find that a correctly performed ACA can have a dramatic effect
on overall well performance.
❒
JEREMY VISCOMI is a strategic
marketing expert for the oil and
gas industry. Among his many
roles, he serves as the MidContinent
regional lead for the
Petroleum Technology Transfer
Council, providing independent
operators with access to emerging
technology aimed at reducing
risk and growing profits.
OCTOBER 2022 29
" With investor pressures mounting,
stimulation optimization will be crucial for
well development.
"
American Oil and Gas Reporter - October 2022

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