American Oil and Gas Reporter - August 2015 - 60

SpecialReport: Hydraulic Fracturing Technology
Operators can deploy cased-hole logging techniques to pinpoint the most productive zones along the lateral and then change
the plug-and-perf program "on the fly" to deploy plugs at the intervals targeted for perforating and completion. The number of
zones and the distance between subsequent stages can be
changed quickly in a well.
This degree of flexibility is not possible with sliding sleeve
completion tools that are opened or closed mechanically using
ball drops or coiled tubing to allow stimulation fluids to selectively fracture zones in the formation. Sliding sleeves are an attractive multistage stimulation option thanks to the efficiency they
bring in terms of reduced rig time, faster operations and limited water usage, especially when windows for scheduling frac jobs
are tight. However, because the sleeves are installed with the casing, sleeves cannot be moved once a casing string is installed.
While the plug-and-perf technique affords cost savings to a
well completion design, operators continue to search for ways to
further optimize well completions in ways that maximize production while minimizing costs. One focus of this optimization initiative has been in the design of new composite frac plugs.
Composite plugs generally contain relatively high amounts of
metal. The length of traditional plugs increases the amount of material that must be milled out across multiple zones. In some cases, a well may contain 55 or more frac plugs set in a lateral that
is one mile or more in length.
Traditional composite plugs also tend to have rough edges and
protrusions that can catch on ledges and casing joints, causing
the plug to preset in the lateral or become stuck. Composite plugs
with primarily metallic slips leave large amounts of debris in the
wellbore, which makes milling operations more complex,
lengthy and costly.
Optimizing Plug Performance

While it is critical to reduce metal content for improved plug
run-in and mill-out, a balance must be struck between optimizing run-in and mill-out performance and achieving the required
anchoring force to set the plug and ensure isolation in the targeted zones.
The newest generation of composite frac plug technology uses
a blend of 97 percent composite material. The low metal content
and advanced composite materials provide a plug that delivers
higher run-in speeds, reliable anchoring, improved seal integrity, and reduced mill-out time with smaller-sized cuttings.
The key to the success of a composite frac plug solution is in
designing and optimizing the individual components-including
the lower slip assembly and cone, molded element system, and
upper slip assembly (Figure 1)-to perform their designated funcFIGURE 1
Individual Composite Plug Components

60 THE AMERICAN OIL & GAS REPORTER

tions in hole, while fitting together into a lightweight and compact design capable of holding a high-pressure, competent seal.
The first component, an upper slip, is machined from a single piece of filament-wound composite and paired with powdered
metal buttons. The buttons provide a high-strength anchoring force
into the casing string. Both the buttons and composite structure
mill easily, allowing each plug to drill out quickly into small pieces
for an efficient return.
The newest composite plug design also incorporates a molded element system with a smooth surface devoid of rough edges
or protrusions, which lowers the risk of the plug presetting or getting stuck while running into the hole. The support structure is
molded directly into the element body, which further reduces the
risk of element flaring during deployment. As a result, the plug
can be run in hole at speeds of up to 500 feet a minute, which is
roughly twice the speed of conventional plugs.
Once the plug is set, the element provides a high-integrity seal
rated for a maximum pressure of 10,000 psi and temperatures to
300 degrees Fahrenheit, making it suited for deploying in diverse
wellbore environments. The element's backup structure prevents
extrusion while holding pressure. And because it is molded into
the element, the backup structure results in smaller element pieces
during mill-out operations.
The lower slip provides the main anchoring force to hold the
plug in place during hydraulic fracturing of the zone. Using a combination of high-strength composite and small hardened inserts,
the lower slip provides superior anchoring force with a minimal
amount of metal. Once the milling operation begins, the slip's
composite material and small buttons break up quickly into a small
size distribution for more efficient well cleaning.
A beveled mule shoe is affixed to the end of the plug next to
the lower slip. During mill out, the beveled shoe acts as a clutch
that locks onto the top of the plug below it to prevent the plug
body from spinning as it is milled, increasing milling efficiency and decreasing mill-out time.
Field Trials

Composite plugs designed for 51⁄2-inch casing were run in a
series of field trials conducted over six months in the winter and
spring of 2014. The field trials uncovered design issues with the
plug that caused issues during run-in. In each instance, design engineers pulled the plugs and conducted a thorough investigation
of the root cause of the problems. Working with the internal design and composite engineering groups, the engineers made the
necessary changes to the design to prevent similar problems on
subsequent deployments.
In the first field trial, 31 plugs were run in a well in Northeast Texas. One plug hung up in the vertical section of the well
during deployment and was pulled on wireline. An examination
at the surface showed that the upper slip band had broken, allowing the lower slips to contact the casing. The operator was not
concerned with this issue and continued deploying the plugs without additional issues.
In another Northeast Texas field trial, two plugs stuck in the
well during deployment, leading to an investigation of problems
with the plug design itself. Collaboration between field and design engineers resulted in a redesign that added a composite pin
to each of the lower slips, which required an additional 1,000
pounds of force to break the lower slip ring and prevent prema-

American Oil and Gas Reporter - August 2015

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