Energy Rubber Group, Inc.
2014 Winter Technical Meeting
C. Andrew Rosenholm, President / Owner
Rockwell Oil and Gas Tool Sales
car@rockwelloil.com
WATER-DISSOLVABLE POLYMERS FOR USE IN
MANUFACTURING FRAC BALLS, SEATS AND PLUGS
Examples of degradable polymers in use today:
Sutures
Tacks
Bone anchors
Golf tees
Flatware
Bags
Foam packaging
Why use water-dissolvable
polymers downhole?
Main reason: To avoid costly mill-outs
Remnants of tools (balls, plugs, seats) will dissolve
over time, increasing product flow and production
Q: Why aren’t degradable plastics more prevalent downhole?
A: Most degradable plastics lack the tensile strength necessary
to hold up to the high pressures used in fracking < 15,000 PSI
DOWNHOLE TOOLS ARE SUBJECTED TO EXTREME PRESSURES!
Fig. 1: Frac ball on seat as used in sliding-sleeve fracking,
note the 1/16” per side shelf spacing
APPLICATIONS FOR DISSOLVABLE DOWNHOLE TOOLS
Fig. 2: Sliding-sleeve frac illustration showing balls and seats
Fig. 3: Composite plug used in plug-and-perf frac completions
PRACTICAL DEGRADABLE POLYMERS:
PETROLEUM-BASED:
Oxo-degradable polymers: These additives (pro-oxidants) use a salt
of a transition metal such as cobalt (Co), iron (Fe), manganese (Mn)
or nickel (Ni) to drive the oxidation process which, under the action of heat
or light, will reduce the molecular weight of the polymer to a level
where bacteria & fungi in the soil or disposal environment can further
reduce the material into water, carbon dioxide & biomass.
Traditional base resins: PE, PP, PS, PET
Polymers with hydrolysable backbones (aliphatic polyesters):
• Polylactide (PLA) (Crystallinity 12.29% to 47.54%)
• PLLA (Crystallinity 37%)
• Polyglycolide (PGA) (Crystallinity 45-55%)
NATURAL/AGRO-POLYMERS
Polymers produced from plants:
• Polysaccharides: Starch and cellulose
• Lipids
Polymers produced from animal sources:
• Collagen, chitin
Crystallinity effects degradation
During the first phase of degradation, water
penetrates the biodegradable device,
initially cutting the chemical bonds and
converting the long polymer chains into
shorter and shorter fragments (hydrolysis).
Ø 3.750” PGA ball failure mode at 7,900 psi
Notice how the ball fractured instead of extruding through
the seat, this failure mode is a hallmark of crystallinity
OBSERVATIONS ON THE DEGRADABILITY OF PGA
Fresh Water
When the frac ball was kept in water for one month at room
temperature little degradation occurred. Raising the temperature
of the water to 200° F initiated the dissolving process. At the
elevated temperature pieces of the frac ball started to flake off,
making a popping sound.
Brine Water
No major changes were observed at any
temperature the first 39 h. At increased
temperatures (200° F), the ball appeared to dissolve
slowly, flaking. Within the next 14 h the diameter
was reduced significantly, The Ø1.70’’ ball was
reduced to Ø1.39’’.
10 % KCl
No major changes were observed over the first 30
hrs. Even at 200° F the ball shrunk only a minimal
amount from Ø2.13’’ to Ø2.11’’. After 30 hrs the ball
started giving off flakes with loud popping noises.
10 % HCl
No changes were observed at room temperature
within the first week.
PGA THICKNESS DECREASE
Tensile Strength ISO 527 (psi)
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
Tensile Strength ISO 527
(psi)
GF PLA
PGA
PEEK 450G
Maximum pressure held Ø 3.750"
ball (psi)
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
Maximum pressure held
Ø 3.750" ball (psi)
PGA
CF-PEEK
G10
ALTERNATIVES TO DEGRADABLE POLYMER
Reactive metals: Magnesium, Aluminum, Calcium
Brand names: InTallic™, ExSolv™ Metall
PROS:
• High pressure capabilities
• Rapid degradation
CONS:
• Very expensive
• Limited supply
REACTIVITY OF METALS
Ø 3.500” REACTIVE METAL BALL
PRESSURE TEST RESULTS
OBSERVATIONS ON THE DEGRADABILITY OF REACTIVE METALS
Brine Water
At room temperature it took the frac ball 9 days to
dissolve completely. Increasing the temperature to
200° F resulted in the frac ball dissolving within 22 hrs.
Figure 2 – ExSolv™ Metall Acid/brinedissolvable metal alloy frac ball dissolving
Figure 3 – ExSolv™ Metall Acid/brinedissolvable metal alloy
Size comparison after 5 days in brine water at
room temperature
CONCLUSIONS:
• DEGRADABLE POLYMER DOWNHOLE TOOLS OFFER A COST-EFFECTIVE
WAY TO PREVENT COSTLY MILL-OUTS
• IN APPLICATIONS WHERE DEGRADABLE POLYMERS ARE NOT STRONG ENOUGH,
DEGRADABLE METALS CAN SUBSTITUTE, I.E.: USE DEGRADABLE POLYMER FRAC BALLS
UP TO Ø 2.500” THEN SUBSTITUTE REACTIVE METAL BALLS FOR LARGER SIZES
• DEGRADABLE POLYMERS NEED ONLY H2O TO BREAK DOWN – HCl CAN ACT AS
AN ACCELERANT
• DEGRADABLE METALS NEED SALTS AND/OR ACID TO BREAK DOWN