What Happens when you Pump F.R. into Formation on a Cross-linked Guar Frac?

Summary of our observations on rubbery polymer accumulations thought to be due to interaction of polyacrylamide and iron:

  • Reduced primary and secondary recovery vs offsets (charts below).

  • “Gummy Bears”: Occurring in multiple formations across North America in situations where polyacrylamide and iron are both present, lab testing supports this theory.

  • Wasted chemical: The proppant suspension mechanism in this frac design is driven by crosslinked-guar. Friction reducer pumped into formation isn’t believed to meaningfully improve proppant suspension or reducing friction with this frac design.

Combing through the data on a well in S.E. Saskatchewan, Canada, we found evidence that there were issues with how the F.R. (polyacrylamide based friction reducer) was being run. The program was designed so that F.R. would be used to reduce friction and horsepower during flush, be re-circulated, and not pumped into formation. However, as we’ve seen time and time again, the program doesn’t necessarily tell you how the frac was actually pumped.

Charting the data after the frac, we saw this on many stages:

Above, F.R. is being pumped during the sand ramp, into formation. This was observed on many stages during this frac, instead of pumping F.R. as programmed (only on flush for circulation horsepower reduction), the F.R. was pumped during a majority of the job. For most of the stages on this well: when the frac slurry or clean was pumping, so was the F.R.

The question becomes: how is this polyacrylamide interacting with the reservoir and formation fluids?

And how does this impact production?

The prognosis

This well, much like the two offsets had a good geological prognosis, the well was anticipated to be a good producer.

However, as production data started to come in, it was obvious this well was underperforming vs geologically analogous offsets immediately to East.

Above, oil production history is charted vs time for the first 8 months and shows the well with F.R. pumped into formation (Red) underperforming versus offsets.

A quick view of the proximity of the wells:

Map showing well with F.R. pumped into formation (on the left) and two geologically analogous offsets to the right.

Secondary Recovery

The well was further incorporated into a waterflood enhanced oil recovery scheme. The well in the middle (Offset 1, directly offsetting the F.R. well to the East) was converted into a water injector.

Both wells showed a waterflood response, unfortunately, the F.R. well continued to underperform vs the offset on the other side of the injector during secondary production.

Confirmation of the mechanism by others

This type of chemistry interaction has been observed in the U.S. in multiple formations, and has also been tested in the lab:

Presented in URTeC 2487:

“When an iron source is added to the fluid a nearly instantaneous development of the accumulation was noted.”

While URTeC 2487 provides a good example of a polymer mass formation, chemistry can vary considerably between different formation fluids and reservoir matrices. Shales, clays, other clasts, and formation fluids have very complex and variable chemistries. We’re not suggesting the problem is cut and dried, but that we know the combination of polyacrylamide and iron has been shown to be problematic in multiple situations.

The well discussed here on the blog is the first time we’ve seen evidence of F.R. being pumped into formation during a cross-linked frac. While we haven’t heard of anything similar happening at other producers, we suspect there are likely many examples lurking in the data waiting to be uncovered.

Pumping F.R. into formations containing Iron is a problem on cross-linked fracs

The significant production underperformance of this well is a good indicator that paying close attention to how the chemicals are being run during your fracs pays off.

The purpose of polyacrylamide in this situation is to reduce friction in the tubulars during circulation, not to suspend proppant. The proppant suspension is being accomplished with a cross-linked guar gel.

This results in a few things:

  • Wasted chemicals, polyacrylamide friction reducer is pumped far above program

  • F.R. pumped into formation, with un-anticipated chemical interactions with formation fluids, and matrix

  • Impacts on load fluid recovery, residual polymer damage to the proppant pack, and potentially lower production (more on this below)

Strategies to avoid polymer mass formation downhole

This type of situation can be avoided, especially in cross-linked guar frac programs. And more generally, it’s good to put a few minutes of thought into a good strategy for identifying and mitigating the potential for F.R. to react with iron.

  • Closely examine the chemical data on your fracs. This situation would not be obvious on a standard post-frac report from a pressure pumping company.

  • Rigorously test for iron. Test frac and formation water for iron content, and also fines recovered during flowback and production.

  • Minimize pumping acrylamide based FR into formation if iron is proven or suspected in formation, for guar-based crosslinked fracs FR may still be run to reduce circulation pressures, but must be carefully timed to avoid pumping it into formation.

References/Related Reading:

Rassenfoss, Stephen. "Solving the Gummy Bears Mystery May Unlock Greater Shale Production." J Pet Technol 72 (2020): 26–29. doi:

Hazra, S., Van Domelen, M., Cutrer, W., Peregoy, N., Okullo, P., and B. Darlington. "Performance of Friction Reducers in Iron-Rich Environments." Paper presented at the SPE/AAPG/SEG Unconventional Resources Technology Conference, Virtual, July 2020. doi:

Ba Geri, M., Imqam, A., and Flori, R. A Critical Review of Using High Viscosity Friction Reducers as Fracturing Fluids for Hydraulic Fracturing Applications . SPE Oklahoma City Oil and Gas Symposium, Oklahoma City, OK. 9-10 April 2019. SPE-195191-MS