Solving the Gummy Bears Mystery May Unlock Greater Shale Production

Abstract

Gummy bears is one of the names for a gross mix of downhole junk that can foul a separator or clog tubing in the Woodford play in Oklahoma. The texture is often rubbery - the only connection to the gummy candy - and the color varies depending on whether the well’s output tilts toward oil or gas. These “unusual semi-solid” accumulations were described in a 2015 Halliburton paper as containing hydrocarbons, sand, iron, fine particles from clay, and sometimes a bit of friction reducer - polyacrylamide to be exact (SPE 173594). The globs generally are seen at the surface after fracturing. When they are observed downhole, they have been pushed by the well’s flow into “choke points” such as “perforations, tubing anchors, gas lift valves” downhole which are noticed when production declines, the paper said. Analysis revealed that those semi-solid amalgams contained iron, which was not in the injected fracturing fluid. While the nature of the stuff made poly-acrylamide a likely ingredient, they did not find acrylamide - the ingredient combined with polymers to make friction reducer. The paper suggested that fracturing “released materials with a strong positive charge (cations)” which were part of “a complex mixture including other solids and the acrylamide, forming a tight emulsion in semi-solid form.” The paper offered a workable description of what was going on below. But the analysis and solutions offered did not seem to make a dent in the problems that were so bad for some operators in the Woodford that they switched from friction reducer to guar to avoid the gunk. Guar, a natural product is not as effective as polyacrylamide at reducing friction. While it has long been used to thicken water-based fluids and allow them to deliver more proppant, laboratory testing concluded guar can hinder production. Substituting guar “is not ideal because the residue guar can [be left] behind on a proppant pack,” said Mark Van Domelen, vice president for technology at Downhole Chemical Solutions (DCS). The maker and supplier of fracture chemicals has been working with Ovintiv on a way to pump friction reducer without the gummy side effects. Ovintiv, formerly known as Encana, saw improved chemistry as a way to increase production from the Oklahoma acreage it acquired from Newfield Exploration. The research partners tested the widely held theory that the gunk was created by a reaction between polyacrylamide and pyrite (iron sulfide or FeS2) in fresh water and found, “When an iron source is added to the fluid a nearly instantaneous development of the accumulation material was noted,” according to their paper (URTeC 2487). A Determined Operator The partners identified the source of the obvious problem - the gummy bears - and some less obvious ones that could undercut the effectiveness of chemicals used for fracturing. One of those is the widely used practice of injecting acid on a site before it is stimulated, which is known as an acid spearhead.