The Fate of Polyacrylamide as a Slick-Water Hydraulic Fracturing Additive, and the analysis of the associated Flow-Back Fluids

Abstract

In the UK, the first well to be fracked onshore used a fracking fluid containing only two additives; an undisclosed salt and polyacrylamide (PAM). This type of simple, predominantly water-based fluid is known as a ‘slick-water’. PAM is used in nearly 100 % of slick-water fluids in the USA, and so it is likely to be a major constituent of UK fluids. The potential for extensive use means the interaction between the PAM and shale must be investigated. Therefore, the aim of this thesis was to investigate the interactions between a PAM slick-water fluid and relevant UK shale gas lithologies, alongside the analysis of associated flow-back fluids from these experiments.
Batch reaction experiments were setup to quantify; 1) the sorption of PAM; and, 2) the composition of associated flow-back fluids. Experiments were conducted at both room temperature (RT) and subsurface conditions (HPHT).
Maximum coverages showed shales to adsorb up to 15,365 mg/kg of PAM, increasing up to 22,972 mg/kg when subjected to increased pressures and temperatures indicative of the UK subsurface. Up to 98 % of the PAM in solution could be removed from solution by adsorption. Of the adsorbed PAM, a maximum of 1.4 % of will desorb during a freshwater flush (typical of the latter stages of a frack). Therefore, PAM is likely to remain downhole, open to degradation.
The addition of PAM to water-based fluid increases the sodium content to ~500 mg/L, producing a slightly saline fluid. Analysis of flow-back fluids from experiments showed that concentrations of the majority of metals analysed, both in RT and HPHT conditions, increased relative to the fracking fluid. Under HPHT conditions, larger concentrations of most contaminants, including Al, Cu, Fe, Mg, Mn and Si, were observed compared to the RT fluids, suggesting subsurface conditions will aid the return of larger amounts of contaminants to the surface.