Hydraulic fracturing fluids (HFF) for unconventional shale reservoirs are formulated to maximize productivity, however, much remains unknown about the fundamental reactions involving HFF additives that occur upon injection into the subsurface. We studied reactions between surfactant and reactive geochemical species present in shale to evaluate whether injected surfactants are chemically stable under unconventional reservoir conditions. A transformation involving commonly used surfactants—alkyl ethoxylates (AEOs) and polyethylene glycols (PEGs)—was examined to determine whether these HFF additives are prone to degradation under abiotic conditions.
Solutions containing AEOs and PEGs were heated (80 °C) and stirred for 24 h in the presence of various components of shale and HFF—including shale powder, pyrite (FeS2), additional Fe(II) (in the form of FeSO4), and proppant. Low molecular weight PEG formation occurred most rapidly in experiments which included pyrite. In the presence of shale alone, the rate of PEG formation diminished over time. However, in the presence of shale powder and FeSO4, greater PEG formation was observed. The presence of FeSO4 alone led to some low molecular weight PEG formation, albeit much less than that observed in the presence of shale or pyrite. The presence of proppant did not stimulate PEG formation, not even in the presence of added FeSO4.
The AEOs and PEGs tested showed rapid degradation in the presence of shale powder and pyrite over the course of 24 hours, well within a normal shut-in period for a well. Such degradation leads to the formation of lower molecular weight, more hydrophilic PEGs that are more likely to return in the produced water and less likely to mobilize the hydrocarbons in the reservoir. Further, little is known about the fate of the other reaction products downhole (e.g. alcohols from AEO central cleavage) and how they may impact reservoir properties like surface wettability and diminish production or stimulate microbial activity that may generate hydrogen sulfide. The instability of AEO surfactants suggests their use in HFF may not be necessary or may not have the intended effects.