Hydraulic Fracturing Additives and the Delayed Onset of Hydrogen Sulfide in Shale Gas

Abstract

As natural gas production has shifted further from deep prolific gas reservoirs to shale gas, several questions are being addressed regarding fracturing technologies and the fate of chemical additives. A less investigated issue is the unexpected increase in produced hydrogen sulfide (H 2 S) from hot shale gas reservoirs. Understanding the source of H 2 S in shale reservoirs and managing low-levels of recovered elemental sulfur affects plans for future treatment, corrosion mitigation, and fracture fluid formulations. In this work we demonstrate that some typical ingredients of hydraulic fracturing fluids are not as kinetically stable as one might expect. Surfactants and biocides such as sodium dodecyl sulfate and glutaraldehyde are shown to undergo hydrolysis and thermochemical sulfate reduction reactions under moderate reservoir conditions, with H 2 S as the final product accompanied with long chain alcohols and hydrogen sulfate as long-lived intermediate species. This finding suggests that fracture fluid additives can be responsible for the delayed production of natural reservoir H 2 S.

Hydrogen sulfide formation in oil and gas

“…Some authors have explained the apparent acid catalysis of TSR by stating that HSO 4 is the more reactive species compared to sulfate, and conclude that conversion of sulfate to bisulfate might be the rate determining step for TSR. 14,23 An alternative explanation is provided here which is self-consistent with the mechanism above and high-pressure aqueous speciation calculations, i.e., where reaction 3 is the limiting reaction.…”

“…33 The geochemical reaction of native and immature organic sulfur compounds could be the major source of the observed H 2 S; 7 however, H 2 S is observed within mature fluids (insignificant hydrocarbon content beyond methane In the case of native H 2 S, the elemental sulfur produced with reaction 5 can react with the 1dodecanol, or other hydrocarbon species in the hydraulic fracturing or from the reservoir, and regenerate H 2 S. With only SDS, our reaction mixtures extracted post-TSR were highly acidic and elemental sulfur had been found in the mixtures. 26,14 These observations explain the rate at which the shale gas wells go sour in the fields.…”

“…21 The reported activation energies for TSR range between E a = 77 and 250 kJ mol -1 , depending on the reaction conditions and reactants/products involved. 20,25,14 Various laboratory results show that the kinetics of TSR depends on the type of organic reductants, dissolved sulfate species and a variety of intermediate sulfur species. 22,23,24,7 The mechanism shown in Reactions (1) to (5) implies that steady-state elemental sulfur (S°) is formed from the reaction between bisulfate and H 2 S (equilibrium), and then S° is kinetically consumed by oxidation of hydrocarbons (or other organic species).…”

Shale Gas: A Review of the Economic, Environmental, and Social Sustainability

“…H 2 Si s am ajor hazard because it is poisonous to humans and it is corrosive;t he latter also means that it could corrodee quipment and pipelines,r eleasing further H 2 Sa nd other chemicals into the environment. Incidents have been recorded in which the delayed releaseo fH 2 Sh as occurred following fracturing activity, [94] but what causes this is unknown. However, it is hypothesized that chemicals used in fracturing fluids may be reacting with H 2 Sa nd microorganisms in the rock formation, leading to the releaseofH 2 S. [94,95] Measuring air emissionsi sadifficult task since variations in weather conditions (temperature,w ind speed and direction, humidity,e tc.)…”

“…Incidents have been recorded in which the delayed releaseo fH 2 Sh as occurred following fracturing activity, [94] but what causes this is unknown. However, it is hypothesized that chemicals used in fracturing fluids may be reacting with H 2 Sa nd microorganisms in the rock formation, leading to the releaseofH 2 S. [94,95] Measuring air emissionsi sadifficult task since variations in weather conditions (temperature,w ind speed and direction, humidity,e tc.) affect the measurements,a sd on earby external sources of emissions,s uch as traffic and farming activities.Af urther issue is that most of the activities,s uch as well pad preparation, drilling,a nd hydraulic fracturing, are episodic.…”