The project goals are to identify geochemical reactions induced in shales upon injection of hydraulic fracturing fluids and to assess the impact of these reactions on shale porosity and release of contaminants, such as uranium.

SLAC National Accelerator Laboratory, Menlo Park, CA

Stimulation of unconventional source rock (shale) using hydraulic fracturing recovers less than 30% of natural gas and less than 5% of oil while consuming large volumes of water and producing similarly large volumes of flowback and produced water. Improving efficiency is imperative not only because of its impact on ultimate hydrocarbon recovery, but also because it provides a route to reducing water demand and problems associated with storing, treating, and disposing flowback and produced water. Formation damage, i.e., the reduction in the permeability of source rock caused by injected fluids, is believed to be an underlying cause of reduced hydrocarbon recovery. The introduction of fluids into shales initiates a myriad of geochemical reactions, including mineral dissolution, oxidation of organic and inorganic species, release of metals, and precipitation of solids [e.g. iron or aluminum (oxyhydr)oxides, carbonates, and sulfates]. For example, oxygen in fracture fluid oxidizes pyrite (which is abundant in shales), leading to precipitation of iron (oxyhydr-)oxides that clog fractures and pore space, potentially blocking flow of hydrocarbons and fluids. These chemical reactions also release metal and radionuclide contaminants such as uranium, lead, and nickel into flowback water, creating the potential for human exposure to radioactive elements and heavy metal contaminants. In spite of the high reactivity of fracture fluid within shale, there is a general lack of fundamental information about fracture fluid-rock geochemical processes in shale. Improved knowledge and control over shale-fluid reactions could be used to reduce formation damage, improve production efficiency, improve overall water usage, and reduce risk of contamination of clean surface and ground water.

The objectives of this research program are to: (1) uncover key chemical processes occurring during shale-fluid reactions; and (2) develop detailed quantitative models that can be used to understand the geochemical behavior of hydraulically stimulated reservoirs, to predict permeability changes, and to mitigate contaminant behavior. To accomplish these objectives, the team characterized the products (dissolved and solid phases) formed by the reaction of fracture fluids with shale minerals and kerogen and identified precipitates at spatial scales down to that of individual precipitate grains (i.e., down to the nm level).

Knowledge developed by this project is helping geochemists and engineers to understand how shale formation damage occurs and to predict the long-term geochemical evolution of hydraulically stimulated shale systems. Ultimately, this knowledge will help improve hydrocarbon recovery, decrease contamination risk, and improve utilization of water resources.

• Three manuscripts in review or published, with additional manuscripts in preparation
• Featured lead article in the NETL Fall 2015 E&P Newsletter
• 23 presentations (seven invited) at (inter)national scientific/industrial venues
• Completion of all project milestones
• Four key scientific discoveries that have emerged from this work:
  1) Carbonate minerals are a master control on geochemical processes. Carbonate mineral content profoundly affects the geochemical response of shale to acidic fracture fluid. Hydraulic stimulation initiated by injection of acid drives dissolution of carbonate, pyrite, and silicate. Rapid carbonate dissolution buffers low-pH fluids, consuming acidity and reducing the overall chemical attack on other minerals. Consequently, carbonate-rich and carbonate-poor rocks respond differently to fracture fluids. The team's work shows that carbonate-rich shales rapidly neutralize acidic fracture fluids, leading to lower contaminant release but much more rapid oxidation and scale precipitation. In contrast, fracture fluids interacting with carbonate-poor, siliceous shales have lower pH values and release larger amounts of uranium and heavy metal contaminants, but result in relatively slow Fe(II) oxidation and dramatically reduced scale precipitation. These findings lead to specific models for chemical-mechanical formation damage, which the team is now linking to matrix permeability evolution and long-term production efficiency.
  
  2) Shale organics dramatically accelerate precipitation of iron scale. Oxygen (and oxidants such as persulfate) in fracture fluid causes pyrite to oxidize, releasing Fe(II) that can also be oxidized and precipitate as insoluble Fe(III) (oxy-)hydroxides (goethite, hematite, and ferrihydrite) that clogs pores and pore necks. The team's research shows that this process is strongly accelerated in the presence of bitumen leached from shale by fracture fluid, as well as by organic fluid additives. These findings indicate that iron control is more difficult to achieve than previously recognized and emphasize the importance of avoiding conditions that cause iron dissolution from Fe-bearing minerals such as pyrite. Moreover, this discovery implies that shale organics may be involved in other precipitation reactions, such as barite scale precipitation. The team's new project is taking direct aim at testing this hypothesis through a series of experiments designed to evaluate the impacts of shale organics on mineral precipitation. 
  
  3) Fracture fluid stimulates and then mitigates uranium release. The team's research shows that reaction of shales with acidic fracture fluid causes large-scale release of uranium. Moreover, kerogen in particular was found to release large amounts of metals, including iron, nickel, and lead. However, after initial reaction, dissolved uranium and metal concentrations decline over time spans of weeks to months. Our geochemical models show that uranium and metals are reabsorbed onto precipitating scale, most likely iron minerals. These findings imply that uranium and metal release can be controlled through manipulation of iron chemistry. 
  
  4) Reactivity of kerogen. Kerogenic pores are believed to be important to oil and gas transport. Fracture fluid was found to have minimal chemical impacts on kerogen. These results suggest that fracture fluid probably is not a significant source of damage to organic pore networks during hydraulic fracturing.

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Over the past two years, the team has used advanced synchrotron CT and spectroscopy techniques, and electron microscopy, laboratory, and modeling techniques to investigate fracture fluid-shale reactions. The team has focused on four shales that span a range of compositions and mineralogies: Marcellus, Barnett, Eagle Ford, and Green River. As noted above, this research has illuminated numerous interactions of practical significance between shale, fracture fluids and organic components of both shale and fracture fluid. All major objectives of the project have been accomplished on-schedule, while expanding the scope or the original proposal.

$500,000 

$0

NETL – David Cercone (david.cercone@netl.doe.gov or 412-386-6571) 
SLAC – John Bargar (bargar@slac.stanford.edu or 650-234-9919)

Chemical Control of Fluid Flow and Contaminant Release in Shale Microfractures (Aug 2017)
Presented by John Bargar, SLAC National Accelerator Laboratory, 2017 Carbon Storage and Oil and Natural Gas Technologies Review Meeting, Pittsburgh, PA

Chemical Control of Fluid Flow and Contaminant Release in Shale Microfractures (Aug 2016)
Presented by John Bargar, National Accelerator Laboratory, 2016 Carbon Storage and Oil and Natural Gas Technologies Review Meeting, Pittsburgh, PA