Unconventional Resources
Fundamental Understanding of Methane-Carbon Dioxide-Water (CH4-CO2-H2O) Interactions in Shale Nanopores under Reservoir Conditions Last Reviewed June 2017

FWP-14-017608

Goal
Project personnel will systematically study CH4-CO2-H2O interactions in shale nanopores under high-pressure and -temperature reservoir conditions, with the ultimate goal of developing new stimulation strategies to enable efficient and less environmentally harmful resource recovery from fewer wells.

Performer
Sandia National Laboratories (SNL)

Background
Shale is characterized by the predominant presence of nanometer-scale (1-100 nm) pores.  The behavior of fluids in those pores directly controls shale gas storage and release in the shale matrix and, ultimately, the wellbore production in unconventional reservoirs.  It has been recognized that a fluid confined in nanopores can behave dramatically differently from the corresponding bulk phase due to nanopore confinement.  CO2 and H2O (either preexisting or introduced) are two major components that coexist with shale gas (predominately CH4) during hydrofracturing and gas extraction.  Liquid or supercritical CO2 has been suggested as an alternative fluid for subsurface fracturing such that CO2 enhanced gas recovery can also serve as a CO2 sequestration process.  Limited data indicate that CO2 may preferentially adsorb in nanopores (particularly those in kerogen) and displace CH4 in shale.  Similarly, the presence of moisture seems able to displace or trap CH4 in the shale matrix.  Therefore, fundamental understanding of CH4-CO2-H2O behavior and their interactions in shale nanopores is vitally important for gas production and related CO2 sequestration.

Impact
The proposed work will address the following knowledge gaps:

  • Most existing gas sorption work has focused exclusively on single component systems. No measurements have been made for a mixture of CH4-CO2-H2O under reservoir conditions.  Molecular dynamics simulations of a CO2-CH4 mixture in carbon nanopores indicate complex interactions between the two components.
  • Sorption-desorption hysteresis is typically observed of fluids in nanoporous materials. We hypothesize that hysteresis may occur during methane sorption-desorption in the shale matrix. If confirmed, this hypothesis will provide us with a completely new perspective for understanding shale gas disposition and release in an unconventional reservoir. Unfortunately, no measurements have been made thus far on the entire cycle of methane sorption and desorption in shale samples. The presence of CO2 or H2O is likely to promote hysteresis for CH4 sorption and desorption.
  • The fundamental question regarding methane disposition and release in shale pores requires an atomistic understanding of the physical and chemical interactions among shale materials, CH4, H2O, and CO2.  However, no systematic simulation study has been performed to specifically examine methane behavior in nanopores of different materials, sizes, and shapes in the presence of other fluid phases for the purpose of understanding the complexity of natural gas recovery from shale reservoirs.
  • The equation of state (EOS) for predicting the thermodynamic behavior of CH4-CO2-H2O in shale is still missing.

Project personnel propose to bridge these gaps by using an integrated experimental and modeling approach to systematically study CH4-CO2-H2O interactions in shale nanopores under high-pressure and -temperature reservoir conditions.

The proposed research will (1) significantly advance fundamental understanding of hydrocarbon storage, release, and flow in shale; (2) provide more accurate predictions of gas-in-place and gas mobility in reservoirs; (3) help to develop new stimulation strategies to enable efficient and less environmentally harmful resource recovery from fewer wells; and (4) provide the basic data set to test the concept of using supercritical CO2 as an alternative fracturing fluid for simultaneous methane extraction and CO2 sequestration. The work will leverage unique SNL capabilities: nanogeochemistry, high-pressure and -temperature geochemistry, numerical modeling, nanoscience, and neutron scattering.

Accomplishments
Project researchers obtained 10 shale core samples (including samples from Mancos, Woodford, and Marcellus) and model materials. They also obtained one relatively pure kerogen isolate from Mancos shale. Additional kerogen extractions from Woodford shale (immature) and Marcellus shale (mature) were prepared using solvent extraction, acid demineralization, and critical point drying. Low angle neutron scattering analysis was performed on Mancos shale samples. The team designed and constructed a unique high-temperature and -pressure experimental system that can measure both of the P-V-T-X properties and adsorption kinetics sequentially. Researchers completed the first set high-temperature, high-pressure (HTHP) measurement for a gas mixture of 90% CH4 and 10% CO2. More measurements for other mixtures with different CH4 and CO2 concentrations are currently underway. Complementary to the HTHP experiments, researchers have completed a set of high-temperature and low-pressure methane adsorption measurements using a thermal gravimetric analyzer. Significant progress has been made on molecular dynamics modeling. The major findings up to date include:

  • Neutron scattering analysis shows the pores in Mancos shale are dominated by those with size < 100 nm, consistent with reported data for shale in general. Specific surface area of kerogen from Mancos shale is determined to be ~ 2 m2/g, smaller than expected.
  • Both low-pressure and high-pressure sorption measurement indicate significant sorption of methane (and CO2) on clay materials. Given the dominant presence of clay materials in shale, the sorption of methane on clays may contribute a significant portion of gas in place in a reservoir.
  • The sorption measurement on activated carbon (used as an analog to kerogen) indicates that sorption capacity of kerogen could be one order of magnitude higher than that of clay materials.
  • The HTHP measurement conducted has demonstrated the applicability of SNL’s hydrothermal system for shale gas study. A thermodynamic model (EOS) for data interpretation was developed for estimating sorption capacity and rate.
  • Various kerogen models have been constructed for molecular dynamics (MD) simulations. The MD simulations have calculated the sorption isotherms for methane sorption on the model kerogen materials. The relative release rates of adsorbed methane molecules on different kerogen sites have also been calculated.
  • Completed full cycle of adsorption-desorption measurements.
  • Performed high-pressure and high-temperature sorption measurements on crushed shale samples.
  • Performed sorption measurements on multicomponent systems to clarify the interactions among different components (CH4-CO2-H2O).
  • Complete the calculations on CH4-CO2-H2O sorption and desorption in kerogens.
  • Experimental data interpretation and atomistic modeling was used to understand how molecular structures of kerogen control gas (CH4, CO2, and H2O) sorption and desorption. Related work was presented at the Annual American Geological Society Meeting, and the American Chemical Society Meeting.
  • Previous research showed that kerogen dried under air has collapsed, damaged pore structure, and does not represent the original state of kerogen in shale. To remedy this situation, critical point drying was accomplished by passing the liquid to a gas state without crossing the phase-boundary. All subsequent analyses will be using these pore-preserved kerogen materials.

Current Status (June 2017)
The existing shale collection is being expanded and characterized for mineral compositions and pore structures. Work is continuing on systematic measurements of CH4-CO2 sorption and desorption on ~6 shale samples or kerogen materials in the presence or absence of H2O under reservoir conditions.

The project is also working on developing a new kerogen model that can provide a better representation of molecular structures of the material as revealed by spectroscopic data. To this end, SNL is focused on the interaction of CH4-CO2-H2O with clay components. The porosity of clay aggregates determines the permeability, ion exchange capacity, gas loading, and fluid migration in shales. Pore spaces in clay-rich rocks include interlayer and interparticle pores. Under compaction and dewatering, the size and geometry of such pore spaces might vary depending on formation conditions. The work focuses on a molecular dynamics simulation method to build complex and realistic clay aggregates with interparticle pores and interparticle boundaries. This model is then used to investigate the effect of dewatering and water content on the micro-porosity of clay aggregates. The results suggest that slow dewatering will create more compact aggregates compared to fast dewatering. The results also indicate that water content affects the porosity of the clay aggregates. In addition, the work indicates that CO2 has a much higher affinity for clay materials than CH4, as compared to matured kerogen. Future work will focus on gas adsorption and release from clay aggregates and compare these results with those obtained for kerogen.

Project Start: October 1, 2014
Project End: September 30, 2018

DOE Contribution: $850,000

Contact Information:
NETL – Bruce Brown (Bruce.Brown@netl.doe.gov or 412-386-5534)
Sandia National Laboratories – Yifeng Wang (ywang@sandia.gov or 505-844-8271)

Additional Information:

Fundamental Understanding of Methane-Carbon Dioxide-Water (CH4-CO2-H2O) Interactions in Shale Nanopores under Reservoir Conditions (Aug 2017)
Presented by Yifeng Wang, Sandia National Laboratories, 2017 Carbon Storage and Oil and Natural Gas Technologies Review Meeting, Pittsburgh, PA

Quarterly Research Progress Report October - December, 2015

Quarterly Research Progress Report July - September, 2015

Quarterly Research Progress Report April - June, 2015

Quarterly Research Progress Report January - March, 2015

Quarterly Research Progress Report October - December, 2014