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.

Sandia National Laboratories (SNL), Albuquerque, NM 87123

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.

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 CH4 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 CH₄ 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 CH₄ 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 CH₄ 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 CH₄ extraction and CO₂ sequestration. The work will leverage unique SNL capabilities: nanogeochemistry, high-pressure and -temperature geochemistry, numerical modeling, nanoscience, and neutron scattering.

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 CH₄ 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 measurements indicate significant sorption of CH₄ (and CO2) on clay materials. Given the dominant presence of clay materials in shale, the sorption of CH₄ 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 CH₄ sorption on the model kerogen materials. The relative release rates of adsorbed CH₄ 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.
• Kerogen extracted from Woodford shale and Marcellus shale have been characterized using Attenuated Total Reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and Brunauer-Emmett-Teller (BET). CO₂ adsorption and desorption isotherms for extracted kerogen were measured at 273 K and 300 K. The specific surface area for kerogen from Woodford shale is 50 cm2/g and 357 cm2/g for Marcellus kerogen, which are consistent with maturity measurement by vitrinite reflectance with Woodford shale being immature and Marcellus shale being mature.
• Marcellus kerogen pore-size distributions, obtained using classic Density functional theory (DFT) model from liquid nitrogen adsorption, shows bimodal distributions with micropore (2 nm or less) accounting for more than 75% of the total pore volume and the rest being 25% pore volume from pores with 20–30 nm diameter. There are no pores between 3 nm and 20 nm (confirmed by scanning electron microscope (SEM) images) from Marcellus shale, which is very unusual for mature shale with maturity of 2.2 (VRo). Kerogen spatial heterogeneity in maturity is being analyzed using FTIR microscope and Raman microscope.
   

The project is 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.

The model also indicates that kerogen may swell upon gas sorption, which may affect the porosity and permeability of shale. Using a hybrid molecular dynamics/Monte Carlo simulation technique, an investigation was done on the swelling properties of kerogen upon helium (He), CH₄, and CO₂ adsorption. The results indicate that kerogen insignificantly swells under helium adsorption. However, kerogen volume increases up to 5.4% and 11% upon methane and carbon dioxide adsorption at 192 atm, respectively. The kerogen volume increases when gas pressure increases, and at the same gas pressure it increases more in carbon dioxide than in methane. Because of the swelling nanostructural properties of kerogen including porosity, surface area, and pore size, distribution change significantly. It is therefore necessary to include kerogen swelling in the porosity, permeability, and adsorption data interpretation, and provide a mechanistic understanding of the interaction between kerogen and gas. This will be useful for shale gas and carbon sequestration applications.

Recently, science has gained a better understanding of fluid flow in the nanopores of shales. Permeability of fluid in nanochannels significantly depends on the wettability, which is usually very low, largely due to the adhesion of fluid at solid interfaces. Using molecular dynamics simulation, SNL has demonstrated that the permeability of water in a hydrophilic nanochannel can be enhanced by an atomistic lubricant. SNL results indicate that water partially wets the kerogen surface. However, when a small quantity of supercritical CO₂ is present in the system, an atomically thin CO₂ film forms at the water-kerogen interface that creates a non-water-wetting interface. Due to the transition from hydrophilic to super-hydrophobic interface, a thin CO₂ film acts like a lubricant that transforms the water flow from a stick to a slip hydrodynamic boundary condition.

$1,021,926

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

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