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:
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:
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.
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