Lab NMR Study on Adsorption/Condensation of Hydrocarbon in Smectite Clay
- Jilin Zhang (Bakerhughes, Inc) | Jinhong Chen (BakerHughes) | Carl M. Edwards (Baker Hughes Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE/EAGE European Unconventional Resources Conference and Exhibition, 20-22 March, Vienna, Austria
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.1.1 Exploration, Development, Structural Geology, 4.3.1 Hydrates, 5.8.2 Shale Gas, 5.2 Reservoir Fluid Dynamics, 5.9.2 Geothermal Resources, 5.1 Reservoir Characterisation, 5.6.1 Open hole/cased hole log analysis, 4.6 Natural Gas, 5.2.1 Phase Behavior and PVT Measurements, 4.3.3 Aspaltenes, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment
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Significant amounts of gas accumulations exist in unconventional gas plays. Current understanding held that in unconventional shale plays, natural gas was stored as "free?? gas in pore spaces and as an "adsorbed?? phase on clay minerals and surface of organic pores material. The adsorption of methane has been confirmed in lab experiments in high-pressured gas chambers. Our lab experiments indicated that hexane vapor could be adsorbed onto organic-rich shale core samples through capillary condensation and the signal could be detected by Nuclear Magnetic Resonance (NMR) instruments. This study further examines the capillary condensation of hexane vapor into clay minerals and the NMR response.
Smectite samples from the Clay Minerals Society were used in the experiments. Two types of capillary condensation experiments were conducted: one with water vapor and the other with hexane vapor, both at room conditions. Weight gains indicated that some of the vapor condensed in the loose powder of smectite clay. NMR experiments were performed on vaporsaturated samples using a Maran 2 MHz spectrometer with an inter-echo time of 300 µsec.
The T2 distributions of the water-vapor and hexane vapor-saturated smectite clay were both unimodal. The water vaporsaturated sample showed a T2 at 0.5 ms, while the hexane vapor-saturated sample showed a T2 between 1 and 6 ms. This was likely due to the fact that the smectite crystallites have a small charge that has a more pronounced effect on polarized molecules such as water, than on non-polarized molecules such as hexane.
In recent years, there has been an increasing need to exploit natural resources in unconventional reservoirs. Coinciding with this are laboratory research activities to understand the gas storage modes of hydrocarbon inside shale samples and the detection and quantification thereof. Lu et al. (1995) carried out experiments on methane in illite clay samples at high pressures and used the pressure drop in the sample cell to quantify the amount of methane that can be adsorbed on the illitic clay. Sigal and Odusina (2011) studied the methane in Barnett shale samples under high pressure at ambient temperatures.
Their experiments indicated that the methane gas in the shale core has a T2 relaxation time of 8 to 9 ms. They concluded that the rapid relaxation of methane gas in a relatively homogeneous gradient was likely attributed to surface relaxation. From their experiments, it can be extrapolated that methane gas at high pressures in unconventional reservoirs exists in a super-critical state, with the physical characteristics between liquid and gas.
In addition to kerogen, another main component in unconventional shale gas samples is clay minerals, which are generally considered water wet (Saada et al., 1995). Recent studies have found that clay minerals can also be hydrocarbon wet. Despite the consensus that the amount of gas storage was largely positively related to the amount of total organic carbon (TOC), Lu et al. (1995) showed that a pure illite sample purchased from the Clay Mineral Society also adsorbed methane gas. Therefore, it was important not to neglect the hydrocarbon adsorbed on clay minerals in addition to that on kerogen material.
Our study shows that smectite clay can be partially hydrocarbon wet and that hydrocarbon can be adsorbed/condensed on the clay surface or in the minuscule pores between clay flakes when the hydrocarbon vapor pressure is less than or equal to the saturation pressure and that the adsorbed/condensed hydrocarbon can be detected by changes in weight and by NMR measurements. In our study, we used a smectite sample purchased from the Clay Mineral Society for hexane vapor saturation.
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