(756e) The Effects of Quartz Crystal Form on the Formation of Residual Water in Brine-CO2-Quartz Systems

The Effects of Quartz Crystal Form on the Formation of Residual Water in Brine-CO₂-Quartz Systems

Geological storage of CO₂ is considered to be a promising option for ensuring the necessary decrease of anthropogenic CO₂ emissions. Suitable geological formations for CO₂ capture and storage include: saline aquifers, un-minable coal seams, and depleted oil or gas reservoirs. Saline aquifers are more broadly distributed worldwide and have the greatest potential for CO₂ storage. Geo-sequestration of CO₂ in deep saline aquifers is achieved by injecting CO₂ into the aquifers and displacing brine. The residual water formed during the drainage process has a strong influence on traps of residual-gas. Moreover, in the context of CO₂ capture and geological storage, brine-CO₂ interfacial tension and brine-CO₂-quartz contact angles directly impact structural and residual trapping capacities. Therefore, we conducted experiments to investigate the effects of quartz crystal form in the rock cores on the formation of residual water, and to determine how much of this residual water remains after CO₂ is injected.

In our experiments, we selected three rock core samples: two of them are natural rock cores extracted from the Ordos Basin, and the other one is a customized artificial rock core. The natural sandstone cores were extracted from the No. 2 monitoring well from the Shenhua Group CCS Project. The well is located in northeastern part of the basin, near Ejin Horo Banner, Inner Mongolia. The cores were collected from the Triassic Heshanggou and Liujiagou Group sandstone formations that are widespread in the basin; they were extracted from depths of 1540.3 (sample #1) and 1690.5 (sample #2) m, respectively. The Heshanggou and Liujiagou Group formations are two of a number of storage formations currently used at the Shenhua Group CCS site. The artificial rock core(Sample #3) is homogeneous isotropic. The particle size and structure of the artificial rock core is similar to the 2 natural sandstone cores. This artificial rock core is composed of quartz sand cemented with epoxy resin. The grains are uniform and around 0.06 mm in diameter (260 mesh). The quartz content of the three samples was determined by X-ray diffraction analysis and it was 58%, 31%, 98% for samples #1, #2 and #3, respectively. Through field emission scanning electron microscope, we observed that the quartz in samples #1 and #2 is tabular, and the quartz in sample #3 is columnar. With the exception of quartz content and quartz crystal form, the 3 sandstone core samples have the same properties (porosity, permeability, mineral and chemical composition and distribution of pore throat sizes).

Firstly the 3 sandstone core samples were all saturated with 35 g/L NaCl brine, then supercritical CO₂ was injected into the rock cores under pressure and temperature conditions of CO₂ geological storage. The amount of displaced water and water-gas mixtures was collected and measured at intervals throughout a 72-h period. The results of the experiments showed that the permeability of rock cores can only influence the drainage efficiency; it does not have a decisive impact on the residual water saturation. The distributions of pore throat sizes and the porosity in the three rock core samples are basically identical. Thus, it is reasonable to conclude that the variation in residual water saturation is independent of the differences in the pore distributions and the porosity. Iglauer, S.(2012) demonstrated that brine-CO₂ interfacial tension(γ) and brine-CO₂-rock surface water contact angles(θ) impact structural and residual trapping capacities in the context of carbon geo-sequestration projects.

Baohe Wang(2014) presented that there is a relationship between interfacial tension and contact angles: cosθ=(γ_sv - γ_sl) / γ_lv, where γ_sv is the interfacial tension of the rock and CO₂, γ_sl is the interfacial tension of the rock and the brine, γ_lv is the interfacial tension of the brine and CO₂. It should be noted that the quartz crystal form is able to alter the surface energies both brine-rock and CO₂-rock. So the γ_sv and γ_sl of the artificial rock core is different from that of natural rock cores. CO₂, brine and pressure, temperature conditions that we used in these experiments were the same, which suggests that the γ_lv in these experiments has never changed. According to Equation: cosθ=(γ_sv - γ_sl) / γ_lv, θ for the natural samples and artificial sample are remarkably different from each other. In addition, θ affects the CO₂ capillary breakthrough pressure (P_c) that can be approximated by the Laplace equation: P_c=(2 γ_lv cosθ)/R , where R is the radius of the largest pore throat in the porous medium. Based on the above discussion we know that γ_lv was consistent in each experiment, and the pore characteristics of the samples are in agreement with each other. Therefore, P_c is dependent on θ.

The experiments show that the irreducible water saturations can be reached after about 60 h. And the residual water saturation was 50.12% for sample #1, 53.28% for sample #2, and 29.05% for sample #3. The results of the experiments demonstrated that the irreducible water saturation was connected with quartz crystal form in the rock core. The amount of the irreducible water was lower in the rock core of tabular quartz than in the rock core of columnar quartz. However, Sample #1, #2’s irreducible water saturation is not exactly the same. The variation seems likely to relate to their quartz content. Nonetheless, the factor would not substantially influence the irreducible water saturation under the experimental conditions described in this study.

Based on the water outflow rate curve of the experiment, we divided the process of drainage into three stages: Pushing Drainage, Portable Drainage and Dissolved Drainage. Meanwhile, we presented a capillary model to interpret the mechanisms that characterize the three stages in these experiments.

This study can help to define the kinds of sandstone aquifers most suitable for the geological storage of CO₂. And to a certain degree, the result gives quantitative evaluation to the volume of trapped CO₂ into the geological formation.