Gas hydrate has become an interesting issue to scientists who are engaged in energy, climate change, and earth science researches. There are several identifications on global and local occurrences of gas hydrate. Moreover, it is needed to do further studies on gas considering the formation, concentration and accumulation. Still the exact origin of gas is not investigated; therefore, transportation of gas through rock mass by fluids is yet a critical element in the present world.
Several researchers such as Etiope and Martinelli (2002) have compiled the geogas theory. One of the important finding of geogas theory said that rapid movement of endogenetic gases through faults and fractures in the crust can form microbubbles and move upward. Accordingly migration of carrier gas by bubbles can be considered as an important transport mechanism in earth surface. They identified that gas is able to migrate in advection or diffusion depending on the condition of the gas-water-rock system in various forms.
Several experiments on microbubble transport through porous medium have been conducted by previous researchers such as Wan et al. (2001). In relation to transport fluid through porous media, several researches by using Berea sandstones have been conducted by previous researchers such as Dey (1986) and Zhou et al. (2010). In addition to the major classical reasons of permeability change such as clay rearrangements, temperature changes, and precipitation of impurities, trapped bubbles was considered as substantial reason (Bona et al., 2009). However, effect of microbubble on fluid flow through the rock is not clear yet. Therefore, in this study, the effect of microbubble existence in the fluid flow through rock pores and fractures were investigated by conducting hydraulic conductivity tests to investigate the intrinsic permeability of the rock. Since there are many parameters that influence the flow of fluids through rock pores, some preliminary tests also had been carried out to investigate rock sample and microbubble in order to check the feasibility of the hydraulic conductivity test.
MATERIAL AND PRELIMINARY TEST
Microbubble (Fig. 1) is defined by several authors as a bubble with a spherical shape, is ≤ 100 μm in size (Sebba, 1987 cited in Choi et al., 2008). Takahashi (2005) was identified that some electrolyte ions could be attracted on microbubble surface. In the present study microbubble was generated by a machine by mixing a gas into filtered tap water using disk impeller which rotates at high speed under some pressure.
Initially, particle size distribution tests were conducted by using SALD-3100 Laser Diffraction Particle Size Analyzer as to measure the microbubble dimensions which were produced by
the generator. In this study, the fluid that was used for the hydraulic conductivity test has 0.463% bubble fractions of the total volume of the fluid. Fig. 2 (a) shows the numbers of microbubble particles were measured immediately after the generator is turned off. It can be observed that the maximum distribution of particle size diameter is 40-50μm. The numbers of microbubble particles were decreasing by the time as can be observed in Fig. 2 (b) and the fluid gradually became transparent. On the other hand, larger sizes of bubbles were attached on the wall of the water container. Subsequently, the microbubble particles were disappeared after twenty hours.
Berea Sandstone is a sedimentary rock with predominantly angular grains. It is composed of quartz sand held together by silica and kaolinite clay matrix. The relatively high porosity (21.64%), permeability (5×10-11-5×10-10 m2), and uniform material properties of Berea Sandstone make it suitable as the standard sample of reservoir rock in oil and gas industry. Obtained average grain density value for the specimens is 2,661 kg/m3.
Pore size distribution analysis as the results of Mercury Intrusion Porosimetry tests of Berea sandstone samples are illustrated in Fig. 3. It can be observed that the maximum distribution of pore size diameter is 30-40μm. It may be smaller than the size of microbubble particles. Nevertheless, the graphs also show the existence of pores greater than 100μm in diameter which greater than the size of microbubble particles. Besides, microscopic image of Berea sandstone was investigated to observe the existence of pores greater than 100μm as shown in Fig. 4. Grey and brown colors are quartz particles, whereas the pores are shown in blue color. Some pores have a relatively large diameter (100μm). Hence, it is possible that the microbubble could pass through pores if the pores are connected enough.
APPARATUS AND TEST PROCEDURES
Rectangular plate samples having 38.5mm length, 9mm height, and 2.45mm width were prepared from a core sample. Sample was placed in the middle of a cylindrical vessel that has a 10mm height and 40mm diameter (Fig. 5), and then its perimeters were sealed by Silicon rubber.
Figure 6 shows the apparatus configuration scheme. First, water or water containing microbubble flows from the storage (A) to the cylindrical vessel (B), then discharged into an output vessel above the electric balance (C). The weight of discharged fluid, the pressure conditions inside the cylindrical vessel, and fluid temperature were recorded by a data acquisition system set (D&E). However, the fluid temperature inside the vessel was not controlled. Pressure condition determined by the water head differences between storage (A) and discharge tube. In addition, to investigate the situation inside the cylindrical vessel during the test, camera (F) was installed on the system.
Two sets of tests were conducted. First is a set of tests that conducted on four different samples. Each sample had an at least twice of each water flow test then followed by water containing microbubble flow tests. Second is a set of repetition tests that conducted on a sample. Each after completion of the test by using water containing microbubble, surface of the sample was cleaned and left at room temperature for 24 hours. Besides, pressure condition of the tests was set to be constant by maintaining persistent water head position for each test.
The hydraulic conductivity and intrinsic permeability values were calculated by using Eq. (1) and Eq. (2) as follows:
where: K = hydraulic conductivity (m/s); Q = quantity of discharged water (m3); L = specimen thickness (m); A = cross sectional area of specimen (m2); t = discharge time (s); h = head
difference (m); к = intrinsic permeability (m2); μ = dynamic viscosity (kg/m-s); ρ = fluid density (kg/m3); and g = gravity acceleration (m/s2). In determining μ value, effect of the temperature and microbubbles volume fractions were also taken into account.
RESULTS AND DISCUSSION
Table 1 and Fig. 7 show the results of entire tests on four different samples. The temperatures for the tests by using water containing microbubble were higher than that using water due to the influence of the heat which is generated by microbubble generator machine. The results of permeability tests by using water show certain different values. Differences in the water content of the samples at the beginning of the tests are expected to be the cause of the discrepancy. However, the results of tests by using water containing microbubble show comparatively similar values since the tests were conducted after the tests by using water while the water content of samples relatively uniform. It also can be observed from Fig. 7 that microbubble caused substantial declines on intrinsic permeability values in the entire tests. It is 0.66-0.76 times of permeability obtained from the tests by using water.
However, the effect is not so significant in the range of 20-30⁰C (Baudracco and Aoubouazza, 1995). Indeed, the permeability change could mainly cause by the existence of microbubble in the fluid. One possible presumption that caused the permeability decline is that the microbubbles were trapped on the grains or blocked small pores causing more resistance on the flow. This presumption is supported by another result shown in Fig. 8. The permeability value was recovered after cleaned. In the other hand, permeability decrements due to changing of clay arrangement and precipitation of impurities could not easily recover. As well temperature also affects the permeability.
Electrical properties of microbubble surface could be the potential reason for microbubble to be attached on other materials such as mineral grains of rock or sediments. However, the physical behavior of the interaction between the microbubble and the rock grains still not investigated yet. Hence, further studies on the behavior of microbubble transport movement through sediment grains are required to investigate into such a presumption.
Results of flow tests performed to investigate the effects of microbubbles on the permeability value of Berea sandstone shows that microbubbles caused the substantial decline in permeability values. It had decreased for more than twenty percent compared with the permeability value of water flow tests. In spite of that fact, physical behavior of the interaction between the microbubbles and the rock grains is not clearly understood. In order to investigate that physical behavior, further studies are required.
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- Masahiko Osada; Geosphere Research Institute, Saitama University
- Takato Takemura; Geomechanics Laboratory, Department of Geosystem Sciences, Nihon University
- Manabu Takahashi; National Institute of Advanced Industrial Science and Technology Japan