Tako Sandstone Research Articles

Numerical modeling and evaluation of lab-scale CO2-injection experiments based on electrical resistivity measurements

This paper presents numerical models for validating electrical resistivity (ER)-derived carbon dioxide (CO2) saturation obtained from lab-scale CO2 flooding experiments. ER assessments have generally been implemented using Archie's empirical equation, but it is only applicable to non-saline water, homogeneous medium, and non-conductive minerals. Therefore, it is necessary to develop a reliable numerical model to scale up experimental results by calibrating uncertain properties, such as capillary pressure, relative permeability, and aquifer heterogeneity. Cylindrical sandstone samples from the homogeneous Berea sandstone and heterogeneous Tako sandstone were prepared to evaluate CO2 transport within brine-saturated media. The reliability of the numerical method was verified by the trajectories of ER-derived CO2 saturation (SCO2) in the homogeneous Berea sandstone, which showed similar profiles and a root mean square error (RMSE) of 0.0503. The aquifer heterogeneity of the Tako sandstone influenced CO2 transport, i.e., its frontal velocity, by producing retardation at the less porous layers, whereas the homogeneous Berea sandstone showed consistent movement of the CO2 front. The numerical simulation of the heterogeneous Tako sandstone confirmed the effects of the less porous layers and matched the experimental profiles of SCO2, with an RMSE of 0.0421. The developed models enable validation of ER-derived SCO2 at the early stage and forecasting of the CO2 distribution in heterogeneous saline aquifers.

Tracking CO2 Plumes in Clay‐Rich Rock by Distributed Fiber Optic Strain Sensing (DFOSS): A Laboratory Demonstration

AbstractMonitoring the migration of pore pressure, deformation, and saturation plumes with effective tools is important for the storage and utilization of fluids in underground reservoirs, such as geological stores of carbon dioxide (CO2) and natural gas. Such tools would also verify the security of the fluid contained reservoir‐caprock system. Utilizing the swelling strain attributed to pressure buildup and the adsorption of supercritical CO2 on clay minerals, we tracked the fluid plume in a natural clay‐rich Tako sandstone at the laboratory core scale. The strain was measured by a high‐resolution distributed fiber optic strain sensing (DFOSS) tool. The strain changes induced by CO2 adsorptions on clay minerals were significantly greater than those caused by pore pressure alone. The distribution of the swelling strain signals effectively captured the dynamic breakthrough of the CO2 plume from the high‐ to low‐permeability regions in the Tako sandstone. Besides revealing the in situ deformation state, the measured strain changes can track the movement of the CO2 plume as it enters the clay‐rich critical regions in the reservoir‐caprock system. The present findings and potential future applications of DFOSS in the field are expected to enhance the monitoring and management of underground fluid reservoirs.

Fiber Optic Sensing for Geomechanical Monitoring: (1)-Distributed Strain Measurements of Two Sandstones under Hydrostatic Confining and Pore Pressure Conditions

In this study distributed optic fiber has been used to measure both the Rayleigh and Brillouin frequency shift of two different sandstone core samples under controlled hydrostatic confining and pore pressure in the laboratory. The Berea sandstone core is relatively homogeneous, whereas the Tako sandstone core is visibly heterogeneous with a coarse-grain and a fine-grain region. Rayleigh frequency has been found to have a superior performance over Brillouin frequency in terms of better consistency (less scattering) in the tests carried out. The strain gauge readings reveal considerable anisotropy in the stiffness of the Berea core between perpendicular (vertical) and parallel to the bedding (hoop) directions. The strains converted from Rayleigh frequency shift measurements agree reasonably well with readings by one of the four hoop strain gauge channels under increasing confining/pore pressure. For the Tako sandstone core, the contrast in the grain-size, and thus rock elastic properties, is clearly reflected in the hoop strain measurement by both strain gauges and distributed optic fiber. The outcomes of the test have demonstrated successfully the use of a single optic fiber for measuring rock strain response at different regions of a heterogeneous core sample along a continuous trajectory.

Monitoring CO2 drainage and imbibition in a heterogeneous sandstone using both seismic velocity and electrical resistivity measurements

In a laboratory fluid-injection experiment, seismic velocity and electrical resistivity were measured simultaneously to monitor injected carbon dioxide (CO2) during CO2 drainage and imbibition within a heterogeneous, clay-containing Tako sandstone sample. In the CO2 drainage process, supercritical CO2 (10 MPa at 40°C) was injected under a condition similar to that of an in situ reservoir. After the CO2 drainage process, water was injected into the CO2-injected sandstone for the CO2 imbibition stage. Employing strategies based on Gassmann fluid-substitution and Archie’s equation, P-wave velocities and electrical resistivities were interpreted to evaluate CO2 saturation (SCO2). Estimated values of SCO2 during the CO2 drainage process were compared with those of volume-derived SCO2, which were obtained by evaluating the volumes of injected and drained fluid. When Tako sandstone SCO2 is < 0.1, SCO2 estimation from P-wave velocity based on the Gassmann-Brie equation (with e = 12) is more precise than resistivity index (RI)-based Archie’s equation estimations from electrical resistivity. For further analysis, a modified RI equation was also employed to estimate SCO2, and the results were compared with those of the original RI-based Archie strategy.

Effect of Sub-core Scale Heterogeneity on Relative Permeability Curves of Porous Sandstone in a Water-supercritical CO2 System

Relative permeability curves of water and supercritical (sc) CO2 were obtained for Berea sandstone and Tako sandstone under steady-state flow conditions. The present measurements provide accurate relative permeability values of water and scCO2, which is important to better understand the behavior of CO2 injected into the deep saline aquifer for carrying out carbon dioxide capture and storage (CCS). The mixed fluid of CO2-saturated water and scCO2 was injected into the rock sample that was previously saturated by CO2-saturated water. The flow volumes of the water and scCO2 were measured for different saturation conditions by changing the mixing ratio of the fluids at the sample inlet. The relative permeability curve is then obtained by plotting the relative permeability values with respect to the degree of water saturation estimated by mass balance methods. However, a typical fitting equation (Books and Corey, 1966) cannot adequately approximate relative permeability curves of water-scCO2 systems. Particularly in Tako sandstone, the deviation of measured relative permeability from the calculated relative permeability curve is most likely to be due to sub-core scale heterogeneity due to inhomogeneous pore size distribution.

多孔質砂岩中の定常流動状態における水‐超臨界ＣＯ２系の相対浸透率

Relative permeability curves of water and supercritical CO2 (scCO2) were obtained for two porous sandstones under steady-state flow conditions. The present measurements provide accurate relative permeability values for water and scCO2, which is important to better understand the behavior of CO2 injected into the deep saline aquifer when carrying out carbon dioxide capture and storage (CCS). We have developed a new method for measuring the volume of scCO2 and water under the same pressure and temperature conditions as in the porous rock employing a high-pressure water-CO2 separator. Water saturation in the sandstone was determined with a mass balance analysis measuring changes of the surface boundary between water and scCO2. We measured Berea sandstone and Tako sandstone, which were cored and shaped into a cylinder form. The mixed fluid of water and scCO2 was injected into a rock sample that was previously saturated with CO2-saturated water. The flow volumes of the water and scCO2 were measured for different saturation conditions by changing the mixing ratio of the fluids at the sample inlet. All measurements were made at 40°C and 10 MPa, including those for the water-CO2 separator. The injection flow rate was 0.5 ml/min. By plotting the ratios of water and scCO2 of the inflow and outflow with respect to elapsed time, we can determine whether the flow is steady or unsteady. The relative permeability curve is obtained by plotting the relative permeability values with respect to the degree of water saturation. The results for Berea sandstone suggest that the relationship between relative permeability and the relative saturation of water and scCO2 correspond well to those of the previous study. However, the results for Tako sandstone deviate from the predicted curve of the previous study. This is likely to be due to the heterogeneity of Tako sandstone.

Supercritical CO 2 core flooding and imbibition in Tako sandstone—Influence of sub-core scale heterogeneity

This paper presents a numerical simulation study of a full CO 2 core flooding and imbibition cycle performed on a heterogeneous Tako sandstone core (measured 14.5 cm long and 3.68 cm in diameter). During the test, supercritical CO 2 (at 10 MPa and 40 °C) and CO 2-saturated brine was injected into one end of the horizontal core and a X-ray CT scanner (with a resolution of 0.35 mm × 0.35 mm) was employed to monitor and record changes in the fluid saturations, which enabled 3D mapping of the saturation profiles throughout the core during the course of core flooding test. The CO 2 flooding test demonstrated that (1) sub-core porosity heterogeneity had a marked impact on the CO 2 migration pattern within the Tako sandstone core at low injection rates (∼0.1 cm 3/min); (2) the influence of the porosity heterogeneity on the mean CO 2 saturation profiles along the core became gradually diminished as the injection rate was increased in steps to 3 cm 3/min. The numerical simulation results have shown that the immiscible displacement processes in the heterogeneous Tako core could not be adequately described by using a single capillary pressure curve in a 1D model of the core. This was found to be the case even when a 3D model (5 × 5 × 24) was used, where the porosity/permeability heterogeneity across the cross-sections, as well as along the core, was taken into account. Furthermore, the apparent correlation between the CO 2 saturation and the porosity (mean) profiles during the CO 2 flooding could largely be accounted for by employing a Leverett J-function type scaling factor, which reflects the influence of porosity/permeability heterogeneity on the capillary pressure.

Direct Observation of Trapped Gas Bubbles by Capillarity in Sandy Porous Media

We investigated the mechanism of residual gas trapping at a microscopic level. We imaged trapped air bubbles in a Berea sandstone chip after spontaneous imbibition at atmospheric pressure. The pore structure and trapped bubbles were observed by microfocused X-ray computed tomography. Distributions of trapped bubbles in Berea and Tako sandstone were imaged in coreflooding at a capillary number of 1.0 × 10−6. Trapped bubbles are of two types, those occupying the center of the pore with a pore-scale size and others having a pore-network scale size. In low-porosity media such as sandstone, connected bubbles contribute greatly to trapped gas saturation. Effects of capillary number and injected water volume were investigated using a packed bed of glass beads 600μm in diameter, which had high porosity (38%). The trapped N2 bubbles are stable against the water flow rate corresponding to a capillary number of 1.0 × 10−4.

Direct measurement of trapped gas bubbles by capillarity on the pore scale

Abstract To utilize aquifers without proven seal structures as a CO2 storage reservoir, CO2 should be trapped by residual gas trapping and solubility trapping as well as physical trapping by cap-rock. We investigated the mechanism of the residual gas trapping from a microscopic point of view by means of micro-focused X-ray CT. First, we imaged trapped air bubbles in a Berea sandstone chip after spontaneous imbibition at atmospheric pressure. With the resolution of 6.52 m/pixel, 610 sheets of slice images of 608×608 pixels were reconstructed. Pore structure and trapped bubbles were clearly visualized. Next, distributions of trapped bubbles in Berea and Tako sandstone were imaged in core-flooding at the capillary number of 1.0×10−6. The trapped bubbles consist of two-type; one occupies the center of pore with a pore-scale size and the other has a pore-network scale size. In low porosity porous media such as sandstone, connected bubbles has high contribution to trapped gas saturation. The distributions of trapped bubbles were uniform throughout the core except for the heterogeneity due to sedimentary layers. With a packed bed of glass beads 600 μm in diameter, the effects of capillary number and the injected water volume were investigated. Even in the packed bed of glass beads, which has a high porosity of 38%, once the gas bubbles are trapped, they are stable against the water flow rate that corresponds to the capillary number of 1.0×10−4. Even at very high capillary number of 4.7×10−5, residual gas saturation is independent of volume of injected water up to 15 pore volumes. Third, distributions of the residual gas saturation in the core were measured at supercritical conditions on core scale. The effect of fracture on the residual gas saturation was also investigated. The fracture reduces the maximum CO2 saturation to about 20% at the end of CO2 injection. However, most of the CO2 were trapped after the water injection. The Land type relationship between trapped CO2 saturation and the maximum CO2 saturation was obtained for supercritical CO2.

History matching of CO2 core flooding CT scan saturation profiles with porosity dependent capillary pressure

This paper presents a numerical simulation study of a CO2 core flooding test performed on a Tako sandstone sample, which measured 14.5 cm long and 3.68 cm in diameter. During the test, supercritical CO2 (at 10 MPa and 40 ∘C) was injected into one end of the horizontal core and a X-ray CT scanner (with a resolution of 0.35 mm×0.35 mm) was employed to monitor and record changes in the fluid saturations at 70 sections evenly spaced along the core length to enable 3D mapping of the saturation profiles throughout the core during the course of CO2 flooding and imbibition. Analysis of the mean saturation profiles along the core length (obtained by averaging the CT values over each cross-section) revealed a strong influence by the (mean) porosity distribution, though this influence became gradually less pronounced after CO2 breakthrough. A 1D model of the core was first constructed to simulate the CO 2 injection process and in particular to history match the evolution of the CT scan CO2 saturation profiles along the core length with time. It was found that a reasonably match could be achieved by using a porosity-dependent capillary pressure multiplier. The multiplier was also dependent on injection time (CO2 saturation level). In order to match the saturation profiles, it needs to be gradually scaled down as more CO2 is injected.

Seismic monitoring and modelling of supercritical CO 2 injection into a water-saturated sandstone: Interpretation of P-wave velocity data

This paper reports on an integrated laboratory and numerical simulation study of ultrasonic P-wave velocity response to supercritical CO 2 displacement of pore water in Tako sandstone. The analysis of dynamic velocity data recorded using an array of piezoelectric transducers mounted on a core sample showed that the P-wave velocities at different positions displayed a similar trend in time, i.e., an initial sharp fall followed by a more gradual decline. Considerable variations observed in the measured P-wave velocity reductions across the sandstone core could largely be attributed to the final state of saturation (e.g. uniform, patchy or in-between) attained by the two-phase fluids. Numerical simulation of the injection test using a simple 1D model was carried out to provide an estimation of the phase saturation changes underlying the measured P-wave velocity reductions. A second order polynomial correlation between the measured ultrasonic P-wave velocity reductions and the estimated CO 2 saturation was established. Comparison with the Gassmann velocities showed that the empirically established relationship marks a clear deviation from both the patchy and uniform saturation velocity curves.

Laboratory Study of CO2 Migration in Water-Saturated Anisotropic Sandstone, based on P-Wave Velocity Imaging

We measured the changes in P-wave velocity that occur when injecting CO2 in gaseous, liquid, and supercritical phases into water-saturated anisotropic sandstones. P-wave velocities were measured in two cylindrical samples of Tako Sandstone, drilled along directions normal and parallel to the bedding plane, using a piezo-electric transducer array system. The velocity changes caused by CO2 injection are typically -6% on average, with maximum values about -16% for the case of supercritical CO2 injection. P-wave velocity tomograms obtained by the differential arrival-time method clearly show that CO2 migration behaviour is more complex when CO2 flows normal to the bedding plane than when it flows parallel to bedding. We also found that the differences in P-wave velocity images were associated both with the CO2 phases and with heterogeneity of pore distribution in the rocks. Seismic images showed that the highest velocity reduction occurred for supercritical CO2 injection, compared with gaseous or liquid CO2 injection. This result may justify the use of the seismic method for CO2 monitoring in geological sequestration.

An experimental study on seismic monitoring of a CO 2 flooding in two sandstones

Laboratory experiments were conducted to measure compressional wave velocities under hydrostatic pressure in Shirahama and Tako sandstone with a porosity of 12 and 24%, respectively. In dry samples under hydrostatic pressure, the compressional wave velocity increased significantly in Shirahama sandstone, as a result of the closure of micropores with low aspect ratio, while the velocity in Tako sandstone was less affected. The pressure dependence of velocity is ascribed to the difference in micropore structure. Velocities were also measured to map the movement of the injected CO 2 within a water-flooded sample during CO 2 flooding. In the water-flooded sample, velocity changes caused by the CO 2 flooding, provided at the bottom end of the sample, are typically on the order of −6%. The compressional wave velocities decreased simultaneously along horizontal paths in two orthogonal directions.

二酸化炭素注入に伴う多孔質砂岩の透気係数と弾性波速度の測定に関する実験的研究

Laboratory experiments were conducted to measure gas permeability and P-wave velocity under hydrostatic pressure in Shirahama and Tako sandstones with porosity of 12% and 24%, respectively. In dry samples, gas permeability and P-wave velocity were significantly changed with increasing hydrostatic pressure in Shirahama sandstone, as a result of the closure of micropores with low aspect ratio, while in Tako sandstone they were less affected. In water-saturated sample, velocity changes caused by CO2 injection, are typically on the order of -6%. P-wave velocities decreased almost simultaneously along horizontal paths in two orthogonal directions. Results of P-wave velocity tomography suggested that cross-well seismic profiling is useful to monitor migrations of the injected CO2 in geological sequestration sites.

流理性および成層性岩石における水圧破砕き裂進展のＡＥによる標定

In this study hydrofracture propagation in discontinuous rocks were investigated by Acoustic Emission (AE) locations. Two kinds of rocks, Inada granite and Tako sandstone, were used in the experiments. Inada granite is an anisotropic rock with rift planes and Tako sandstone has discontinuous planes. AE measurements were carried out by 12 piezoelectric transducers.Hydrofracture propagation in Inada granite were controlled by rift planes. Under unconfined condition hydrofractures were oriented to rift plane. Under confined condition, however, the initiation fracture were caused by the stress condition. In the stage of fracture extension, the fracture changed its direction depending on the rock anisotropy.In the case of Tako sandstone discontinuous plane had a great influence on the propagation of hydrofracture. Discontinuous planes prevented fractures from propagating through these planes. And, the majority of the AE events were located below the discontinuous plane and predominant fracture growth was downwards.