Ziqiu Xue

Seismic monitoring and modelling of supercritical CO2 injection into a water-saturated sandstone: Interpretation of P-wave velocity data

Abstract This paper reports on an integrated laboratory and numerical simulation study of ultrasonic P-wave velocity response to supercritical CO2 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 CO2 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.

Lessons from the First Japanese Pilot Project on Saline Aquifer CO2 Storage

Several key questions need to be answered when CO2 geological storage is to be undertaken worldwide. How should CO2 be stored underground? Can trapping be assumed in saline formations and can CO2 be retained for long periods safely in the subsurface? The first Japanese pilot-scale CO2 sequestration project in Nagaoka was undertaken to provide answers to these questions. The injection site is located at the Minami-Nagaoka gas field in Nagaoka City, 200km north of Tokyo. Supercritical CO2 was injected into an onshore saline aquifer at a depth of 1,100m. CO2 was injected at a rate of 20 to 40 tonnes per day over an 18-month period, with a cumulative amount of 10,400 tonnes. A series of monitoring activities, which consisted of time-lapse well logging, crosswell seismic tomography, 3D seismic survey and formation fluid sampling, was carried out successfully to monitor CO2 movement in the sandstone reservoir. This paper presents an overview of the results obtained from both field and laboratory studies to examine the spatial-time distribution of CO2 and various trapping mechanisms in the reservoir. CO2 breakthrough at two of the three observation wells was clearly identified by changes in resistivity, sonic P-wave velocity and neutron porosity from time-lapse well logging. Each velocity difference tomogram obtained by crosswell seismic tomography showed a striking anomaly area around the injection well. As the amount of injected CO2 increased, the low-velocity zone expanded preferentially along the formation up-dip direction during the first two monitoring surveys and less change around the CO2-bearing zone could be confirmed from the following surveys. Unfortunately there was no significant change in 3D seismic results due to CO2 injection. The pilot-scale project demonstrated that CO2 can be injected into a deep saline aquifer without adverse health, safety or environmental effects. The Nagaoka project also provides unique data to develop economically viable, environmentally effective options for reducing carbon emissions in Japan.

Monitoring and detecting CO2 injected into water-saturated sandstone with joint seismic and resistivity measurements

As part of basic studies of monitoring carbon dioxide (CO2) storage using electrical and seismic surveys, laboratory experiments have been conducted to measure resistivity and P-wave velocity changes due to the injection of CO2 into water-saturated sandstone. The rock sample used is a cylinder of Berea sandstone. CO2 was injected under supercritical conditions (10 MPa, 40°C). The experimental results show that resistivity increases monotonously throughout the injection period, while P-wave velocity and amplitude decrease drastically due to the supercritical CO2 injection. A reconstructed P-wave velocity tomogram clearly images CO2 migration in the sandstone sample. Both resistivity and seismic velocity are useful for monitoring CO2 behaviour. P-wave velocity, however, is less sensitive than resistivity when the CO2 saturation is greater than ~20%. The result indicates that the saturation estimation from resistivity can effectively complement the difficulty of CO2 saturation estimations from seismic velocity variations. By combining resistivity and seismic velocity we were able to estimate CO2 saturation distribution and the injected CO2 behaviour in our sample.

LABORATORY MEASUREMENTS OF SEISMIC WAVE VELOCITY BY CO2 INJECTION IN TWO POROUS SANDSTONES

Publisher Summary This chapter presents a preliminary result of measurements on velocity changes while injecting CO2 into water-saturated Shirahama and Tako sandstone. Wave velocity and attenuation in porous sandstone are widely studied in fields of reservoir engineering and geo-engineering. Seismic survey provides substantial information concerning positions for new wells and modification of the existing depletion strategy. Cross-well seismic tomography is considered as a promising monitoring method to map the movement of CO2 in the subsurface. The formation water, which existed in pore spaces within reservoir rocks, will be partially displaced by the injected CO2. This process will affect the propagation characteristics of the seismic waves. Seismic properties depend on the mineralogical composition of the rock as well as factors such as porosity, fluid content, and in situ stress. Previous works on effects of CO2 flooding on seismic wave velocity clearly show that CO2 flooding caused compressional wave (P-wave) velocities to substantially decrease. Interpretation of seismic monitoring of CO2 flooding requires an understanding of the effects of pore pressure buildup caused by the CO2 injection and CO2 saturation. Experimental studies, such as converting field measurements of wave velocities and attenuations to CO2 saturation, support the interpretation of the survey results. A series of seismic tomography experiments on porous sandstone samples to demonstrate the use of cross-well seismic profiling for monitoring the migration of CO2 in geological sequestration projects have been conducted.

Time-Lapse Seismic Crosswell Monitoring of CO2 injected in an Onshore Sandstone Aquifer

We present a case study of time-lapse seismic monitoring of CO2 injection by time delay tomography. During 550 days, 10.400 tonnes of CO2 were injected into a porous reservoir sandstone at 1100 m depth. Crosswell data were acquired before and after the CO2 injection. The estimated tomographic velocity image shows a clear time-lapse velocity anomaly on the order of -10 % below the CO2 injection well head.

Effects of fluid displacement pattern on complex electrical impedance in Berea sandstone over frequency range 104–106 Hz

ABSTRACT To better understand the effect of fluid distribution on the electric response of rocks saturated with oil and brine, we conducted experimental studies on the complex electrical impedance in a Berea sandstone, together with in situ acquisitions of oil distribution images employing a high‐resolution medical X‐ray computed tomography. We performed two tests of brine displacement by oil under high (10 MPa) and low (5 MPa) pressures, which were accompanied by fingering and stable displacement patterns, respectively. The measured complex impedance data were fitted to the Cole model to obtain the resistance, capacitance, peak frequency of the imaginary impedance, and the exponent α of the rock–fluid system. With increasing oil saturation, the resistance showed an increasing trend, whereas the other three parameters decreased. The fingering displacement exhibited lower resistance and capacitance than the stable displacement. The analysis of the resistance changes using a simple parallel connection model indicates that there are more components of residual brine in parallel connections in the fingering pattern than in the stable displacement pattern at the same saturation. We also interpreted the normalised changes in the capacitance (or apparent dielectric constant) with respect to the oil saturation via an analysis of the shape factor of fluid distribution based on the Maxwell–Wagner–Brugermann–Hanai model. The changes in the shape factor suggest that the pinch‐off of the brine in parallel connection by the oil is a dominant mechanism reducing the capacitance. In the stable displacement, most of the connections in the brine phase are immediately pinched off by oil displacement front at a local oil saturation of 65%. Conversely, in the fingering displacement, there is a transition from the bulk or layered brine to the pinched‐off at a local oil saturation below 60%. The analyses indicate that the difference in the fluid distribution under different fluid conditions is responsible for the non‐Archie behaviour.

Applying differential analysis to cross-well seismic survey for monitoring CO2 sequestration

Summary Capturing CO2 directly from large stationary sources such as thermal power plants and subsequently storing it in a nearby aquifer can be the most efficient way to reduce CO2 emissions into the atmosphere at a lower cost, and monitoring injected CO2 is important to verify practical effectiveness of the CO2 storage. The pilot-scale CO2 sequestration experiment has been undertaken at the Nagaoka gas and oil field, in Niigata Prefecture, Japan and we conducted time-lapse cross-well seismic tomography to detect the spread of injected CO2. When super-critical CO2 spreads into porous media saturated with brine water, the seismic velocity always decreases, which is confirmed theoretically and experimentally. Therefore, CO2 flood area can be estimated from seismic tomography records before and after CO2 injection. However, the tomography records before and after CO2 injection include not only the variation due to CO2 flood, but also differences occurred from the location and property of sources and receivers and they result analysis errors. Thus, in this study, we applied the differential analysis to cross-well seismic tomography for reducing the difference due to measurement conditions and getting more accurate monitoring results. In the normal procedure, velocity differences are calculated after applying inversion processing to each record separately, but in this differential analysis, first forward modeling is applied using the velocity distribution inverted from one referential record, second the set of the first arrival time is obtained from the modeling result and finally the inversion processing is analyzed from the data set of the first arrival time added with the difference of travel-times from the referential record. In the result, we can have an analysis result in which the distribution of CO2 flooding is clearly identified along with a caprock under the inversion analysis with the constrained condition of no velocity enhancement.

Coal matrix swelling caused by adsorption of carbon dioxide and its impact on permeability

Publisher Summary This chapter demonstrates involving laboratory measurements on coal matrix swelling that were carried out during carbon dioxide (CO2) injection, by monitoring strain and P-wave velocity changes. Sequestration of carbon dioxide (CO2) into coal seams has been proposed as one of the most attractive option of all geological sequestration possibilities. When injecting CO2 into coal seams, the CO2 is stored and the recovery of coalbed methane is enhanced concurrently. Permeability of coal is a very important factor in controlling the CO2 injection and the production of methane. Coal matrix swelling associated with CO2 adsorption is markedly greater than the shrinkage due to the methane desorption and, hence, there is a net reduction in permeability. This creates challenge in understanding the subsurface behaviors of injected gas and the coal. Swelling is a time dependent phenomena associated with the diffusion of the injected CO2 from cleats or fractures into micropores within coal matrix. To minimize the influence of the effective stress change due to the injection of CO2, samples with a distinct fracture passing through both top and bottom ends of the sample were used in the study presented in the chapter. Results of the intact coal sample showed that the permeability decreased by about 50% associated with CO2 adsorption.

The Pathway‐Flow Relative Permeability of CO2: Measurement by Lowered Pressure Drops

We introduce a simple method to measure the relative permeability of supercritical CO2 in low-permeability rocks. The method is built on the assumption of the stability of formed CO2 percolation pathway under lowered pressure drops. Initially, a continuous CO2 flow pathway is created under a relatively high-pressure drop. Then, several subsequent steps of lowered pressure drops are performed while monitoring the associated flow rates. When the pressure drop is lower than a threshold value, the created flow pathway is assumed to be adequately stable and does not vary significantly during successive flows, with the average saturation and flow rate achieving a quasi-steady state. The relative permeability of CO2 is then calculated from the relationship between the pressure drop and flow rate at several lowered pressure drops according to the extended form of Darcys law. We demonstrate this method using both numerical modeling and an experimental test using X-ray CT imaging. The results indicate the validity of the assumption for the stability of flow pathway under lowered pressure drops. A linear relationship between the lowered pressure drops and the corresponding CO2 flow rate is found. Furthermore, the measurement results suggest that the relative permeability of CO2 can still be high in low-permeability rocks if the CO2 saturation is higher than the threshold value required to build a flow pathway. The proposed method is important for measuring the pathway-flow relative permeability of non-wetting fluids in low-permeability rocks.

Monitoring and Quantification of Stored CO2 with Combined P-wave Velocity and Resistivity

al., 2008, Xue et al., 2002, 2005; Xue & Ohsumi 2004a, 2004b, 2005; Onishi et al., 2006). In the most CO2 storage sites, seismic survey has been conducted to monitor the injected CO2. From the recent injection projects, seismic survey shows great results for monitoring the migration of the CO2 in the reservoirs such as Sleipner or Nagaoka site (Arts et al., 2002, 2004; Bunge et al., 2000; Davis et al., 2002; Xue et al., 2006). In Nagaoka project, studies have attempted to estimate the CO2 saturation around the observation wells by using the results of well logging and laboratory studies (Kim et al., 2009b, Nakatsuka et al., 2009, Xue et al., 2006). When estimating CO2 saturation from seismic survey, Gassmann’s theory which consisted of bulk modulus of the saturated porous rock has been often used (Gassmann, 1951). When the saturation was less than 20%, P wave velocity shows good response but when the saturation was more than 20%, P-wave velocity became less sensitive to CO2 saturation. P-velocity is not sensitive to gas saturation in high gas saturation regime (Sg>20%) for either homogenous saturation or patchy saturation with patchy size << wavelength (Xue & Lei 2006, Lei & Xue 2006). To overtake this weak point of seismic monitoring for the estimation of CO2 saturation, there is a need for estimating accurate CO2 saturation using resistivity. In this paper, laboratory experiments have been conducted to monitor combined P-wave velocity and resistivity simultaneously in porous sandstone during CO2 injection process.