Supercritical CO2 Saturation Research Articles

Are changes in time‐lapse seismic data due to fluid substitution or rock dissolution? A CO2 sequestration feasibility study at the Pohokura Field, New Zealand

ABSTRACTSeismic methods are commonly used to monitor the subsurface when carbon dioxide (CO2) is injected into a reservoir. Besides fluid saturation and pressure changes, CO2–water mixtures may cause rock alteration. In this petrophysical study, we compare the elastic property changes due to fluid replacement and those due to mineral dissolution for carbonate‐cemented sandstones at the Pohokura Field, New Zealand. We quantify the effects of fluid substitution from fully brine to fully supercritical CO2 saturation and carbonate cement dissolution on the seismic signatures of the reservoir rocks by combining laboratory results, petrographic analyses, and geophysical well log data. We conclude that elastic property changes due to mineral dissolution are significantly greater than those due to fluid substitution alone. The northern part of the Pohokura Field has coarser‐grained sandstones, which experience the largest changes in wave speeds. Our hypothesis is that these changes result from carbonate cement dissolution in the presence of CO2–water–sandstone reactions. If time‐lapse seismic data were to be acquired in this field, the northern area could show P‐wave velocity reductions of up to 20% and a 131.7% increase in seismic amplitude from a brine‐saturated rock to an altered, fully CO2‐saturated rock. In comparison, the southern part of the field, where sandstones are mostly fine‐grained, we expect a P‐wave velocity decrease of 6% if such dissolution process took place. Finally, we show that the elastic properties of the reservoir rocks can be described with the constant‐cement model. The model is used to predict that the dissolution process reduces the volume of grain contact cement, on average, from 2.5% to 1.75% of the total rock mineral volume. Our analysis suggests that changes to the rock frame, which includes carbonate minerals, cannot be ignored for a CO2 injection scenario.

Experimental laboratory study on the acoustic response during injection of supercritical CO2 into brine saturated sandstones

Quantitative knowledge of the acoustic response of rock from an injection site on supercritical CO2 saturation is crucial for understanding the feasibility of time-lapse seismic monitoring of CO2 plume migration. A suite of sandstones with similar composition but different petrophysical properties has been tested to reveal the effects on acoustic responses of supercritical CO2 injection into brine saturated sandstones. CO2 is first injected into dry samples, flushed out with brine and then injected again into brine saturated samples. Such experimental protocol allows us to obtain acoustic velocities of the samples for the wide range of CO2 saturations from 0 to 100%. On injection of supercritical CO2 (scCO2) into brine-saturated samples, some of samples exhibit observable perturbation of ~10% of compressional velocities with the increase of CO2 saturation form 0% to maximum (~50%). However for some sample effect of scCO2 injection on acoustic properties is negligible.

Constraining CO2 simulations by coupled modeling and inversion of electrical resistance and gas composition data

This study investigates how model predictions of subsurface CO2 migration can be constrained and improved with time-lapse electrical resistance tomography (ERT) data for a pilot experiment located at Cranfield, Mississippi. To this end, we first invert the time-lapse ERT dataset using structurally constrained and unconstrained inversions. With the ERT time-lapse inversions, we image the increasing supercritical CO2 saturation in the reservoir and find that including the reservoir boundaries as structural constraints significantly improves the images. We then use ERT-derived changes in subsurface electrical resistivity along with gas composition data to constrain and calibrate hydrological models. We use the inversion framework iTOUGH2 and test several simplified conceptual models for the reservoir. Our analysis shows that the reservoir response cannot be adequately reproduced with a radial model; rather, the system exhibits 1D behavior. A model with three 1D layers, whose permeability values and width were estimated by inversion, is able to explain the ERT and gas composition data. Derived permeabilities agree with those from core measurements and a well test. Despite high noise levels, the ERT data provided crucial information in the inversion thanks to its high sensitivity at the inter-well scale, its stabilizing effect on the inversion, and the direct link it provides between electrical resistivity and CO2 saturation.

High-resolution characterization of a CO2 plume using crosswell seismic tomography: Cranfield, MS, USA

We present the results of a high-resolution time-lapse crosswell seismic survey carried out at a large-scale CO2 injection pilot located in Cranfield, MS, USA. This dataset, spanning an injector and two monitoring wells, provided a detailed view of the boundaries and internal structure of the injection unit, the Tuscaloosa D/E sand. Time-lapse tomographic processing of one well pair revealed the signature of the injected plume, two zones of decreased P-wave velocity which spatially correspond to higher permeability sections of the reservoir unit. We then used White's model for patchy saturation, combined with secondary information from core and well-log measurements, to convert the imaged changes in P-wave velocity into estimates of supercritical (sc) CO2 saturation in the interwell region. Our estimate is relatively consistent with existing lithologic and core data as well as independent measurements of scCO2 saturation derived from pulsed neutron logging. While effective in mapping the zone of CO2 invasion in the interwell region, several sources of uncertainty still exist in quantifying saturation using crosswell seismic methods, including (but not limited to) the degree of mesoscale mixing or “patchiness” at scales below a seismic wavelength.

Experimental Laboratory Study on the Acoustic Response of Sandstones During Injection of Supercritical CO2 on CRC2 Sample from Otway Basin Australia

Quantitative knowledge of the acoustic response of rock from an injection site on supercritical CO2 saturation is crucial for understanding the feasibility of time-lapse seismic monitoring of CO2 plume migration. A suite of shaley sandstones from the CRC-2 well, Otway Basin, Australia is tested to reveal the effects of supercritical CO2 injection on acoustic responses. The sandstone samples were cut in different directions with respect to a formation bedding plane and varied in porosities between 14% and 29% and permeabilities between 0.2 mD and 10,000 mD. Pore pressures and temperatures were varied from 4MPa to 10MPa, and 23°C to 45°C respectively to cover both vapour and supercritical regions of CO2 phase diagram. CO2 is first injected into dry samples, flushed out with brine and then injected again into brine saturated samples. Such experimental protocol allows us to obtain acoustic velocities of the samples for the wide range of CO2 saturations from 0 to 100%. On injection of supercritical CO2 (scCO2) into brine-saturated samples, they exhibit observable perturbation of ∼7% of compressional velocities with the increase of CO2 saturation form 0% to maximum (∼50%). Changes of the dry samples before and after the CO2 injection (if any) are not traceable by acoustic methods. An applicability of implementation of fluid substitution using Gassman theory for CRC2 well has been proved in the experiments. CO2 Residual saturation of about 50% was measured by monitoring of the volume of brine displaced from the sample and was independently confirmed by computer tomography (CT) imaging of the sample before and after experiments.

Fabrication of well-controlled porous foams of graphene oxide modified poly(propylene-carbonate) using supercritical carbon dioxide and its potential tissue engineering applications

Poly(propylene carbonate) is a new amorphous, biodegradable and biocompatible aliphatic polyester. It has a potentially wide range of applications, such as packing materials and biomedical materials. However, the low glass transition temperatures (Tg) and poor mechanical property have limited its applications. In this paper, poly(propylene-carbonate)/graphene oxide nanocomposites with a 10°C increase in Tg and a 50 times increase in storage modulus at 30°C were firstly fabricated, then the nanocomposites were foamed using supercritical CO2 to widen their applications, particularly in the area of tissue engineering. It was demonstrated that the nanocomposite foams had good dimension stability and the final pore features were depended on supercritical CO2 saturation conditions. In addition, cytotoxicity and in vitro cell culturing tests of selected foams showed that the fabricated porous materials were non-cytotoxic and able to support cellular adhesion within the 3D structure, suggesting that these are promising materials for tissue engineering applications.

Assessing the monitorability of CO2 saturation in subsurface saline aquifers

We propose a method to assess the geophysical monitorability of petrophysical parameters (in this case CO2 saturation) in subsurface geological reservoirs. The approach is based on measuring and inverting up to six geophysical parameters: P- and S-wave impedances and quality factors (1/attenuation), density and electrical resistivity. We use Shannon's information measure to quantify the information obtained from the inversion of different combinations of geophysical parameters. We thus develop a generic approach to assess the joint monitorability of supercritical CO2 saturation when CO2 is stored in subsurface reservoirs. Uncertainty analysis shows that expected information is in general nonlinearly related to the level of a priori or measurement uncertainty in the geophysical and petrophysical parameters, to the true CO2 saturation, and also to the measurement frequency in the case of seismic measurements. Prior uncertainties in petrophysical parameters such as porosity have a significant effect on the monitorability, highlighting the need for accurate benchmark (pre-injection) measurements and reservoir characterization in the case of time-lapse (4D) monitoring. We show that estimates of P-wave seismic attenuation contribute most to the overall information obtained, and that in principle the joint application of different geophysical methods can significantly improve monitorability. We applied the approach to assess the monitorability of the CO2 saturation after 10 years of simulated injection into a saline aquifer reservoir in the near-shore UK North Sea. Application of the approach to design optimal data combinations for hydrocarbon saturation assessment and monitoring is straightforward.

Visualization experimental investigations of supercritical CO2 inject into porous media with the fissure defect

Experimental studies about supercritical CO2-brine displacement are necessary to understand the detailed flow process of injected supercritical CO2 in deep saline aquifers. This article adopts experimental method to research the supercritical CO2 and water flow in porous media, using MRI as visualization and measurement method. Supercritical CO2 and water are injected into the sintered glass porous media structure by different mixture ratios. The sintered glass porous media structure is designed with the fissure defect. Visualization and analysis results simultaneously demonstrate that supercritical CO2 saturation and distribution vary with different ratios. The CO2 saturation increases with elevation in CO2-water ratio. And the CO2 distribution is significantly influenced by the buoyancy force and the supercritical CO2 will tend to migrate to the top of the test section. This paper also investigates the fissure effect. More supercritical CO2 will migrate to the top when encounters the fissure defect, and therefore the cap rock has to withstand bigger pressure, which will reduce the safety of carbon geological storage.