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 4 MPa to 10 MPa, 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

A (not so) shallow controlled CO2 release experiment in a fault zone

The CSIRO In-Situ Laboratory Project (ISL) is located in Western Australia and has two main objectives related to monitoring leaks from a CO2 storage complex by controlled-release experiments: 1) improving the monitorability of gaseous CO2 accumulations at intermediate depth, and 2) assessing the impact of faults on CO2 migration. A first test at the In-situ Lab has evaluated the ability to monitor and detect unwanted leakage of CO2 from a storage complex in a major fault zone. The ISL consists of three instrumented wells up to 400 m deep: 1) Harvey-2 used primarily for gaseous CO2 injection, 2) ISL OB-1, a fibreglass geophysical monitoring well with behind-casing instrumentation, and 3) a shallow (27 m) groundwater well for fluid sampling. A controlled-release test injected 38 tonnes of CO2 between 336-342 m depth in February 2019, and the gas was monitored by a wide range of downhole and surface monitoring technologies. CO2 reached the ISL OB-1 monitoring well (7 m away) after approximately 1.5 days and an injection volume of 5 tonnes. Evidence of arrival was determined by distributed temperature sensing and the CO2 plume was detected also by borehole seismic after injection of as little as 7 tonnes. Observations suggest that the fault zone did not alter the CO2 migration along bedding at the scale and depth of the experiment. No vertical CO2 migration was detected beyond the perforated injection interval; no notable changes were observed in groundwater quality or soil gas chemistry during and post injection. The early detection of significantly less than 38 tonnes of CO2 injected into the shallow subsurface demonstrates rapid and sensitive monitorability of potential leaks in the overburden of a commercial-scale storage project, prior to reaching shallow groundwater, soil zones or the atmosphere. The ISL is a unique and enduring research facility at which monitoring technologies will be further developed and tested for increasing public and regulator confidence in the ability to detect potential CO2 leakage at shallow to intermediate depth

Lessons learned : the first in-situ laboratory fault injection test

The CSIRO In-Situ Laboratory has been a world first injection of CO2 into a large faulted zone at depth. A total of 38 tonnes of CO2 was injected into the F10 fault zone at approximately 330 m depth and the process monitored in detail. The site uses a well, Harvey-2, in SW Western Australia (the South West Hub CCS Project area). The top 400 m section of Harvey-2 was available for injection and instrumentation. An observation well, ISL OB-1 (400 m depth) was drilled 7 m to the north east of Harvey-2. ISL OB-1 well was cased with fibreglass to provide greater monitoring options. The CSIRO In-Situ Laboratory was designed to integrate existing facilities and infrastructure from the South West Hub CCS Project managed by the West Australian Department of Mines, Industry Regulation and Safety. While new equipment was deployed for this specific project, the site facilities were complemented by a range of mobile deployable equipment from the National Geosequestration Laboratory (NGL). The geology of the area investigated poses interesting challenges: a large fault (F10) is estimated to have up to 1000 m throw overall, the presence of packages of paleosols rather than a contiguous mudstone seal, and a 1500 m vertical thickness of Triassic sandstone as the potential commercial storage interval. This unique site provides abundant opportunities for testing more challenging geological environments for carbon storage than at other sites. While details of this first project are described elsewhere, lessons were learned during the development and execution of the project. A rigorous risk register was developed to manage project risk, but not all events encountered were foreseen. This paper describes some of the challenges encountered and the team's response. Relocation of the project site due to changes in landholder ownership) and other sensitivities resulted in the need for rapid replanning of activities at short notice resulting in the development of the site at Harvey-2. The relocation allowed other research questions to be addressed through new activities, such as the ability to consider a shallow/controlled release experiment in an extensive fault zone, but this replanning did cause some timing stress. The first test at the In-Situ Laboratory was reconfigured to address some of those knowledge gaps that shallow/controlled release experiments had yet to address. Novel approaches to drilling and completing the monitoring well also threw up unanticipated difficulties. Loss of containment from the wellbore also posed significant challenges, and the team's response to this unintended release of gas and water from the monitoring well at the conclusion of the field experiment will be discussed. Other challenges that we encountered, their impacts, and our response are also catalogued here (Table 1 and below) to enable broad knowledge exchange

A controlled CO2 release experiment in a fault zone at the in-situ laboratory in Western Australia

A controlled-release test at the In-Situ Laboratory Project in Western Australia injected 38 tonnes of gaseous CO2 between 336-342 m depth in a fault zone, and the gas was monitored by a wide range of downhole and surface monitoring technologies. Injection of CO2 at this depth fills the gap between shallow release (600 m) field trials. The main objectives of the controlled-release test were to assess the monitorability of shallow CO2 accumulations, and to investigate the impacts of a fault zone on CO2 migration. CO2 arrival was detected by distributed temperature sensing at the monitoring well (7 m away) after approximately 1.5 days and an injection volume of 5 tonnes. The CO2 plume was detected also by borehole seismic and electric resistivity imaging. The early detection of significantly less than 38 tonnes of CO2 in the shallow subsurface demonstrates rapid and sensitive monitorability of potential leaks in the overburden of a commercial-scale storage project, prior to reaching shallow groundwater, soil zones or the atmosphere. Observations suggest that the fault zone did not alter the CO2 migration along bedding at the scale and depth of the test. Contrary to model predictions, no vertical CO2 migration was detected beyond the perforated injection interval. CO2 and formation water escaped to the surface through the monitoring well at the end of the experiment due to unexpected damage to the well’s fibreglass casing. The well was successfully remediated without impact to the environment and the site is ready for future experiments

Feasibility of CO2 plume detection using 4D seismic: CO2CRC Otway Project case study - Part 2: Detectability analysis

A key objective of stage 2 of the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway Project is to explore the ability of the seismic reflection method to detect and monitor injection of a small amount of greenhouse gas into a saline formation. Development of a seismic monitoring program requires an understanding of expected time-lapse (TL) seismic signals. Hence, before such an injection experiment is undertaken, we assessed the feasibility of seismic monitoring in a modeling study. Considering realistic gas distributions inferred from reservoir simulations, we analyzed the influence of various factors (injection volume, time after injection, and realizations of the reservoir flow model) on the TL seismic signal. However, the applicability of seismic monitoring depends not only on the strength of the TL seismic signal but also on the noise level of the seismic data. Hence, to estimate the detectability of gas in the subsurface, we have developed a workflow that integrated actual data repeatability observed at the Otway test site into the seismic feasibility study. Although we observed differences between the considered scenarios, all of the scenarios indicated a high likelihood of successful plume detection with the observed noise level and surface 4D seismic acquisition geometry used in stage 1 of the CO2CRC Otway Project at the same site. However, a thin layer of gas spreading out from the edges of the main plume below the seal in all scenarios would be a challenge for surface seismic monitoring

CO2 storage in a depleted gas field: An overview of the CO2CRC Otway Project and initial results

The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway Project in Australia is the first heavily monitored pilot site for CO2 storage in a depleted natural gas reservoir. With the site characterisation and risk analysis complete, the new CRC-1 injection well was drilled in April 2007. An updated static and dynamic model forecast an injected gas transit time of between 4 and 8months between CRC-1 injection and Naylor-1 observation wells. Injection began on March 18th 2008 and was halted on August 29th 2009 with 65,445 tonnes of CO2 mixed gas stored. Two pulses of tracer compounds were added to help identify the injected CO2 from other naturally occurring CO2 and to track dispersion and diffusion. Assurance monitoring included surveillance of the atmosphere, soil gas and shallow groundwater. To date, no tracer compounds have been detected above background levels in samples taken as part of the assurance monitoring system. Monitoring of the reservoir has been accomplished with a combined geophysical and geochemical approach. Formation fluids are sampled at pressure with the multilevel U-Tube system. The transient geochemistry at the observation well has: (1) recorded injected gas arrival at the Naylor-1 observation well; (2) recorded tracer compound arrival at Naylor-1; (3) shown a mixing trend between the isotopic signature of the Naylor indigenous CO2 and that of the injection supply gas; and (4) provided an estimate for the dynamic storage capacity for a portion of the Naylor reservoir. The data collected are compared with the pre-injection dynamic model forecasts and provide a means of calibration. The CO2CRC Otway Project has successfully demonstrated the storage of CO2 in a depleted gas field. Geochemical assurance monitoring and reservoir surveillance will continue post injection. Continued analysis of the data will serve to reduce uncertainty in forecasting long term fate of the injected CO2 mixed gas

Feasibility of CO2 plume detection using 4D seismic: CO2CRC Otway Project case study — Part 1: Rock-physics modeling

A key objective of stage 2 of the Cooperative Research Centre for Greenhouse Gas Technologies Otway Project is to evaluate the seismic detection limit of greenhouse gas injected into a saline aquifer. For this purpose, injection of a small amount of CO2-rich gas into the Paaratte Formation, a saline aquifer located at a depth of approximately 1.5 km, is planned. Before the injection experiment is undertaken, we assessed the detectability of injected gas with seismic methods in a modeling study. A key objective of this study was to model changes in elastic properties caused by CO2-saturation effects using predictions of reservoir simulations. To this end, we established an elastic property/porosity relation to link the reservoir flow model and the elastic properties of the subsurface. Predicting changes in elastic properties requires suitable velocity-saturation relations. To choose an appropriate velocity-saturation relation, we analyzed the effect of fluid distribution on the time-lapse seismic response by performing 1.5D poroelastic and elastic modeling based on reservoir simulations. The modeling results emphasized the importance of taking the variability of rock properties into account and to carefully estimate dry bulk moduli to adequately represent the sensitivity of rock properties to fluid changes. Furthermore, we determined that the Gassmann-Wood relation was an appropriate velocity-saturation relation at seismic frequencies for the Paaratte Formation. However, changes in acoustic contrasts caused by CO2 saturation between layers below the seismic resolution had to be considered. In this sense, an appropriate velocity-saturation relation also depends on the scale at which we model the seismic response

Validating Subsurface Monitoring as an Alternative Option to Surface M&V - The CO2CRC's Otway Stage 3 Injection

Future CO 2 storage projects will require Monitoring and Verification (M & V) operations at the CO 2 storage site to understand the behavior of the CO 2 plume, including the assurance that leakage of the CO 2 has not occurred. Current surface based monitoring technologies may be unable to yield sufficient resolution or accuracy. CO2CRC, in conjunction with its Australian partners, is developing the Otway Stage 3 Project to identify and validate sub-surface monitoring techniques and configurations as a key element of a risk-based M & V program in large scale CCS projects. Subsurface monitoring approaches will be tested on a plume of CO 2 from an array of monitoring wells. Primary monitoring methods will be pressure tomography and downhole seismic, although other modalities (gravity, electromagnetic) are also being considered

Seismic monitoring of CO2 geosequestration: CO2CRC Otway case study using full 4D FDTD approach

© 2016 Elsevier Ltd. Stage 2C of the Otway project by CO2CRC Limited was designed as a feasibility study of seismic monitoring to detect and characterise small-scale leakage of CO2-rich gas into a saline aquifer. Design of the monitoring program is based on a series of simulations conducted in 2007-2014. The gas plume is likely to be small in size and the contrast in elastic properties is also predicted to be relatively low. To maximise the chances of detecting the low-amplitude time-lapse signal we optimise the current time-lapse processing workflow using synthetic datasets for the entire baseline and monitor surveys. The datasets were obtained by an elastic 3D FDTD modelling approach for the actual field acquisition geometry and the most realistic model of the subsurface and distribution of elastic properties in the gas plume. To this end we built a full-earth static geological model of the Otway site with resolution typical for reservoirs in petroleum exploration. Distributions of the seismic properties were obtained from geostatistical interpolation between wells within the static model. The analysis of the synthetic datasets gives an estimate of the magnitude of the time-lapse signal and illustrates effects of the conventional processing procedures on the signal in the presence of the band limited random noise. We have found that the anticipated intensity of the time-lapse signal is comparable to the average intensity of the reflections observed within the target interval, and hence should be sufficient for the detection of the signal. We believe that the proposed modelling workflow is of methodological value since it provides a reliable basis for seismic feasibility studies and development of modelling-driven processing workflows