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Instrumenting wells with distributed acoustic sensors (DASs) and illuminating them with passive or active seismic sources allows precise tracking of temporal variations of direct-wave traveltimes and amplitudes, which can be used to monitor variations in formation stiffness and density. This approach has been tested by tracking direct-wave amplitudes and traveltimes as part of a carbon capture and storage project where a 15 kt supercritical CO2 injection has been monitored with continuous offset vertical seismic profiling using nine permanently mounted surface orbital vibrators acting as seismic sources and several wells instrumented with DAS cables cemented behind the casing. The results indicate a significant (from 15% to 30%) increase of strain amplitudes within the CO2 injection interval, and traveltime shifts of 0.3–0.4 ms below this interval, consistent with full-wave 1.5D numerical simulations and theoretical predictions. The results give independent estimates of the CO2 plume thickness and the associated P-wave velocity reduction.

With new developments of fiber-optic sensing and rotary sources, continuous active seismic monitoring for onshore applications has now the opportunity to be fully realized and applied to enhance subsurface resource extraction and use. To date, conventional seismic monitoring deployments have primarily consisted of receiver arrays, either fixed or not, with periodic data acquisition campaigns using temporarily deployed sources, resulting in time-lapse data with poor temporal resolution. Only a few niche efforts have demonstrated continuous acquisition using fixed source-receiver networks. We have evaluated the initial results of a network of fixed rotary seismic sources, referred to as surface orbital vibrators (SOVs), coupled with a permanent distributed acoustic sensing (DAS) network at the CO2CRC Otway Field Site. Although rotary seismic sources are not new, our development of the SOV focused on simplifying the cost and complexity of the source hardware while delivering broad frequency spectrum of the source signal. The upgraded hardware is aligned with a robust methodology for autonomous operation and data processing. At the Otway Site, we deployed SOVs at nine locations, monitoring seismic response in seven DAS instrumented wells. Baseline operation of the DAS/SOV sensor array and source system demonstrates its capability with near offsets attaining a signal-to-noise ratio approaching 100 dB with a normalized root mean square of 10%. Furthermore, analyses of traveltime repeatability indicate that the DAS/SOV system can deliver time resolution of ±500 μs.

Time‐lapse seismic reservoir monitoring is used as a way to gain insight into subsurface processes. Yet, the application of standard 4D technology onshore faces challenges, such as high cost, significant environmental footprint and, consequently, relatively infrequent surveys. As part of the Otway Project Stage 3 CO2 injection study, continuous automated borehole‐based monitoring using distributed acoustic sensing has been paired with permanently deployed surface sources, referred to as surface orbital vibrators, as a way to monitor the spreading CO2 plume. The injection of 15,000 tonnes of CO2 in a saline reservoir at a depth of 1550 m is monitored using five boreholes instrumented with enhanced sensitivity fibre optic cables and nine surface orbital vibrators, creating an array of 45 well–source pairs. The data are processed with an offset vertical seismic profiling processing workflow developed to address key challenges of the continuous distributed acoustic sensing acquisition using surface orbital vibrators. The processing flow includes deconvolution with a source sweep recorded by a pilot geophone installed below the surface orbital vibrators. A second deconvolution with a wavelet estimated from direct arrivals compensates for the difference between distributed acoustic sensing measurements and the pilot geophone as well as near‐surface variations. Image quality is noted to be best for short offsets and decreases with increasing offsets and well deviations. As surface orbital vibrators generate unique sweeps in two rotation directions, further processing is applied to stack these rotation signals together, which further improves the images. The resulting 2D transects of each well–source pair visually provide good illumination of the subsurface, suggesting continuous monitoring of the spreading CO2 plume should be possible with some further tuning of the processing workflow for time‐lapse repeatability.

The effective monitoring of hydraulic fracturing in unconventional oil and gas production requires tools to quantify elastic property variations even in the absence of microseismic activity. To track the subtle time-lapse variations in reservoir properties during such activities, monitoring techniques with high repeatability and high resolution, spatially and temporally, are required. Distributed acoustic sensing (DAS) is a rapidly maturing fiber-optic technology for low-cost, permanent, high density, in-well monitoring. Surface orbital vibrators (SOVs) are inexpensive fixed rotary seismic sources that offer the opportunity to frequently interrogate the subsurface with energies comparable to vibroseis sources. We have evaluated a field vertical seismic profile test, conducted in the Eagle Ford play, pairing an SOV source recorded by DAS behind casing in a deviated well to better evaluate the potential of the technology set for unconventional reservoir monitoring. We determine the data processing workflow for reservoir monitoring using the SOV-DAS system. We analyze the data characteristics of the SOV-DAS system, including the signal-to-noise ratio characteristics and source repeatability. High-quality P- and S-wave reflections, as well as mode conversions, are visible in the vertical section. In addition, clear P-P reflections are also observable along the horizontal well sections. Time shifts with a mean value of 10 μs between different data sets demonstrate the high repeatability for the semipermanent SOV source, which is crucial for time-lapse analysis. We also apply reflection imaging on P and S to reveal reflection depths. In a first-of-its-kind deployment, we implemented a rotating SOV with a slewing bearing and discuss the possibility to optimize S-wave construction along the horizontal well with specific SOV orientation directions. Our preliminary results indicate that the combination of repeatable surface sources such as SOVs with DAS has significant potential for providing a low-cost approach for high-resolution seismic monitoring of unconventional reservoirs.

Due to their granular nature and presence of fluids, elastic moduli of most crustal rocks show a strong stress dependency. This means that the relationship between stress and strain is nonlinear, which should cause nonlinear wave phenomena. In particular, interaction of seismic waves of different frequencies should generate higher harmonics and combinational frequencies. Analysis of these effects in field data can potentially help find areas of anomalous nonlinear properties, such as fractured zones, mixed saturation or overpressure. To better understand the potential of nonlinear seismology, we observed and analyzed nonlinear seismic effects in field and laboratory experiments. The field experiment was performed using two seismic vibrators generating monochromatic signals of different frequencies. The wavefield was recorded with a fiber optic distributed acoustic sensing (DAS) cable cemented in a 900 m deep borehole. The signals recorded both on the surface and in the borehole show combinational frequencies, harmonics, and other intermodulation products of the fundamental frequencies. The laboratory experiment, which was designed to replicate the setup of the field experiment, shows similar nonlinear products of the fundamental frequencies. Furthermore, the nonlinear effects show a dependency on the saturating fluid. These tests confirm that nonlinear components of the wavefield propagate in a form of body waves, are likely to be generated in rock formations, and have the potential for reservoir fluid characterization.

It is crucial to ensure the safety and integrity of underground gas storage (UGS) infrastructure for energy reliability in California, and many other places around the world. To address the risk management need in UGS industry, we take advantage of recent advances in downhole fiber optic monitoring and coupled well-reservoir simulation to provide unprecedented understanding of gas flow in wells at UGS sites. We have combined advanced monitoring and simulation of UGS operations into a decision-support system called the Integrated Risk Management and Decision Support System (IRMDSS). The IRMDSS framework includes three components: (i) mechanistic models, (ii) continuous and frequent monitoring data, and (iii) a supervisory interface for performing analyses using the models and monitoring data. The goal of the IRMDSS is to equip UGS operators with real-time monitoring data and simulation tools that can alert them to potential failures, detect early leakage, and support mitigation decision-making to prevent otherwise larger failures. We demonstrate an application of the IRMDSS by analyzing the temperature and pressure response to a hypothetical leak. Through a review of distributed temperature sensing (DTS) data collected at an operating UGS facility we show that DTS can uniquely and precisely identify the depth of the gas-water-contact in the well annulus, and that DTS can provide an early warning signal of upward gas flow as would occur in a well blowout scenario. When combined with modeling analysis, a rough leak rate can be roughly estimated to understand the severity of the leakage conditions and to support the mitigation decision needed.

Well integrity is critical to the safety and success of subsurface energy exploration and management, as leakage of fluids from subsurface reservoirs is often induced by compromised wells. This study investigates bending deformation of a mockup of an oil/gas well that is subjected to loads expected in deviated wells under reservoir compaction and assesses the feasibility of utilizing distributed fiber optic strain sensing to monitor the deformation. A well mockup, which consists of outer and inner steel pipes with a cemented annulus, is tested under three-point bending loading and its strain and curvature development is monitored by Brillouin optical time domain reflectometry/analysis (BOTDR/A) as well as optical frequency domain reflectometry (OFDR). The primary objective of this research is to assess the strain sensing performance of newly fabricated fiber optic cables and to identify key cable characteristics which could improve the quality of distributed strain measurements with BOTDR/A. Results show that the tight-buffered cable is best suited for strain sensing as its maximum errors in the strain measurement were −36% and −24% against conventional sensors at the maximum elastic and plastic bending loads, respectively, whereas those of the non-tight-buffered cable were −45% and −71%, respectively. Similar trends were obtained in the bending curvature measurement. The detailed design of the tight-buffered cable is presented to elucidate key characteristics of such a cable, which will facilitate accurate distributed strain sensing in oil and gas wells.

In the CO2CRC Otway Project, seismic monitoring has evolved from traditional campaign-based seismic acquisition using a large array of seismic receivers and mobile sources, towards the techniques u...

An in-hole electrode array was installed and cemented behind a fibreglass casing in a vertical monitoring well as part of an in-situ laboratory test for CO2 migration in a shallow fault zone in Har...

This study aims to assess the ability of shallow distributed acoustic sensing (DAS) to serve as a cost-effective seismic sensor array for permanent monitoring applications. To this end, as part of the CO2CRC seismic monitoring program, a fibre-optic DAS array was deployed alongside a permanently buried geophone array at the Otway Project site (Victoria, Australia). The DAS array consisted of a standard commercially available tactical fibre-optic cable, which was deployed in 0.8m deep trenches. A custom-designed helically wound (HW) cable was also deployed in one of the DAS trenches for comparison of the cable designs. Simultaneous acquisition of the seismic data was carried out using 3000 vibroseis source points and geophones, DAS standard and HW cables. For initial assessment of the seismic images acquired with DAS and to compare different cable designs, preliminary 2D seismic reflection processing is conducted on both DAS cables and geophone data along a single 2D line. The geophone data processing guided processing of the DAS data. Several shallow structures (100-450ms) and some important reflectors at 450-600ms are observed on the final DAS images. Comparison of the two different DAS cable types demonstrated that seismic imaging would benefit DAS technology. However, the benefit of utilising HW cable is modest compared with the standard cable. The workflows and results of this study pave the way for processing of the 3D seismic data set acquired with the DAS array, as well as further detailed analysis of the DAS cables and the system itself.

A permanent automated continuous seismic CO2 geosequestration monitoring system for was installed at CO2CRC Otway Project site (Victoria, Australia) in early 2020. The system is composed of five deviated ∼1600 m deep wells equipped with distributed acoustic sensing (DAS) acting as seismic receivers and nine seismic orbital vibrators (SOV) as seismic sources. DAS recording is performed continuously by three iDASv3 units. Each SOV operates for 2.5 h at a time, and hence all SOVs operating sequentially (during daytime only) produce in a single vintage every two days. Each vintage consists of 45 offset VSP transects covering predicted CO2 plume migration paths over ∼0.7 km2 area. An automated data processing implemented on-site reduces data size from ∼1.3 TB/day to ∼500 MB/day with the results transmitted to the office daily. The repeatability analysis based on pre-injection data (acquired from May to October 2020 before the injection start in December 2020) shows that variability of SOV performance is the main source of non-repeatability while borehole measurements are stable. An SOV waveform could reach NRMS value from 20 to 100 % within a few days. However, deconvolution of the seismograms with the waveform of the direct wave reduces the repeatability to within 10–15 % NRMS.

The CO2CRC Otway Stage 3 project is developing low-impact methods for near-continuous monitoring of storage sites. This paper reports on the design, drilling, instrumenting, and early results from an array of an injector and five monitoring wells, spread over a km2. Highlights include the deployment and use of distributed acoustic sensors on casing, and baseline injections and interpretation of pressure tomography.

[enter Abstract Body]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.

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.

Summary Time-lapse seismic was performed as part of the Otway Stage 3 Project (Victoria, Australia) to monitor the spread of an injected plume of CO2. A permanent seismic monitoring system based on surface orbital vibrators and DAS VSP technology was used to acquire a new vintage survey every two days. SOV/DAS data was collected from the end of May 2020 to April 2021 (~150 vintages), with the CO2 injection commencing in December 2020. We acquired more than 60 vintages to utilize as a baseline, which provided an opportunity to investigate how a selection of a baseline survey impacts the interpretation of time-lapse changes. Two baseline vintages from June 2020 and November 2020 were compared to a monitor survey acquired in April 2021. The NRMS was calculated to be 45% and 60% for the June and November baselines, respectively. We suggest that this non-repeatability is created by seasonal variations affecting near-surface conditions. The pairwise analysis of all possible vintage combinations highlights seasonal changes in the NRMS value. Analysis of the time-lapse VSP wavefield suggests that the main source of non-repeatability is a seasonal variation of near-surface conditions. This variation manifests itself in changes in source performance and the pattern of surface-related multiples.

Time-lapse (TL) (4D) seismic monitoring of injected CO2 in geological formations is being increasingly employed as the principal method for ensuring containment of CO2 and testing conformance of predicted plume behavior. However, to bring further confidence in this method, the CO2 volume detection limit in the seismic monitoring and key factors controlling it need to be quantitatively understood. The CO2CRC Otway Project attempts to improve this understanding by exploring the capability of seismic reflection method to detect and monitor a 15-kt injection of supercritical CO2/CH4 mixture in a saline aquifer at a depth of 1500 m. The monitoring program consists of TL 3D seismic surveys using a buried geophone array, TL 3D vertical seismic profiling (VSP), and offset VSP. Seismic acquisition was carried out at injection intervals of 5, 10, and 15 kt over a 5-month period and also, 9 and 23 months after the end of injection. The TL seismic images clearly show the distribution and evolution of the stored CO2/CH4 plume. The results demonstrate the potential of TL reflection seismic to provide key information to both operators and regulators for confirming the security and behavior of stored CO2 at very small volumes.

Summary In recent years, distributed acoustic sensing (DAS) has gained large popularity in applied geophysics. Development of helically wound cables (HWC) has pushed the technology forwards for land applications. HWCs are designed to mitigate the directional sensitivity of fibre cables and provide a suitable and cost-effective tool for permanent subsurface deployments for reservoir monitoring tasks. However, a number of field experiments have revealed that seasonal variations of near surface conditions significantly affects the quality of data. We study these effects over an extended period of time and demonstrate how data quality of a buried HWC is changing over a year.

Summary Fibre-optics Sensing and Permanent Sources for Seismic Monitoring of a Large-scale CCS site in Decatur, Illinois: Preliminary Results and Lessons Learnt

Distributed Acoustic Sensing (DAS) is an optical interferometric method for acquisition of acoustic and seismic signals. It uses laser pulses that travel along the length of a fibre-optic cable and backscatter as they encounter small inconsistencies in the fibre. Impinging seismic waves cause strain on the cable, resulting in differences in phase of the backscattered light. Interest in DAS has increased significantly in the past decade as it is particularly suited for VSP acquisitions, including for permanent reservoir monitoring. Fibre-optic cables can be installed permanently in the well, cemented behind the casing or attached to tubing; they offer a relatively cheaper and efficient solution when compared to conventional borehole sensors. This study is part of the CO2CRC Otway Project. The Otway Project site is located approximately 240 km south-west of Melbourne, Australia. The Stage 2C of the project aims to monitor a small injection (15 kt) of CO2/CH4 gas mixture at a depth of approximately 1500 m. Here, we show the results of a 3D VSP survey acquired using DAS cable deployed on production tubing in the injector well. The DAS up-going wavefield shows a high level of noise. However, DAS is able to record the main reflections, including at the injection depth. After 3D migration of the data, noise levels reduce significantly. Events on DAS inline match with events on a corridor stack produced from a geophone check-shot data. Due to the directionality pattern, DAS was only able to image up to approximately 300 m radius from the well.

The In-Situ Laboratory Project entails completing, instrumenting and pump testing five intervals in an existing well and injecting a small volume of CO2 for testing purposes into the Lesueur Formation at the South West Hub project in Western Australia. The project commenced in the middle of October 2016 and is scheduled to run until the end of April 2019, with hydraulic testing and CO2 injection planned towards the end of 2018. The purpose is to aid demonstration of the commercial viability of geologically storing carbon and contribute to broadening the portfolio of globally evaluated geological settings for storage via testing of a more than 1000 m thick injection reservoir in which CO2 migration is largely governed by residual saturation and dissolution trapping. The project will develop the first part of an enduring research facility at the South West Hub to enable further research of a geological environment that has more uncertainty than many other current projects; i.e. in the case of the South West Hub there is uncertainty around the extent of a regional seal.

The In-Situ Laboratory Project (In-situ Lab) entails a configuration of wells at approximately 400 m depth for monitoring the controlled release of CO2 in a fault zone at the South West Hub CCS Fla...

Carbon Capture and Storage (CCS) remains one of the most viable short-term options for halting the increase in atmospheric concentration of greenhouse gases. Assurance of storage safety is an essen...

Active time-lapse seismic is widely employed for monitoring CO2 geosequestration due to its ability to track the distribution of fluids in space and time. However, standard 4D seismic monitoring suffers from several challenges, including high cost, disruption to other land uses, and, consequently, relatively large intervals between monitor surveys. Some of these challenges can be mitigated using permanently installed sources and receivers. Such an approach was tested at the CO2CRC Otway site by continuous offset VSP monitoring of 15,000 t of supercritical CO2 injected into an aquifer 1,500 m deep with nine permanent seismic sources (surface orbital vibrators or SOVs) and five downhole fibre-optic receivers. This continuous monitoring is complemented by multi-well 4D VSP using a mobile vibroseis source and the same DAS receivers, which included one baseline and two monitor surveys after injection of 4,000 and 12,000 t of CO2. The continuous DAS-SOV monitoring detected an abrupt increase of travel times below the injection interval on the second day of injection (after injection of 300 t of CO2) and tracked the growth of the areal CO2 plume by mapping changes of reflection amplitudes. The plume is also detected by time-lapse changes of reflection amplitudes in multi-well 4D VSPs. The plume images obtained from continuous offset VSP and 4D VSP are broadly consistent with each other but with some differences due to differences in illumination, lateral variations of velocities and seismic anisotropy. These differences also serve as a measure of uncertainty of 4D VSP images.

Time-lapse (4D) seismic monitoring of injected CO2 in geological formations is being increasingly employed as the principal method for ensuring containment of the CO2 and testing conformance of predicted plume behaviour. However, to bring further confidence in this method, the CO2 volume detection limit in the seismic monitoring and key factors controlling it need to be quantitatively understood. The CO2CRC Otway Project attempts to improve this understanding by exploring the capability of seismic reflection method to detect and monitor a 15,000 t injection of supercritical CO2/CH4 mixture in a saline aquifer at a depth of 1500 m. To increase the signal to noise ratio and to reduce the disruption to land users, seismic acquisition is performed using a buried geophone array. Seismic acquisition occurred at injection intervals of 5000, 10,000 and 15,000 t over a 5-month period. The seismic images clearly show the distribution and evolution of the stored CO2/CH4 plume. The analysis confirms that signal from pure CO2 would be of similar magnitude to the signal from CO2/CH4 mixture. The results demonstrate the potential of time-lapse reflection seismic to provide key information to both operators and regulators for confirming the security and behaviour of stored CO2 at very small volumes.

The South West Hub CCS project (SW Hub) in Western Australia is proceeding to reduce uncertainties related to injectivity, capacity and containment through a well drilling, coring and logging program. This study provides reviews of well designs for in situ tests and well-based monitoring methods at CO 2 storage sites. Wells are expensive and complex engineering undertakings, and their design including size, geometry and materials, greatly impacts on the type of data that can be collected and techniques for monitoring that can be performed at a site. There is no ‘one size-fits-all’ monitoring well, but there is a tool-box or ensemble of solutions that can achieve a broad range of relevant monitoring objectives given constraints of site characteristics and budgetary limitations. For the SW Hub, a multi-well, multi-use and multi-completion monitoring scheme is proposed that combines the benefit of four different types of monitoring wells in addition to equipping the injector: 1) a well completed in the reservoir for conformance monitoring with additional completion above the storage complex, 2) a well completed above the confining layer for ensuring containment, 3) a well completed in the reservoir in the vicinity of an identified fault for monitoring potential across-fault migration and fault re-activation risks and 4) a well for fault leakage surveillance above the storage complex.

The CO2CRC's “Otway Stage 2C project” is a test injection of 15,000 tons of supercritical gas mixture (80 mole% CO 2 & 20 mole% CH 4 ) at the CO2CRC Otway site in the Australian state of Victoria. The objective of this test is to examine the limits of surface seismic detection and to conduct detailed pressure monitoring of the injection. A key design feature of the project is injection near the bottom of a highly permeable formation, thereby allowing for buoyancy driven flow to thicken the plume and enhance seismic response. The project is specifically targeting observation of the development of the gas plume through a combination of multi-vintage 4D seismic surveys acquired with a buried seismic receiver array, 4D Vertical Seismic Profiling (VSP) and pressure measurements. Using both time-lapse seismic data and reservoir simulations, we aim to not only detect the plume but also demonstrate its eventual stabilisation. In this paper we discuss the technical aspects of the Otway Stage 2C seismic monitoring program and the initial results.

Several recent studies have demonstrated that Distributed Acoustic Sensing (DAS) can utilize existing subsurface telecom fiber (i.e. dark fiber) for high quality seismic measurements. Researchers to date have shown that this sensing combination, coupled to ambient noise interferometry techniques, can effectively image the shallow subsurface (< 30 m) using vehicle and infrastructure noise (f = 8 - 30 Hz). We present the first long-offset surface wave inversion study targeting deeper (⁓ 500 m) structure using DAS and dark fiber. This study utilizes a previously acquired dataset collected on a 23 km fiber section between West Sacramento and Woodland, CA, part of the DOE Energy Science Network (ESnet). By targeting noise generated by a co-linear rail line, broadband and rich in low frequencies (down to f=1 Hz), and long array offsets, we generate high-quality interferometric gathers suitable for inversion. Subsequent surface wave inversions using a multimode Monte Carlo (MC) sampling algorithm are consistent with geology and available confirmatory datasets derived from co-located sonic logs. The relatively sparse confirmatory data demonstrates, by comparison, the utility of the high spatial sampling provided by DAS. These results open the door to larger regional DAS studies targeting deeper targets but with resolutions higher than those afforded by the use of persistent low frequency (f

Distributed acoustic sensing (DAS) can revolutionize the seismic industry by using fiber-optic cables installed permanently to acquire on-demand vertical seismic profile (VSP) data at fine spatial sampling. With this, DAS can solve some of the issues associated with conventional seismic sensors. Studies have successfully demonstrated the use of DAS on cemented fibers for monitoring applications; however, such applications on tubing-deployed fibers are relatively uncommon. Application of tubing-deployed fibers is especially useful for preexisting wells, where there is no opportunity to install a fiber behind the casing. In the CO2CRC Otway Project, we acquired a 3D DAS VSP using a standard fiber-optic cable installed on the production tubing of the injector well. We aim to analyze the quality of the 3D DAS VSP on tubing, as well as discuss lessons learned from the current DAS deployment. We find the limitations associated with the DAS on tubing, as well as ways to improve the quality of the data sets for future surveys at Otway. Due to the reduced coupling and the long fiber length (approximately 20 km), the raw DAS records indicate a high level of noise relative to the signal. Despite the limitations, the migrated 3D DAS VSP data recorded by cable installed on tubing are able to image interfaces beyond the injection depth. Furthermore, we determine that the signal-to-noise ratio might be improved by reducing the fiber length.

We present one of the first case studies demonstrating the use of distributed acoustic sensing deployed on regional unlit fiber-optic telecommunication infrastructure (dark fiber) for broadband seismic monitoring of both near-surface soil properties and earthquake seismology. We recorded 7 months of passive seismic data on a 27 km section of dark fiber stretching from West Sacramento, CA to Woodland, CA, densely sampled at 2 m spacing. This dataset was processed to extract surface wave velocity information using ambient noise interferometry techniques; the resulting VS profiles were used to map both shallow structural profiles and groundwater depth, thus demonstrating that basin-scale variations in hydrological state could be resolved using this technique. The same array was utilized for detection of regional and teleseismic earthquakes and evaluated for long period response using records from the M8.1 Chiapas, Mexico 2017, Sep 8th event. The combination of these two sets of observations conclusively demonstrates that regionally extensive fiber-optic networks can effectively be utilized for a host of geoscience observation tasks at a combination of scale and resolution previously inaccessible.

The CO2CRC Otway Project “Stage 3” is planned to be an injection of between 15,000 and 30,000 tonnes, to be carried out at the Otway site in South Western Victoria, Australia. The objective of the project is to test unobtrusive, continuous monitoring methods that would operate with a small environmental footprint and limited impact on other land use activities such as farming. Progress since GHGT13 falls into three broad areas; reservoir characterisation, dynamic modelling, and feasibility and design of the monitoring methods. The appraisal well CRC-3 was successfully drilled and a wide range of core and log data was collected. While only 600-700 metres from existing logged wells, it was crucial to confirm that the injection concept was feasible and that the plume would migrate towards the proposed monitoring wells. In addition, injection tests were performed that confirmed pressure continuity to the existing wells. As there are many small faults in the area this was an important outcome. The geological and dynamical models for the experiment have been updated with the data from CRC-3; and model predictions have benefited from the ongoing performance history match to the stage 2C time-lapse seismic monitoring experiment. Currently the Stage 3 project is in its Define phase, having completed its Opportunity Definition and Evaluate phases. By the time of GHGT14 we expect to be commencing the Execution phase, with designs finalised and the drilling programme imminent.

Summary The CSIRO In-Situ Laboratory is a testing facility focused on a series of experiments on the controlled-release of a small amount (38 tonnes) of CO2 into a shallow fault zone. To understand the behaviour of movements of CO2 in the shallow formations and its interaction with faults and fracture zones it is necessary to gain detailed knowledge about structures in the vicinity of an injection well. Here we demonstrate the processing results of DAS 3D VSP data from a characterisation survey at the In-Situ Lab site acquired to obtain the detailed structural information around the injection and observation wells.

Residual CO2 trapping is a key mechanism of secure CO2 storage, an essential component of the Carbon Capture and Storage technology. Estimating the amount of CO2 that will be residually trapped in a saline aquifer formation remains a significant challenge. Here, we present the first oxygen isotope ratio (δ18O) measurements from a single-well experiment, the CO2CRC Otway 2B Extension, used to estimate levels of residual trapping of CO2. Following the initiation of the drive to residual saturation in the reservoir, reservoir water δ18O decreased, as predicted from the baseline isotope ratios of water and CO2, over a time span of only a few days. The isotope shift in the near-wellbore reservoir water is the result of isotope equilibrium exchange between residual CO2 and water. For the region further away from the well, the isotopic shift in the reservoir water can also be explained by isotopic exchange with mobile CO2 from ahead of the region driven to residual, or continuous isotopic exchange between water and residual CO2 during its back-production, complicating the interpretation of the change in reservoir water δ18O in terms of residual saturation. A small isotopic distinction of the baseline water and CO2 δ18O, together with issues encountered during the field experiment procedure, further prevents the estimation of residual CO2 saturation levels from oxygen isotope changes without significant uncertainty. The similarity of oxygen isotope-based near-wellbore saturation levels and independent estimates based on pulsed neutron logging indicates the potential of using oxygen isotope as an effective inherent tracer for determining residual saturation on a field scale within a few days.

Monitoring techniques capable of deep subsurface detection are desirable for early warning and leakage pathway identification in geologic carbon storage formations. This work demonstrates the feasibility of a pulse-testing-based leakage detection procedure, in which the storage reservoir is stimulated using periodic injection patterns and the acquired pressure perturbation signals are analyzed in the frequency domain to detect potential deviations in the reservoir's frequency domain responses. Unlike the traditional well testing and associated time domain analyses, pulse testing aims to minimize the interference of reservoir operations and other ambient noise by selecting appropriate pulsing frequencies such that reservoir responses to coded injection patterns can be uniquely determined in frequency domain. Field demonstration of this pulse-testing leakage detection technique was carried out at a CO2 enhanced oil recovery site—the Cranfield site located in Mississippi, USA, which has long been used as a carbon storage research site. During the demonstration, two sets of pulsing experiments (baseline and leak tests) were performed using 90-min and 150-min pulsing periods to demonstrate feasibility of time-lapse leakage detection. For leak tests, an artificial leakage source was created through rate-controlled venting of CO2 from one of the monitoring wells because of the lack of known leakage pathways at the site. Our results show that leakage events caused a significant deviation in the amplitude of the frequency response function, indicating that pulse testing may be deployed as a cost-effective active monitoring technique, with a great potential for site-wide automated monitoring.