Roman Pevzner

Ground Roll Repeatability Analysis - CO2CRC Otway Project Case Study

Time-lapse (TL) seismic is an essential tool for the monitoring of changes in reservoir conditions induced by reservoir production, reservoir stimulation and more recently CO2 sequestration. The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently conducting a carbon dioxide geo-sequestration pilot study in the Otway Basin, onshore Victoria, Australia. An extensive TL seismic monitoring is a part of monitoring and verification program of the project. Being a well-developed methodology for offshore applications, time-lapse seismic is also gaining popularity as an onshore reservoir monitoring technique. However its onshore use is limited by issues of seismic repeatability. This is primarily attributed to the higher near-surface variability as compared to marine seismic. Analysis of several repeated Otway 2D surveys show that different frequency bands have different repeatability. Low frequencies (< 30 Hz), contaminated by the ground roll noise, are characterised by particularly poor repeatability. In this paper we analyse three repeated 2D surveys obtained with same geometry but different sources and near-surface conditions in order to investigate ground roll repeatability by evaluation of surface waves phase velocity spectra.

The CO2CRC otway project deployment of a distributed acoustic sensing network coupled with permanent rotary sources

Summary We have deployed a novel permanent monitoring system at the Australian CO2CRC Otway Site that includes a surface and borehole distributed acoustic sensing (DAS) network with orbital vibrator (rotary) surface seismic sources. DAS is an emerging technology for performing seismic acquisition based on optical interferometric techniques, which allows for data collection with a wide spatial aperture and high temporal resolution using commercially available telecommunications fibres. DAS sensitivity currently lags behind conventional discrete geophone and hydrophone sensor technologies. Our implementation of surface rotary seismic sources is based on open-loop controlled asynchronous motors. This avoids the complexity of feedback loops for phase control, instead using deconvolution of the source function as measured by a shallow source-monitor sensor. Initial data analysis shows that the amount of energy available from long source sweeps overcomes limitations in DAS sensitivity. The combination of relatively inexpensive but powerful permanent surface sources with permanent DAS deployment in an areal array provides a new paradigm for time-lapse seismic monitoring. The methodology we describe has broad applicability for long-term reservoir surveillance, with time-lapse change sensitive to many subsurface properties.

Using Fresnel Zone to Characterise and Image Different Types of Diffractors in Low S/N Situations

We investigate the possibility to image and characterise the different types of diffractors – such as point, line, and edge – and separate them from reflections by limiting the aperture around the tangent point between the diffraction hyperbola and the relfection traveltime surface. To this end, we look at the tangents of the diffraction hyperbola with typical traveltime surfaces corresponding to the different diffrator types. We do this by studying the coherency of the recorded wavefield along the diffraction hyperbolae. The different types of diffractors show different energy distributions that can be used to characterise the diffractors and separate them from specular reflections. These areas of high energy correspond to the Fresnel zones of the particular diffractor type. Limiting the aperture of the diffraction hyperbola to the particular Fresnel zone results in improvement in S/N, while omitting the Fresnel zone corresponding to the reflections allows for imaging of the diffractors, which have larger Fresnels zones.

CO2 storage site characterisation at the location of harvey-3 well, harvey, Western Australia

The South West Hub is the first commercial scale CO2 capture and sequestration project in Australia. The project aims to capture CO2 from several significant polluters that are located around the town of Harvey, Western Australia. The potential CO2 reservoir is the prominent Lesueur sandstone formation while the Harvey-3 well is likely to be utilised for the first geo-sequestration test. Unfortunately, due to survey restrictions imposed in 2014, the area around the Harvey-3 well is void of seismic information. New developments have given rise to new seismic investigations which had to be designed in a unique manner due to limited access to the site. The data acquisition program is comprised of 2D surface and borehole surveys (550 OVSP points) and 3D surface and VSP surveys. Results will be discussed in detail.

Stochastic Time-lapse Inversion of a CO2 Sequestration Synthetic Seismic Data

The objective of this work is to assess the effect of noise and parameterisation on the performance of the stochastic time lapse inversion. To do so, a noise-free synthetic dataset created for a feasibility study of an actual CO2 sequestration project (CO2CRC Otway Project) was inverted and used as a baseline. Noise (random and coherent) was added to the seismic data, input parameters changed and the results were compared with the baseline case. The findings for wrong parameterisation cases were very encouraging and consistent with the theory. When random noise was added to the input seismic data the algorithm was able to recover the true model within an acceptable margin of error. However, addition of coherent noise affected the inversion result significantly. Only when the root-mean-square (RMS) amplitude level was comparable to the one in the difference volume the algorithm was able to actually differentiate the noise from the signal. These findings support the idea of a careful processing to avoid coherent noise and a judicious interpretation when it is unavo idable. Finally a new indicator was developed to calculate the improvement in detectability after the input of new data using the stochastic time lapse inversion.

Borehole seismic monitoring of a small-scale CO2 injection - The CO2CRC Otway project feasibility study

The CO2CRC Otway project is the first Australian demonstration of safe geological storage of carbon dioxide. The first stage of the project, concluded in 2010, was focussed on injection of more than 66,000 tonnes of CO2/CH4 gas mixture (~80% of CO2) into a depleted gas reservoir located at approximately 2 km depth through a purposely drilled CRC-1 well. The next stage of the project, which is currently underway examines small scale (up to 15,000 tonnes) gas injection into a saline aquifer located at 1.5 km depth using CRC-2 well. In this paper we use full elastic 3D finite difference modelling to evaluate the level of time-lapse signal on walk-away VSP data acquired in CRC-1 and CRC-2 wells. The results are used to optimise borehole seismic acquisition parameters.

Application of Diffrction Imaging and Steered Migration to 3D Seismic Data from the South West Hub CCS Project

The assessment of subsurface architecture and location of faults is key information in CO2 geosequestration projects. The 3D seismic survey has been acquired as a part of the SW Hub CCS project in vicinity of potential future CO2 injection sites. The acquisition was carried out within the Harvey and Waroona Shires about 150 km south-east from Perth, Western Australia. In order to support the fault detection and decrease uncertainties on the structural framework two additional imaging techniques have been applied to the 3D seismic volume. One is a diffraction imaging algorithm, the other is an alteration to the post-stack Kirchhoff migration – the steered migration. The application of these methods to the seismic volume enhanced the signal to noise ratio of the final migrated images and supported the understanding of fault distribution in the study area.

Quantifying Time-lapse Seismic Signal Detection for the Otway Project Using Prestack Migration

During Stage 2 of the Otway CCS Australian project it is planned to inject a small, up to 15,000 tonnes of gas, into a saline aquifer located at depth of 1500 m. In CO2 sequestration, the ability to detect CO2 plumes is one of the main purposes of using time-lapse seismic imaging. The detectability of CO2 in seismic time-lapse surveys relies on two main factors: a sufficiently strong signal and sufficiently small noise. Therefore, to model time-lapse seismic records, we need to model not only the seismic response of the geology and the plume but also the time-lapse noise. Because plume detection is determined by the S/N, the ability to model realistic time-lapse noise is crucial in any feasibility study. In this work, we propose a more realistic approach by adding band-limited random noise to the pre-stack data (shot gathers) to match the S/N of field data. Using these noisy gathers we then compare the detectability of CO2 plume by using pre- and post-stack Kirchhoff migrations.

3D Diffraction Imaging of Linear Features and its Application to Seismic Monitoring

Many subsurface features, such as faults, fractures, cracks, or fluid content terminations are defined by geological discontinuities. Seismic response from such features is encoded in diffractions. We develop an algorithm for imaging such discontinuities by detecting edge diffractions. The algorithm exploits phase-reversal phenomena of edge diffractions, and uses them as a criterion to separate diffractions from specular reflections. The performance of the method has been demonstrated on both synthetic and real 3D seismic data. The output image focuses the diffracted energy back to its origin, and shows high semblance values at the edge of the object. The method is applied on conventionally stacked data producing an image contains only diffraction events called (D-volume). We also reveal the potential of diffractions to image and track the changes of the CO2 plume using time-lapse analysis and detect any possible CO2 seepage from its primary containment.

Time-lapse Seismic Anisotropy Analysis for CO2 Geosequestration Using 3D 3C VSP Data

We present a quantitative analysis of the change of the seismic anisotropy observed after CO2 injection within the CO2CRC Otway Basin Project. We invert two 3C 3D VSP datasets for the density scaled stiffness tensors and compute the difference of the resulting compliance tensors. The results are in good agreement with the observed changes in the time-lapse zero-offset VSP experiments and with the available sonic log data.