Milovan Urosevic

A simple approach to calculating refraction statics corrections

The higher the resolution demanded from the interpretation of seismic data, the more care required in the acquisition and processing of that data. Of the processing steps that directly influence resolution, the one that is having the most impact on land data is the statics correction. Geophysical conferences have devoted entire technical sessions to statics. Seismic crews frequently undertake specific refraction surveys just to obtain good statics data. Here we outline a very simple solution to the statics problem that produces remarkable results.

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.

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.

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.

Seismic Monitoring of CO2 Injection into a Depleted Gas Reservoir–Otway Basin Project

Within the Stage I of the Otway Basin Project, of the Australian Cooperative Research Centre fornGreenhouse Gas Technologies (CO2CRC), approximately 65,000 tons of CO2/CH4 mix in the ratio ofn80/20 was injected into the Waarre C formation (depleted Naylor gas reservoir) over the last twonyears. The CO2 was produced and transported from a nearby natural accumulation, via pipeline andninjected into a sandstone reservoir.nThe use of depleted gas fields for CO2 storage as well as CO2-based enhanced gas recovery are ofnglobal importance. Thus, the CO2CRC Otway Basin Pilot Project provides important experience innestablishing whether such scenarios can be monitored by geophysical techniques, in particular seismicntime-lapse methodology. Indeed injection of CO2 into a depleted gas reservoir (within residual gasnsaturation window) does not present favourable conditions for the application of geophysicalnmonitoring techniques. Numerical simulation of the CO2 injection process at Otway show thatnchanges in elasticity of the reservoir rock will be quite small and difficult to monitor even with thenmost powerful time-lapse (TL) seismic methodologies. Consequently, the design and implementationnof the monitoring program had to address these issues. The monitoring program had two objectives:n(1) to ensure detection of possible gas leakages out of the reservoir into other formations and (2) tonattempt to detect changes of seismic response due to CO2 injection into the reservoir.nTo increase the sensitivity of TL seismic we combined 3D VSP with 3D surface seismic. For a landnseismic case, we achieved excellent repeatability with 3D time lapse surveys, which at the reservoirnlevel produced normalised RMS difference values of about 20% for surface seismic and 10% for 3DnVSP, respectively. The location of the time-lapse anomaly detected at the reservoir level is broadlynconsistent with CO2 flow simulations. However borehole seismic measurements showed that timelapsenis very small to be reliably evaluated from repeated surface seismic measurements as thenanomaly is of a similar magnitude to noise, making its unique attribution to the CO2 plume difficult.nOne of the important outcomes of these studies is evaluation of land time-lapse seismic capabilities.nNew understanding and new methodologies for assessment of 3D seismic data repeatability werendeveloped during Otway Basin tests. This helped us understand and validate time-lapse signal formnthe reservoir. It also enabled us to demonstrate that quality of time-lapse land surveys can be highnenough to be able to detect very small, up to five thousand tonnes, leakages which is of essentialnimportance to any monitoring program as it provides possibility for rapid mitigation.