Boris Gurevich

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.

Full Waveform Inversion of Time-lapse Offset VSP Data - CO2CRC Otway Project Case Study

Summary Vertical Seismic Profile (VSP) is a useful tool for time-lapse monitoring. We conduct elastic 2D Full Waveform Inversion (FWI) on offset Vertical Seismic Profile (VSP) data to detect and quantify the time-lapse anomaly introduced by the CO2 geosequestration. Two datasets are being studied: a 2D synthetic dataset and the field dataset acquired during the Stage 2C of the CO2CRC Otway project. FWI proves capable of detecting the time-lapse anomaly on both datasets, however, the strength of the anomaly in the field data inversion results is lower than expected from theoretical predictions. We attribute this to 3D effects, which are not taken into account. In the end, FWI proves to be an instrument that fits the monitoring problem well. The inversion of the monitor dataset is significantly quicker than the baseline inversion, which enables the time-lapse anomaly identification shortly after the data acquisition.

Estimation of Intrinsic Q in Finely-layered Media by Wavefield Inversion of VSP Data - Australian North West Shelf Case-study

Seismic waves propagating through attenuative subsurface exhibit amplitude loss and distortion of frequency spectra. Proper description of attenuation process is required to compensate for these effects. Moreover, inelastic attenuation contains information on rock properties and could be utilized in attribute analysis for subsurface characterization. We propose to quantify inelastic (intrinsic) attenuation in horizontally-layered media by wavefield inversion of VSP data with respect to effective interval Q-factors. Impact of short-path multiples in finely-layered subsurface, i.e. scattering at stratigraphic boundaries, and other interference effects are taken into account by forward simulation over a high-resolution elastic model acquired from the well logs (resolution of 1.5m). We present a case study of approximate-zero-offset vertical seismic profile data from Wheatstone offshore site (Northern Shelf of Western Australia). First we describe the algorithm of 1D waveform Q-inversion. Then we validate it on the full-wave synthetics computed for the field survey geometry using a Global Matrix approach (OASES MIT code). Finally, we discuss the results of the Q-inversion application to the field ZVSP dataset versus Q-estimates by Centroid Frequency Shift method.

A Link between the Pressure Dependency of Elastic and Electrical Properties of Porous Rocks

We present a technique to invert for the stiff and compliant porosity from velocity measurements made as a function of differential pressure on saturated sandstones. A dual porosity concept is used for dry rock compressibility and a squirt model is employed for the pressure and frequency dependent elastic properties of the rocks when saturated. The total porosity obtained from inversion shows satisfactory agreement with experimental results. The electrical cementation factor was determined using the inverted porosity in combination with measured electrical conductivity. It was found that cementation factor increased exponentially with increasing differential pressure during isostatic loading. Elastic compressibility, electrical cementation factor and electrical conductivity of the saturated rocks correlate linearly with compliant porosity, and electrical cementation factor and electrical conductivity exhibit linear correlations with elastic compressibility of the saturated rocks under loading. The results show that the dual porosity concept is sufficient to explain the pressure dependency of elastic, electrical and joint elastic-electrical properties of saturated porous sandstones.

An Experimental Evidence of the Squirt-flow Effect in Glycerol-saturated Berea Sandstone at Seismic Frequencies

In sedimentary rocks saturated with fluid characterized by low mobility, which can be determined by low intrinsic permeability or high fluid viscosity, relative motion between pore fluid and rock skeleton may produce a significant impact on acoustic wave attenuation and dispersion of the elastic moduli of rocks at seismic frequencies. To investigate the influence of low-mobility pore fluid on elastic and anelastic parameters of sedimentary rock, the seismic-frequency laboratory measurements on dry and glycerol saturated Berea sandstone were carried out. The elastic moduli and extensional attenuation of the sandstone were measured at differential pressures of 5 MPa and 10 MPa under temperatures of 23 and 31 °C. Noticeable peaks of attenuation and significant dispersion of the moduli were detected for the glycerol-saturated sample. It was found that the frequencies of the peaks increase approximately twofold when the viscosity of glycerol was reduced by half with the temperature changing from 23 °C to 31 °C. Our analysis shows that the quantitative relationship between the extensional attenuation and the Young’s modulus measured for the glycerol-saturated sandstone is consistent with the causality principle presented by the Kramers-Kronig relationship.

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.