Konstantin Tertyshnikov

Burying receivers for improved time‐lapse seismic repeatability: CO2CRC Otway field experiment

4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO2 sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time-lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount(around 15,000 tonnes) of CO2/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability. A small-scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3mdeep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1–12m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30–90 Hz.

Diffraction Imaging in Hard-rock Environments

Hard rock seismic exploration normally has to deal with rather complex geological environments. These types of environments are usually characterized by a large number of local heterogeneities. The seismic data from such environments often have a poor signal to noise ratio because of the complexity of hard rock geology. In such situations, the processing algorithms that are capable of handling data with a low signal/noise ratio and are able to image geological discontinuities and subvertical structures are essential. Herein we present a modification of the 3D Kirchhoff post-stack migration algorithm and diffraction imaging. The modification utilizes coherency attributes obtained by the diffraction imaging algorithm in 3D to weight or steer the main Kirchhoff summation. We applied diffraction techniques to a number of 3D seismic datasets from different hard rock mine sites.

Steering Migration with Diffractions in Seismic Exploration for Hard Rock Environments

Hard rock seismic exploration normally has to deal with rather complex geological environments. These types of environments are usually characterized by a large number of local heterogeneities (e.g., faults, fracture zones, steeply dip interfaces). The seismic data from such environments often have a poor signal to noise ratio because of the complexity of hard rock geology. In such situations, the processing algorithms that are capable of handling data with a low signal/noise ratio are essential for a reflection seismic exploration. In this paper we describe an alteration of the 3D Kirchhoff post-stack migration algorithm that utilizes coherency attributes obtained by the diffraction imaging algorithm in 3D to weight or steer the main Kirchhoff summation. We apply the method to a 3D synthetic model with a presence of high level of random noise; and test the algorithm on the 3D seismic volume acquired on a mine site located in Western Australia.

Time-lapse full waveform inversion of vertical seismic profile data: Workflow and application to the CO2CRC Otway project

Vertical seismic profile (VSP) is one of the technologies for monitoring hydrocarbon production and CO2 geosequestration. However quantitative interpretation of time-lapse VSP is challenging due to its irregular distribution of source-receiver offsets. One way to overcome this challenge is to use full waveform inversion (FWI), which does not require regular offsets. We present a workflow of elastic FWI applied to offset vertical seismic profile data for the purpose of identification and estimation of time-lapse changes introduced by injection of 15,000 tonnes of CO2-rich gas mixture at 1.5 km depth. Application of this workflow to both synthetic and field data shows that elastic FWI is able to detect and quantify the time-lapse anomaly in P wave velocity with the magnitude of 100-150 m/s.

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.

Estimation of seismic attenuation and prediction of VTI anisotropy parameters from VSP and log data: a case study from the Middle East

Quantitative interpretation of seismic data depends upon the amplitude analysis of reflected waves. However, the quality of the image obtained can be significantly affected by attenuation and anisotropy in the overburden. Therefore, the insights into the magnitude, sources and spatial distribution of these parameters may prove to be substantial for improving the quality of seismic image and reservoir characterisation. Both seismic vertical transverse isotropy (VTI) anisotropy and scattering attenuation could be caused by the layering. In order for these phenomena to play a major role, strong elastic contrasts between them are needed. In this paper, we present the case study from a typical setting in the Middle East where such a contrast between stiff carbonates and relatively soft siliciclastic sediments significantly deteriorates the quality of the seismic image. Vertical seismic profiling (VSP) and log data from one of the wells in the region are used to demonstrate this phenomenon. Apparent attenuation from VSP data is estimated using a modified centroid frequency shift method. The log data for seismic forward modelling is used in order to show that scattering could be a significant contributor to the apparent attenuation. In addition, by using both Backus averaging and synthetic walk-away VSP data analysis, we demonstrate that the same layering can be responsible for a significant TI anisotropy.

Subsurface Imaging Using Buried DAS and Geophone Arrays - Preliminary Results from CO2CRC Otway Project

A permanent geophone array along with a fibre optic distributed acoustic sensing (DAS) array were deployed at the CO2CRC Otway Project site in order to conduct seismic monitoring of a CO2 plume during a small-scale injection test. This study aims to assess the ability for a permanent geophone array to overcome issues related to different acquisition (receiver) designs, high ambient noise level and seasonal variations in the near surface, as well as to test the DAS system for performing cost-effective time lapse seismic measurements. The acquisition of 3D seismic data is performed for this purpose using ~3000 vibroseis source points. We show the preliminary results of seismic reflection imaging conducted using DAS data. We observe the differences in performance between a standard commercially available tactical fibre optic cable and a custom helically wound cable. The results of this study and the workflows established will be used for processing a complete 3D seismic dataset acquired with a DAS array before being compared to a conventional geophone array.

Offset VSP for the Reservoir Monitoring

Summary Offset Vertical Seismic Profile (VSP) is often used for time-lapse seismic monitoring in oil/gas exploration or CO2 geosequestration. Standard offset VSP processing technologies allow the imaging and qualitative interpretation of time-lapse changes in the medium. However, quantification of the observed time-lapse anomalies is often needed. We suggest using Full Waveform Inversion (FWI) for quantitative interpretation of the offset VSP data. We apply conventional time-lapse processing and time-lapse FWI to the offset VSP dataset acquired in the Otway field and compare the results. We observe a match in the images of the CO2 plume obtained by both methods and suggest that using FWI increases the value of offset VSP data.

DAS Versus Geophones: a Quantitative Comparison of a VSP Survey at a Dedicated Field Laboratory

Summary Distributed Acoustic Sensing (DAS) is a novel technology to acquire acoustic data. DAS is noticeably promising for Vertical Seismic Profiling (VSP) applications as it offers highly sampled data, acquired simultaneously at all levels, at relatively low cost. In this study, DAS data is acquired using two different cable deployments: cemented behind the casing, and deployed suspended in the well. Both datasets were compared to conventional 3-component geophone VSP. DAS acquired with cemented cable presented similar quality and signal to noise ratio as geophone data, although it suffers from loss of high frequencies. The suspended cable presented predominantly tube noise, however reflections can still be recognized. We hope the results from this study contribute to a broader use of DAS for VSP acquisition.

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.