Applicability of Zero-offset and Offset VSP for Time-lapse monitoring - CO2CRC Otway Project Case Study

Time-lapse seismic technology is proven to be a powerful tool for monitoring of reservoir depletion or fluid (such as gas, water or steam) injection into subsurface formations. While being almost standard technique for offshore reservoirs, time lapse surveys are still relatively rare used onshore due to poor repeatability of land seismic data. Borehole time-lapse seismic surveys could be a good alternative because of very stable receiver, and, in certain cases, source conditions, acquisition and processing of zero-offset and offset VSP is reasonably fast. In this paper we analyse repeated zero-offset and offset VSP acquired in within CO2CRC Otway pilot project scientific program in year 2007 (pre-injection) and year 2010 (postinjection). We address repeatability of the data and possibility to use simple VSP techniques for monitoring purposes

Estimation of Azimuthal anisotropy from VSP data using multicomponent velocity analysis

Observation of azimuthal shear wave anisotropy can be useful for characterisation of fractures or stress field. Shear wave anisotropy is often estimated by measuring splitting of individual shear-wave events on VSP data; however this method may become unreliable for zero-offset (marine) VSP where the seismogram often contains no strong individual shear events but many low-amplitude PS conversions. In this paper we introduce a new approach to estimation of fast and slow shear wave velocities and orientation of polarization planes based on the multi-component velocity analysis. This technique is applicable to zero-offset VSP data and should take advantage of the presence of a large number of shear wave events with the same velocity. The main idea is to estimate the velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity. The algorithm was tested on marine 3C VSP acquired in the North-West Shelf of Australia. These tests show good agreement between anisotropy parameters (magnitude and orientation) derived from the VSP and cross-dipole sonic log data

Estimation of azimuthal anisotropy from VSP data using multicomponent S-wave velocity analysis

Observation of azimuthal shear wave anisotropy can be useful for characterization of fractures or stress fields. Shear wave anisotropy is often estimated by measuring splitting of individual shear wave events in vertical seismic profile (VSP) data. However, this method may become unreliable for zero-offset (marine) VSP where the seismogram often contains no strong individual shear events, such as direct downgoing shear wave, but often contains many low-amplitude PS mode converted waves. We have developed a new approach for estimation of the fast and slow shear wave velocities and orientation of polarization planes based on the multicomponent linear traveltime moveout velocity analysis. This technique is applicable to zero-offset VSP data, and should take advantage of the presence of a large number of shear wave events with the same apparent velocity (which, for a horizontally layered medium, should be close to the interval velocity).The approach assumes that the VSP data are acquired in a vertical well drilled in an orthorhombic medium with a horizontal symmetry plane (including horizontal transverse isotropy). The main idea is to estimate the dominant apparent velocity for a given polarization direction by measuring the coherency of the seismic signal of a large number of events as a function of the apparent velocity. The algorithm was tested on marine three-component (3C) VSP acquired in the North West Shelf of Australia, and on land 3C VSP acquired with different sources in the same borehole located in Otway Basin, Victoria. These tests show good agreement between anisotropy parameters (magnitude and orientation) derived from the VSP and cross-dipole sonic log data

Numerical Testing of Virtual Source Method

Cross-well profiling is an important technique that produces a high resolution image between wells and provides a better delineation of rock properties. Yet, using a real downhole source is considered a major obstacle in a well logging operation. Thus, the Virtual Source Method (VSM) was introduced as an emerging technique that can virtually place a downhole source without damaging the well. This allowed us to passively use different source types on the surface with no velocity correction required for the medium between sources and downhole receivers.Using a numerical model, we produced our synthetics and created the Virtual Source gathers. A comparison was then made between the real downhole shot gathers and the VS gathers, and we found that the direct arrivals and the reflections are matching in time. A tomogram section was then produced using the direct P-wave arrivals. Our study demonstrated that cross-well tomography can be used with the Virtual Source Method, as the inversion model that was produced showed a velocity distribution matching with the geological model

Processing of land 4D seismic data in case of limited area of repeated survey - a case study from Otway Basin, Australia

Time lapse 3D seismic is an important part of monitoring and verification program of the Otway Project - an Australian first demonstration of the deep geological storage of CO2 located on-shore in Victoria. From March, 2008 CO2-rich gas is being injected into a depleted gas reservoir at a depth of around 2050 metres. In year 2000 3D pre-production seismic data were acquired over large area, covering Naylor field, our CO2 sequestration site and also several adjacent small gas fields. Baseline 3D seismic data were acquired in December 2007, however due to logistical problems and cost of the survey size of the area was much smaller (only 3 sq. km). Processing of land time-lapse seismic surveys is a challenging task and in this case it was additionally complicated since limited migration aperture was determined to be one of main problems affecting imaging of the target horizon. To overcome this limitation we adopted an approach which is based on joint processing of two different 3D vintage sets with different size of survey area

Feasibility of borehole reflection seismology for hard rock mineral exploration

Complex geological models typical of hard rock mineral exploration in the Yilgarn Craton of Western Australiahave been created. Through full waveform synthetic modeling, borehole reflection seismology has been tested inthese environments with borehole geometries typical of hard rock exploration techniques. One such example ispresented here. Synthetic testing has shown that borehole reflection seismic sections suffer from lack of aperture in the down-dip direction. Thus Large offsets and higher shot density is required on the down-dip side of the borehole to compensate for this. However at large offsets wavefield identification is complex and correct separation of wavefields for imaging is difficult. These limitations and acquisition specific geometries and processing are discussed here. Initial field studies conducted during a pilot study show potential for seismic imaging from angled boreholes

Seismic Monitoring of CO2 Geosequestration in Otway Basin, Australia

CO2CRC Otway Project is the Australia s first demonstration of the deep geological storage of CO2. CO2 has been injected in a depleted gas field at the depth of 2050 m and then will be injected in saline aquifer at the depth of around 1 km. For time lapse studies, we had four different 3D seismic surveys available. Besides of a large 3D seismic volume acquired in the year 2000 prior gas production of the reservoir, three sequential 3D surveys were acquired at the same site but over a smaller area: baseline survey in 2008 and two monitoring surveys in 2009 and 2010. We concentrated on repeatability of 4D seismic data acquisition and processing. This led us to the results which allow to define CO2 location in the reservoir and proves that time-lapse seismic is a valuable tool in CO2 monitoring even in on-shore case

Estimation of Scattering Attenuation from Zero-offset VSP Data: CO2CRC Otway Project Case Study

Seismic attenuation consists of anelastic absorption and scattering loss. Due to the dominance of stratification, the scattering attenuation in the sedimentary crust is dominated by 1-D scattering. In this study we applied an integrated workflow for estimation of attenuation from ZVSP and log data to a comprehensive dataset acquired at Otway basin. Both 1D reflectivity modeling and application of generalized O’Doherty-Anstey theory to the Otway log data shows that the 1-D scattering component of attenuation gives Q of over 200. At the same time, average Q estimated from field VSP data value is close to 60. Hence we conclude that scattering plays a relatively minor role in the study area. Further research is required to understand whether this conclusion holds in other areas. In particular, scattering attenuation might be larger in environments with larger variability of elastic properties between layers, such as in areas with laminated coal layers