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

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

Finite-difference numerical experiments and laboratory testing of attenuation and dispersion in patchy-saturated media

Accurate numerical modeling tools are of practical importance for understanding of seismic wave propagation in reservoir rocks. 2D finite-difference (FD) poroelastic code is used for simulation of acoustic wave propagation through water saturated medium with regular distributed circular gas inclusions. The results of the numerical simulations are validated with the newly derived theoretical solution for the same geometry of patches. Finally, the numerical results for a media with randomly distributed patches are used to verify the interpretation of the results of a laboratory experiment in which saturation and compressional velocity are simultaneously measured in a sandstone sample

Repeatability of land time-lapse seismic surveys - Otway project 2D test line case study

Full range of 2D and 3D borehole and surface seismic methods are utilised for time-lapse surveys for monitoring of CO2 sequestration at Otway Basin pilot project site. Conducting land time-lapse surveys is challenging task because of typical variability of ground conditions, source-receiver coupling, ambient noise which results in poor repeatability of land seismic data. We analysed the major factors influencing repeatability of land seismic data. We also analysed both synthetic and field data for possible limitations of the surface reflection seismic method when it comes to repeatability. We show that changes in near surface conditions will produce kinematic differences but also different ground roll patterns. This will in general require slightly different parameters for processing of two successive surveys if they are acquired for vastly different soil conditions. We then conduct extensive numerical and field tests to show that the S/N variability as function of the source strength relative to the background noise level is crucial. Source type is less important for time lapse surveys as long as S/N ratio is high. Our tests included impact (weight drop) and vibrating (IVI Mini-Buggy) sources

Microstructural characterisation of organic-rich shale before and after pyrolysis

Organic-rich shales, traditionally considered as source rocks, have recently become an ambitious goal for the oil and gas industry as important unconventional reservoirs. Understanding of the initiation and development of fractures in organic-rich shales is crucially important as fractures could drastically increase the permeability of these otherwise low-permeable rocks. Fracturing can be induced by rapid decomposition of organic matter caused by either natural heating, such as emplacement of magmatic bodies into sedimentary basins, or thermal methods used for enhanced oil recovery. In this work the authors study fracture initiation and development caused by dry pyrolysis of Kimmeridge shale, which is characterised with a high total organic carbon content of more than 20%. X-ray diffraction (XRD) analysis exhibits high carbonate (both calcite and dolomite) and low clay (illite) content. Field emission gun scanning electron microscopy (FEG-SEM) shows that kerogen is presented either as a load-bearing matrix or as a filling of the primary porosity with pores being of micron size. Cylindrical samples of the Kimmeridge shale are heated up to temperatures in the range of 330–430°C. High-resolution X-ray microtomographic (micro-CT) images are obtained. The microtomographic images are processed using AVIZO (Visualization Sciences Group) to identify and statistically characterise large kerogen-filled pores and pre-existing and initiated cracks. The relationship between the total area of fractures and the temperature experienced by the sample has been obtained. Total organic carbon content is determined for samples subjected to heating experiments. This approach enables a quantitative analysis of fracture initiation and development in organic-rich shales during heating

Feasibility of time-lapse seismic methodology for monitoring the injection of small quantities of CO2 into a saline formation, CO2CRC Otway Project

A key objective of Stage 2 of the CO2CRC Otway Project is to explore the ability of geophysical methods to detect and monitor injection of greenhouse gas into a saline formation. For this purpose, injection of some 10,000 30,000 tonnes of CO2-rich mixture into the Paaratte formation, a saline aquifer located at a depth of about 1,400 m, is planned. Before such an injection experiment is undertaken, we assess the feasibility of geophysical monitoring using computer modelling. To examine the detectability of the plume we need to estimate the time-lapse signal and time- lapse noise. The time lapse signal is modelled using flow simulations, fluid substitution and seismic forward modelling. In order to assess the applicability of time-lapse seismic to monitor the injection, the predicted signal is compared to the time-lapse noise level from the recent 4D seismic survey acquired at the Otway site in 2009-2010. The methodology is applied to two alternative reservoir intervals located at a depth of 1392-1399 m and 1445-1465 m below the sea level, respectively. These intervals are considered to be the two possible options for the injection. The results show that injection into the lower interval will produce a plume of a larger thickness and smaller lateral extent, and a seismic response that is more likely to be detectable. The developed feasibility assessment workflow, and the results of its application to the Otway site, can be used to assess the ability of seismic methods to detect and monitor greenhouse gas leakage in other CCS projects