Land seismic repeatability prediction from near-surface investigations at Naylor Field, Otway

Time-lapse seismic is a powerful methodology for remotely monitoring changes in oil and gas reservoirs. Its high sensitivity and resolving power make it the methodology of choice for monitoring CO2 sequestration in deep saline aquifers or depleted oil and gas fields. This method is now routinely applied offshore but rarely onshore because of inherently poor repeatability of land seismic data. Considering that CO2 sequestration on land is becoming a necessity, there is a great need to evaluate the feasibility of this method for land based CO2 sequestration projects. A feasibility study, onshore Otway Basin, Australia, aims at evaluating the viability of monitoring methodologies for the case of CO2 storage into a depleted gas field. Since injection of CO2 into a depleted gas field at a depth of around 2 km causes very subtle changes in elastic properties of the reservoir rock, it is critical to achieve high repeatability of time-lapse seismic surveys if they are to be implemented into a monitoring program.The goal of this thesis is to analyse the main factors affecting seismic repeatability at the Otway site. I aim to achieve this goal through the deployment of pre-base line measurements and combining the results with detailed numerical modelling studies. Such measurements have to be rapid, effective and quantitative so that a seismic monitoring team can decide whether to use time-lapse methodology when processing their data.To find the most likely repeatability at the Otway site I used so-called micro-arrays (surface and borehole) in a time-lapse manner to determine the seasonal variation of elastic properties of the near surface. The measurements were aimed at determining directional P-wave velocity and attenuation (Q-factor). The top soil (0.5m thick agricultural layer or elasto-plastic zone) had a low velocity and low Qfactor and hence significantly attenuated seismic energy.The elastic parameters obtained were then used to numerically simulate real timelapse surveys. The results obtained were compared and verified against conventional time-lapse studies conducted at the Otway site over a three year period, at different times of the year and with different sources. The agreement between numerical and field data, expressed through a normalised root mean square (NRMS) difference confirms that the effect of the near surface variation in the time-lapse land seismic can be predicted with minimum cost and through the deployment of small, inexpensive experiments

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

Assessing the repeatability of reflection seismic data in the presence of complex near-surface conditions CO2CRC Otway Project, Victoria, Australia

This study utilises repeated numerical tests to understand the effects of variable near-surface conditions on time-lapse seismic surveys. The numerical tests were aimed at reproducing the significant scattering observed in field experiments conducted at the Naylor site in the Otway Basin for the purpose of CO2 sequestration. In particular, the variation of elastic properties of both the top soil and the deeper rugose clay/limestone interface as a function of varying water saturation were investigated. Such tests simulate the measurements conducted in dry and wet seasons and to evaluate the contribution of these seasonal variations to seismic measurements in terms of non-repeatability. Full elastic pre-stack modelling experiments were carried out to quantify these effects and evaluate their individual contributions. The results show that the relatively simple scattering effects of the corrugated near-surface clay/limestone interface can have a profound effect on time-lapse surveys. The experiments also show that the changes in top soil saturation could potentially affect seismic signature even more than the corrugated deeper surface.Overall agreement between numerically predicted and in situ measured normalised root-mean-square (NRMS) differences between repeated (time-lapse) 2D seismic surveys warrant further investigation. Future field studies will include in situ measurements of the elastic properties of the weathered zone through the use of ‘micro Vertical Seismic Profiling (VSP)’ arrays and very dense refraction surveys. The results of this work may impact on other areas not associated with CO2 sequestration, such as imaging oil production over areas where producing fields suffer from a karstic topography, such as in the Middle East and Australia

Land Seismic Repeatability Prediction from Near Surface Investigation

High seismic repeatability is critical to the monitoring program of the Naylor Field because of the small time-lapse effect related to CO2 injection into a depleted gas reservoir (Naylor). To understand the effect of changing ground conditions on repeatability, we conducted so-called “micro-array” investigation of the near-surface layers at this site. A feature of the injection test site area is the near-surface karst topography. In such geological terrain, a change in water table level can influence the seismic response and cause changes in the seismic wave scattering pattern. Hence the aim of micro-array measurements was to determine the properties of the near surface layers during the wet and dry seasons. This could help us understand and ultimately predict the seismic response and hence survey repeatability at a given site.Measurements of seismic response due to near surface property changes (seasonal) could help optimise the design of time lapse surveys which ultimately yields improved survey differencing. Measured elastic properties of the near surface are used to produce seismic response and predict repeatability as a function of the variable soil conditions. Comparison of numerical and field data is finally used to verify the validity of this approach. In this study we investigated the variation of elastic properties of both top soil and the deeper rugose clay-limestone interface as a function of water depth level. Such tests in fact simulate the measurements conducted in dry and wet seasons and help evaluate the effect of these seasonal variations on the seismic signature, which is then analysed in terms of non-repeatability. In this study, we use both micro-borehole (micro VSP) and micro-refraction arrays to analyse directional properties of the near-surface. Finally, numerical tests were performed with calibrated soil parameters

Short time-lapse seismic repeatability test, CO2CRC Otway Project, Victoria, Australia

High seismic repeatability is critical to the monitoring program of the Naylor Field because of the small time-lapse effect related to CO2 injection into depleted gas reservoir (Naylor). To be able to obtain reliable seismic from a time-lapse seismic survey, the repeatability of the time-lapse survey must be determined. This can be achieved by performing zero-time repeatability tests through the acquisition and reacquisition of data in the same area before any changes occur in the reservoir. By comparing the analysis of multiple repeated pre-stack 2D surface seismic and VSP data, differences maybe established to minimize near-surface system effects

The properties of the near surface layers at Naylor Field, Otway, Australia

A feature of the injection test site of Naylor Field is the near-surface karst topography. In such geological terrain, a change in water table level can influence the seismic response and cause changes in the seismic wave scattering pattern. High seismic repeatability is critical to the monitoring program of the Naylor Field because of the small time-lapse effect related to CO2 injection into a depleted gas reservoir (Naylor). To understand the effect of changing ground conditions on repeatability, we conducted so-called “micro-array” investigation of the near-surface layers at this site. Hence the aim of micro-array measurements was to determine the properties of the near surface layers during the wet and dry seasons. This could help us to understand and ultimately predict the seismic response and hence survey repeatability at a given site.Measurements of seismic response due to near surface property changes (seasonal) could help optimise the design of time lapse surveys which ultimately yields improved survey differencing. Measured elastic properties of the near surface are used to produce seismic response and predict repeatability as a function of the variable soil conditions. In this study we investigated the variation of elastic properties of both top soil and the deeper rugose clay-limestone interface as a function of water depth level. Such tests in fact simulate the measurements conducted in dry and wet seasons and help evaluate the effect of these seasonal variations on the seismic signature, which is then analysed in terms of non-repeatability. In this study, we use both micro-borehole (micro VSP) and micro-refraction arrays to analyse directional properties of the near-surface