Rodney Calvert

The virtual source method: Theory and case study

We present a way to image through complex overburden. The method uses surface shots with downhole receivers placed below the most complex part of the troublesome overburden. No knowledge of the velocity model between shots and receivers is required. The method uses time-reversal logic to create a new downward-continued data set with virtual sources (VSs) at the geophone locations. Time reversal focuses energy that passes through the overburden into useful primary energy for the VS. In contrast to physical acoustics, our time reversal is done on a computer, utilizing conventional acquisition with surface shots and downhole geophones. With this approach, we can image below extremely complex (realistic) overburden — in fact, the more complex the better. We recast the data to those with sources where we actually know and can control the waveform that has a downward-radiation pattern that may also be controlled, and is reproducible for 4D even if the near-surface changes or the shooting geometry is altered sl...

Virtual Source: New Method For Imaging And 4D Below Complex Overburden

We propose an alternative solution that does not require knowledge of the near-surface velocity model. The price to pay is placing geophones in the Earth below the most complex near-surface part while keeping sources at the surface. Receivers may sit in horizontal or slanted wells, which may be producers/injectors or dedicated sidetracks. Utilizing time reversal logic, we convert surfaceto-downhole data into a new dataset with downhole Virtual Sources (VS) located at geophone positions. The resulting VS dataset with both downhole sources and receivers can be conventionally imaged requiring only the bottom portion of the velocity model below the receivers that is more simple to obtain. To illustrate the technique, we show application to one synthetic data set and one field case study.

Improving the virtual source method by wavefield separation

The virtual source method has recently been proposed to image and monitor below complex and time-varying overburden. The method requires surface shooting recorded at downhole receivers placed below the distorting or changing part of the overburden. Redatuming with the measured Green’s function allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. There are still some challenges that need to be addressed in the virtual source method, such as limited acquisition aperture and energy coming from the overburden. We demonstrate that up-down wavefield separation can substantially improve the quality of virtual source data. First, it allows us to eliminate artifacts associated with the limited acquisition aperture typically used in practice. Second, it allows us to reconstruct a new optimized response in the absence of downgoing reflections and multiples from the overburden. These improvements are illustrated on a synthetic data set of a complex laye...

Acquisition geometry requirements for generating virtual-source data

Using model and field data, this article reviews the virtual-source method and its acquisition geometry requirements. Before we go into the details of the acquisition geometry requirements, let us briefly review the basic concept and the advantages of the virtual-source method. A typical surface seismic experiment has sources on the surface to excite waves that propagate through the subsurface. Surface receivers record the reflected waves. In order to image the subsurface, we migrate the reflected wavefield recorded by the receivers, using an estimate of the subsurface velocity model. However, the near surface is usually complex, and the velocity is difficult to estimate. These near-surface inhomogeneities, if not represented in the migration velocity model, defocus the deeper image. In order to avoid the estimation of the near-surface velocity model, Bakulin and Calvert (2006) proposed the virtual-source method, a technique that uses cross-correlation of the wavefield recorded by a given pair of receiver...

Strengthening the virtual-source method for time-lapse monitoring

Time-lapse monitoring is a powerful tool for tracking subsurface changes resulting from fluid migration. Conventional time-lapse monitoring can be done by observing differences between two seismic surveys over the surveillance period. Along with the changes in the subsurface, differences in the two seismic surveys are also caused by variations in the near-surface overburden and acquisition discrepancies. The virtual-source method monitors below the time-varying near-surface by redatuming the data down to the subsurface receiver locations. It crosscorrelates the signal that results from surface shooting recorded by subsurface receivers placed below the near-surface. For the Mars field data, redatuming the recorded response down to the permanently placed ocean-bottom cable (OBC) receivers using the virtual-source method allows one to reconstruct a survey as if virtualsources were buried at the OBC receiver locations and the medium above them were a homogeneous half-space. Separating the recorded wavefields ...

Virtual shear source makes shear waves with air guns

We demonstrate a novel application of the virtual source method to create shear-wave sources at the location of buried geophones. These virtual downhole sources excite shear waves with a different radiation pattern than known sources. They can be useful in various shear-wave applications. Here we focus on the virtual shear check shot to generate accurate shear-velocity profiles in offshore environments using typical acquisition for marine walkaway vertical seismic profiling (VSP). The virtual source method is applied to walkaway VSP data to obtain new traces resembling seismograms acquired with downhole seismic sources at geophone locations, thus bypassing any overburden complexity. The virtual sources can be synthesized to radiate predominantly shear waves by collecting converted-wave energy scattered throughout the overburden. We illustrate the concept in a synthetic layered model and demonstrate the method by estimating accurate P- and S-wave velocity profiles below salt using a walkaway VSP from the deepwater Gulf of Mexico.

Overview of global 4D seismic implementation strategy

For many Shell-operated fields around the world, time-lapse reservoir monitoring (4D) is now an integral part of field management and some 25 dedicated 4D surveys were acquired for this purpose by the end of 2002. This widespread application is the result of a focused implementation effort aimed at global deployment to maximize the value extracted from the surveys in terms of saved costs, increased production, increased recovery and improved HSE management, where effective implementation is achieved through a combination of global operatorship and technology capability. Apart from ensuring global deployment, there is the challenge to extend the range of 4D applicability. To achieve this the project has three application portfolios: todays‘ proven’ 4D technology portfolio, which is about monitoring fluid movements in thick clastic oil reservoirs offshore. Results come largely from comparing reservoir simulator output with difference maps derived from repeat 4D streamer surveys; a‘ stretch’ portfolio where the technology is applied to gas reservoirs, land data, stacked reservoirs, carbonate fields and to the monitoring of pressure changes; a‘ tomorrows technology’ portfolio, which has the potential to increase the application base even further. The new technologies are about the use of permanent arrays, downhole acquisition, passive listening and ‘smart fields’ where semi-continuous 4D monitoring provides eyes and ears between the wells. More and more value is realized as 4D becomes fully integrated with subsurface work flows and modelling tools. Benefits from 4D technology for individual fields can be in the tens or hundreds of millions of dollars. A large fraction of these come from 4D surprises, illustrating that we tend to underestimate our uncertainties and suggesting different approaches to field management.

Virtual Shear Source: a New Method For Shear-wave Seismic Surveys

Shear-wave seismology holds great promise but always remains small niche activity due to a variety of operational and subsurface limitations. In this study we demonstrate that the Virtual Source method can overcome many of these limitations and revive shear-wave seismology. With an array of sources at the surface this method allows us to obtain a Virtual Shear Source at the location of each downhole geophone in a well. Firstly, in certain cases it allows us to generate pure shear-wave energy without P -wave contamination using conventional P -wave sources even in a marine environment. Secondly, we can construct SS images of the subsurface even through a complex nearsurface for which the velocity model is unknown. Thirdly, we can control the polarization. All this is at a price of placing geophones in the subsurface and making downhole recordings. This price tag is expected to decrease with greater use of permanent downhole monitoring, cheap wells and instrumented oilfields.

Virtual source gathers and attenuation of free‐surface multiples using OBC data:implementation issues and a case study

Virtual source imaging is a technique based on extracting the Green’s function that characterizes wave propagation between two receivers by cross-correlating the wave-fields recorded by these receivers. We focus on implementation issues in generating a virtual source gather from a multi-component OBC data recorded at the Mars field. The implementation issues include choice of the receiver that acts as the virtual source and the number of sources over which the cross-correlated data is stacked. The pre-stack correlated data (correlation gather) is a useful diagnostic for quality control and for assessing the source locations that give a stationary phase contribution. By stacking over specific source locations, we restrict the direction of the incoming energy and generate virtual source gathers containing arrivals within a specified horizontal slowness interval. We compare the virtual source gather generated by using a small number of sources to the virtual source gather generated by using a larger source aperture for stacking. Artifacts due to the traces at the edges of the source aperture can be suppressed by applying a taper before stacking the correlation gather. Another artifact observed in virtual source gathers is due to side-lobes of the auto-correlation of the source-time function. We show the use of dualsensor summation to separate the upand the down-going energy in the raw data and using that to generate virtual source gathers containing only the upgoing energy, hence attenuating the free-surface multiples.

The virtual‐source method applied to Mars field OBC data for time‐lapse monitoring

The virtual source method has recently been proposed to image and monitor below a complex and time-varying overburden. The method requires surface shooting recorded by subsurface receivers placed below the distorting or changing part of the overburden. Redatuming the recorded response to the receiver locations allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. The ability to redatum the data independent of the knowledge of time-varying overburden velocities makes the virtual source method a valuable tool for time-lapse monitoring. We apply the virtual source method to the Mars field OBC data acquired in the deepwater Gulf of Mexico with 120 multi-component sensors permanently placed on the seafloor. Applying to the virtual source method, a combination of up-down wavefield separation and deconvolution of the correlation gather by the source power spectrum suppresses the influences of changes in the overburden (sea water), thus strengthening the virtual source method for time-lapse monitoring.