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In this study we investigate methods of elastic-wavefield interferometry to obtain P- and S-wave velocity information from the 3D P-wave vibrator VSP at Wamsutter Field in Wyoming. The Wamsutter Field consists of naturally fractured tight gas sand reservoirs where geophysical objectives are lithology discrimination and fracture identification. We examine P-wave pseudo-source VSP data to eliminate strong tube-wave noise generated by sources near the borehole and converted P- to S-wave (C-wave) pseudo-source VSPs for S-wave splitting analysis. However, data at the stationary-phase point is not conducive to conventional interferometry due to tube-wave noise and C-wave polarity reversals. Removing physical sources at the stationary-phase point, perturbing the integration path of the integrand and applying a radiation condition effectively recovers P- and C-wave pseudo-source VSPs, reduces tube-wave noise and separates wavefields. The retrieved wavefields correspond to vertical incidence and can be used for conventional P- and S-wave velocity analyses. Four-component C-wave VSPs retrieve S-wave splitting in transmitted and reflected converted waves that agrees with both C-wave and P-wave azimuthal anisotropy measurements from surface-seismic data.

BP conducted the first field trial of ION's FireFly cableless seismic acquisition system in the Wamsutter Field, Wyoming, during 2006/2007. The acquisition geometry of the survey provided exceptional azimuthal coverage for anisotropy analysis. The data is processed in the offset-vector tile domain to facilitate extraction of P-wave velocity anisotropy and azimuthal AVO variation data. Seismic-derived azimuthal AVO results are consistent with the dipole-sonic derived data. Integration of borehole measurements with the azimuthal anisotropy data is expected to have significant impact on our understanding of target reservoir rocks. Results show that the measured azimuth of the fast P-wave velocity can be utilized to determine present-day stress and paleo-stress conditions. The post-Cretaceous stratigraphic interval shows the effects of the present-day stress condition, whereas the Cretaceous interval which includes the target Almond Formation exhibits both present-day stress as well as paleo-stress conditions.

The BP Wamsutter Seismic Integration team conducted two seismic field trials within 2006/2007- a surface seismic field trial and a borehole seismic field trial. The borehole seismic field trial consisted of a 3D VSP as well as a four-well crosswell seismic campaign. Significantly higher frequencies (double the bandwidth) were successfully achieved with the 3D VSP as compared to existing and newly acquired surface seismic data. Prestack depth migration yielded excellent imaging results, which have allowed enhanced stratigraphic description of a very complex reservoir. Analysis, interpretation and integration of the VSP data has greatly progressed our understanding of the potential increase in the value of 'designer' or fit-for-purpose seismic across the Wamsutter Field and beyond. The Wamsutter 3D VSP represents a true success story, from definition of the problem, safe field acquisition, through to data interpretation and integration.

Use of Distributed Acoustic Sensing (DAS) fibre-optic technology for acquisition of vertical seismic profile (VSP) data is increasing rapidly. Since 2012 BP has carried out and participated in several field studies using DAS across different basins. These tests have included acquisition on land, in deep and shallow water, with sources deployed as a walk-above spread and shot patches of 60,000, using pre-installed fibre in producing wells and highly deviated wellbores. This paper will discuss the results and experience gained from acquisition and processing of the dual-well 3D DAS VSP over the Valhall field in the North Sea in 2015. This survey had a larger shot patch than previous trials, was acquired in shallow water and one of the wells remained on production throughout; a scenario presenting various challenges and opportunities. Successful imaging of the data confirmed the technical utility of DAS in achieving a targeted subsurface image and results indicate that recording data acceptable for imaging may be possible in producing wells.

In this study, elastic-wavefield interferometry was used to recover P- and S-waves from the 3D P-wave vibrator VSP data at Wamsutter field in Wyoming. S-wave velocity and birefringence is of particular interest for the geophysical objectives of lithology discrimination and fracture characterization in naturally fractured tight gas sand reservoirs. Because we rely on deconvolution interferometry for retrieving inter-receiver P- and S-waves in the subsurface, the output fields are suitable for high-resolution, local reservoir characterization. In 1D media where the borehole is nearly vertical, data at the stationary-phase point is not conducive to conventional interferometry. Strong tube-wave noise generated by physical sources near the borehole interfere with S-wave splitting analyses. Also, converted P- to S-wave (PS-wave) polarity reversals occur at zero offset and cancel their recovery. We developed methods to eliminate tube-wave noise by removing physical sources at the stationary-phase point and perturbing the integration path in the integrand based on P-wave NMO velocity of the direct-arrival. This results in using nonphysical energy outside a Fresnel radius that could not have propagated between receivers. To limit the response near the stationary-phase point, we also applied a weighting condition to suppress energy from large offsets. For PS-waves, a derivative-like operator was applied to the physical sources at zero offset in the form of a polarity reversal. These methods resulted in effectively recovering P-wave dipole and PS-wave quadrupole pseudosource VSPs. The retrieved wavefields kinematically correspond to a vertical incidence representation of reflectivity/transmissivity and can be used for conventional P- and S-wave velocity analyses. Four-component PS-wave VSPs retrieve S-wave splitting in transmitted converted waves that provide calibration for PS-wave and P-wave azimuthal anisotropy measurements from surface-seismic data.

As part of a significant seismic technology effort, the BP Wamsutter Seismic Integration team conducted two field trials during 2006/2007- a surface seismic field trial and a borehole seismic field trial. The borehole seismic field trial consisted of a 3D Vertical Seismic Profile (VSP), as well as a four-well crosswell seismic campaign. The 3D VSP was acquired to further our understanding of seismic technical limits within the field, demonstrate the value of enhanced temporal resolution to reservoir characterization, and to test the viability of borehole seismic as a development tool for infill planning. The complex, heterogeneous, and thin-bedded nature of these tight reservoir sandstones make detailed reservoir characterization from surface seismic data extremely challenging. For Wamsutter, pre-acquisition FD modeling from existing well data allowed theoretical limits of vertical seismic resolution to be compared to existing seismic data. Significantly higher frequencies (double the bandwidth) were successfully achieved with the 3D VSP as compared to existing and newly acquired surface seismic data. Prestack depth migration of the 3D VSP yielded excellent imaging results, which have allowed enhanced stratigraphic description of a very complex reservoir. Additional work explored the potential to use 3D VSP in conjunction with the surface seismic data for seismic imaging without a velocity model. Analysis, interpretation, and integration of the VSP data has greatly progressed our understanding of the potential increase in the value of "designer" or fit-for-purpose seismic across the Wamsutter Field and beyond.

In 2009, BP acquired a 7 square mile 3D3C pilot dataset utilizing ION's FireFly cableless acquisition system over BP's East Texas Blocker Field. This acquisition delivered a high density, full azimuth, 3-component dataset that aided in the planning and execution of a full-field production survey over the field. Very careful processing was applied to the full wavefield pilot dataset to optimize resolution and retain robust amplitude and azimuthal information. In addition to conventional full stack seismic attributes, AVO (Amplitude with Offset) and AVOAz (AVO with Azimuth) attributes, and seismic inversion volumes and have been generated and analyzed. Learnings from the analysis of the pilot datasets are being used to optimize the interpretation and analysis workflows of larger, more recent East Texas production 3D datasets.

The US Onshore Wamsutter Field Development Team executed two seismic field trials in 2006: a borehole seismic field trial and a cableless surface seismic field trial. The borehole seismic field trial consists of a 4-well crosswell tomography program, and acquisition of a massive 3D VSP with 8000 ft of 3C receiver tool deployed in the wellbore and 6000 surface vibroseis source points. The surface seismic field trial is the world's first deployment of the FireFly cableless seismic acquisition system, with 8300 3C digital sensors deployed, and 7200 explosive source points. These field trials were designed to increase seismic resolution, improve imaging and identification of higher porosity zones, and increase fault and fracture detection, with the ultimate goal of optimizing field performance. The data that was previously available for interpretation had a dominant frequency of 25 Hz, which allowed us to image reservoir thicknesses of greater than 50 feet. The producing reservoirs have much lower average thicknesses, which have been targeted with the new higher resolution crosswell, 3D VSP and dense surface seismic data. Rock properties understanding of the Wamsutter reservoirs has been fundamental to well-to-seismic calibration and interpretation and analysis of the seismic datasets. Seismic forward modelling has been undertaken to calibrate the seismic responses, as part of a larger integrated effort to understand porosity and lithology responses and fracture anisotropy. Field acquisition presented many challenges which were safely and successfully overcome. The crosswell data has resolved reservoir thicknesses of less than 20 feet. Frequencies from the 3D VSP are better than historically recovered in surface seismic data. This integrated borehole/surface seismic approach has allowed us to better understand the seismic responses and calibrate rock properties to different levels of seismic resolution. The Wamsutter Seismic Field Trials is a true success story worth sharing globally.

In the Winter of 2006/7, the BP Wamsutter team conducted an extensive seismic field trial including a world-first deployment of cable-less full-wavefield single-sensors in a 3D onshore survey, in addition to a 3DVSP and cross-well seismic. Despite weather and mechanical challenges, the acquisition operation successfully delivered a unique high density, full azimuth, full-wavefield dataset for analysis. Special care & ''designer'' processing was applied to this unconventional data to optimize resolution and retain robust amplitude and azimuthal information. This has successfully yielded a full stack image comparable in quality to that of conventional geophone arrays, despite the lower-than-anticipated trace recovery, boding well for future surveys where significantly greater recovery can be expected. Additional Prestack AVO (amplitude with offset) and AZAVO (azimuth with offset) attributes are shedding new light on a reservoir previously only imaged with full stack data; the converted-wave component is also of unusually high quality and is ultimately expected to contribute to describing the reservoir to new levels of detail. Learnings from the field trial are already impacting the onshore seismic strategy both within the Wamsutter Field and across BP’s tight gas business.

The Wamsutter Field in Wyoming, USA is a massive tight gas accumulation covering more than 1600 sq. miles (Figure 1). Discovered in the 1960s and brought on production in the 1970s, the field is currently being developed at 80-acre spacing and is now moving to 40-acre spacing. To date, development has been primarily led by step-outs from good producing wells and geological reservoir correlation (though with some operators more recently incorporating seismic). While some areas of the field have been successful in this approach, others have experienced higher proportions of poorer quality well rates and EURs and it is in these areas that future development will dominate.

As part of a larger 2006-2007 technology program, the world’s first onshore cable-less full-wavefield 3D survey was acquired over 25 sq. miles, combined with a massive 3D VSP and cross-well borehole survey. The intent was to test the viability of emerging seismic technologies and in doing, further our understanding of the value of seismic data as a development tool for reservoir and fracture characterization in both the Wamsutter Field and tight gas plays in general.

Primary reservoirs are the Cretaceous Almond sands with a gross interval thickness of 500ft at an average depth of 10,000ft. The Almond play is a heterogeneous and thinly bedded reservoir made up (in the seismic locality) of lower coastal plain - shallow marine sands, shales and coals. These present a complex challenge to both well-based predictions and to seismic imaging in the inter-well areas. In particular, the presence of low density coal is known to dominate the seismic response (Figure 2). Pre-acquisition 1D modeling from existing well data suggested that higher bandwidth data (~90Hz) had the potential to enhance imaging of individual sand units. The survey was designed with the objective of acquiring high bandwidth (>40 Hz) wide azimuth, full-wavefield data with uniform offset distribution, focused on Section 7 at the center of the survey area.

In this study we investigate methods of elastic-wavefield interferometry to obtain P- and S-wave velocity information from the 3D P-wave vibrator VSP at Wamsutter Field in Wyoming. Wamsutter field consists of naturally fractured tight gas sand reservoirs where geophysical objectives are lithology discrimination and fracture identification. We examine P-wave pseudo-source VSP data to eliminate strong tube-wave noise generated by sources near the borehole, and converted P- to S-wave (C-wave) pseudo-source VSPs for S-wave splitting analysis. However, data at the stationary-phase point is not conducive to conventional interferometry due to tube-wave noise and C-wave polarity reversals. Removing physical sources at the stationary-phase point, perturbing the integration path of the integrand and applying a radiation condition effectively recovers P- and C-wave pseudo-source VSPs, reduces tube-wave noise and separates wavefields. The retrieved wavefields correspond to vertical incidence and can be used for conventional P- and S-wave velocity analyses. Four-component C-wave VSPs retrieve S-wave splitting in transmitted and reflected converted waves that agrees with both C-wave and P-wave azimuthal anisotropy measurements from surface-seismic data.