Amazingly long, wireline-conveyed, high-fidelity vertical seismic profiling (VSP) arrays are delivering better data at reduced rig time cost, opening the door to more comprehensive use of the resulting datasets.

Operators' interest in improving the recording fidelity of vertical seismic profiles (VSPs) while reducing the rig time/cost of their acquisition has driven the development of larger, more efficient, wireline-conveyed arrays of borehole seismic tools and related technologies. Today VSP surveys are providing more accurate data and better-processed images of the subsurface with dramatic reductions in the amount of rig time it takes to record such surveys. In the last few years the development of large-borehole seismic array tools has made previously inconceivable applications for borehole seismic data a current reality.

Progress over time

Until the late 1980s borehole seismic surveys were made with simple, mandrel-type downhole tools containing one or more geophones in a single sonde that was run stand-alone on wireline for specific surveys or logging runs. Typically, the acquisition aim was a simple measure of transit time from source initiation on the surface until the signal was recorded downhole at a known depth. This knowledge of time versus depth, and hence velocity - commonly referred to as a checkshot survey - was used to calibrate surface seismic interpretation.

The breakthrough in borehole seismic came with the ability to process the full seismic wavetrain recorded during a checkshot survey. A VSP survey is simply the result of processing the data recorded by a downhole geophone when a source is initiated on the surface. A series of closely spaced geophone levels are generally required for accurate VSP processing and interpretation.
Later advancement added three-component (3-C) downhole recording, enabling VSP techniques to evolve into what are called offset and walkaway surveys, whose objective is to produce high-resolution images of reflectors all around the well. The imaging capability then quickly led to the demand for larger arrays of 3-C sensors that could be deployed on conventional wireline while delivering improved signal fidelity and large data volumes in reduced time.

Borehole seismic imaging applications have grown to include large-scale 3-D projects delivering, in addition to checkshot and correlation information, products such as direct measurement of attenuation and spherical divergence, identification of interbed multiples, 2-D phase analysis of surface seismic, 3-D match filtering of surface seismic, and acoustic impedance inversion, among others.

Re-engineering the sensor package

Borehole seismic array tools began emerging in the 1990s and have continued to evolve. A key factor in their evolution and improved efficiency has been the re-engineering of sensor packaging.
One integrated borehole seismic system provides improved signal fidelity through the use of small, 3-C sensor packages that are acoustically isolated from the main body of the tool (Figure 1). The isolated sensor package, which houses the three-axis geophones, is designed to keep all tool resonances outside the useful frequency of the seismic signal as well as to achieve optimal coupling in all hole conditions. This design ensures accurate recording of the seismic signal and results in true amplitude measurement on all three axes, ensuring outstanding vector fidelity.

Improvements in the seismic signal and reduction in noise on data acquired in poor hole conditions are achieved both through the downhole and integrated surface equipment. The integrated surface equipment also records reference signals and parameters such as source depth, pressure and position.
Isolated sensor packaging also has led to improvements in surveying efficiency as VSP acquisitions can be conducted in combination with other logging activities in single-well runs.

Lengthening the array

As operators have gained experience using high-fidelity 3-C borehole seismic data, the demand for even longer 3-C VSP arrays has grown. The most recent response to this demand has been the enhanced Versatile Seismic Imager (VSI) tool. It allows the deployment of arrays as long as 4,100 ft (1,250 m) weighing almost 2.5 tons and including up to 40 individual seismic sensor packages. Sensor (shuttle) arrays can be configured to suit the task at hand. Intershuttle spacing can be customized from 9.2 ft (2.5 m) to 98.4 ft (30 m), depending on survey objectives, and new configurations can be crafted quickly using ordinary wireline logging cable.

The ability to anchor a higher number of shuttles simultaneously with this tool has resulted in 30% to 40% improvements in connection efficiency, thereby significantly shortening the time it takes to clamp the array to the borehole wall once it is in place. Operators using expensive deepwater rigs are especially appreciative of this capability.

The latest tool includes shear-pin weak points with improved tolerances in anchor arms and heads, which allows better control in recovery operations. In addition, re-engineered electronics have contributed to higher reliability through redundant systems.

Of course, generating large volumes of downhole data is not attractive unless a high-capacity telemetry system is available to transmit data to the surface. During large 3-D VSP surveys offshore, for example, with tens of thousands of shot points, it is very important to keep the source boat moving and shooting at sufficient speed. Therefore, the telemetry system must be able to keep pace with the volume of data collected. The telemetry system currently used with the VSI tool allows a complete dataset to be retrieved in 12 seconds from a 20-level array recording 10-second records at 2-millisecond sampling.

Advanced applications

Many of the newer borehole seismic applications of interest to geophysicists require high fidelity of all three geophone components. These applications include:
• compressional and shear (P and S) wave analysis;
• microseismic (HFM) event location;
• P and S velocity ratio (Vp/Vs) for amplitude variation with offset (AVO) modeling, as well as multicomponent surface seismic survey design and processing calibration;
• reliable attenuation estimation;
• S-VSP and P-VSP for lithology and fluid interpretation;
• shear sonic calibration for use in modeling; and
• azimuthal anisotropy measurement.

Among the notable shear wave applications made possible by this technology is the ability to acquire a large amount of useful shear wave information, even from near zero-offset source VSP data. This includes accurate S-wave interval velocities for calibration of S-wave sonic log data and S-wave reflectivity estimates for comparison with traditional P-wave corridor stacks and synthetic seismograms. Such information is of value when processing and interpreting multicomponent surface seismic data, AVO modeling and time-lapse seismic calibration.

Recent projects provide further insight into the improved results being obtained with today's VSP arrays.
Gulf of Mexico. In 2003 a major operator undertook a 6-day spiral 3-D VSP survey in the Gulf of Mexico. A remotely operated vehicle was used to guide a 2,000-ft (610-m), 20-shuttle VSI toolstring from a false derrick through 4,500 ft (1,373 m) of water to a riserless borehole with a subsea completion. The rig was simultaneously drilling another well on the same template during this period, saving US $3 million in rig time. Two walkaway VSP surveys were acquired with a shuttle spacing of 100 ft (30 m). The data were processed at the rig. More than 32,000 shots were acquired in 143 hours, making this the operator's largest 3-D VSP by data volume at the time and a success in terms of continuous operational hours - not a single minute was lost during the 6 days of acquisition.

Also in the Gulf of Mexico, in early 2004 another major operator used a 24-shuttle, 2,400-ft (732-m) VSI design for walkaway VSP surveying. In one well, walkaway VSPs were recorded into four consecutive settings of a 24-level, array giving a total of 96 levels in the borehole. The objective was to image under salt; hence the need for many levels. The high-resolution survey was conducted in 49 continuous operating hours without lost time. The operator's goal of a 12-second cycle time with a 10-second record length and 2-millisecond sampling interval was successfully attained.

Onshore United States. The first hydraulic fracture-monitoring job was conducted in the Denver Julesburg basin in 2003 using VSI technology. An 8-shuttle VSI tool was used in a continuous passive monitoring mode to record 28 hours of downhole data with 100% operating efficiency while a nearby well was hydraulically fractured. The data were processed to produce a 3-D movie of the fracture as it developed over time. In addition, the resulting accurate fracture height, length and azimuth data were used by the operator to redesign its completion, well placement and fracturing programs. Additional fracture monitoring jobs are planned by the operator.

Offshore Mexico. In a final example, the tool was chosen for offset VSPs in two offshore exploration wells. The objective was to obtain high-resolution P-wave and converted S-wave seismic images as well as investigate how the P-wave and S-wave amplitudes were affected by the presence of hydrocarbons. In the first well, VSI receivers were spaced every 32.8 ft (10 m) up to 656 ft (200 m) in depth, allowing converted shear waves to be picked close to the sea floor and providing the first shear checkshot survey offshore Mexico. In the second well, VSP processing was integrated with drillbit seismic data recorded in the same well to deliver a complete borehole seismic answer.

As illustrated by these case histories, the capability to configure reliable, high-fidelity arrays of VSP sensors to suit a project's particular objectives and challenges, and run them cost-effectively, is enabling the broadest range of VSP applications ever possible. VSP technologies will continue to advance as successful new applications will unleash new demand for yet more technical capability. With the VSI tool in particular, development activities will soon provide even faster telemetry systems, which will allow higher sample rate recording and reduced time between shots, resulting in improved data quality and even more rig time savings. Stay tuned.

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