The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by M. Ray Thomasson.
This month's column, the second of a two-part series on VSP, is titled "Additional Computed Product Utility."
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VSP Survey Meets Accuracy Demands
By R.J. Brewer
Editor's note: Robert J. Brewer is an account representative, Borehole Seismic Services for Halliburton Energy Services in Houston.
VSP surveys are routinely performed in many parts of the world -- especially in Europe, because of the recognized superiority and versatility of the VSP over the simpler and less expensive check shot survey.
More and more VSP surveys are being done -- especially offset surveys and a more detailed variation of the offset survey called the walk away VSP survey -- as the advantages become clearer and survey reliability increases.
Pre-survey ray trace modeling has gained wide acceptance and is used to more accurately design offset VSP surveys and offset energy source placement. The computed product of the offset VSP is known as a VSP/CDP transform -- basically a high-resolution, mini-seismic section resembling a surface seismic CDP (Common Depth Point) stack display.
The VSP/CDP transform has been converted or "transformed" from its original recorded one-way time to two-way time, and displayed at a convenient scale to match the surface seismic data it is to be correlated with (figure 1). The VSP/CDP transform data set can be migrated, filtered and processed just like any surface seismic data set.
Because VSP data has a broader bandwidth and contains high frequency events, subtle features like small faults, stratigraphic changes and amplitude anomalies can be observed in the vicinity of the well bore that were not discernable on the surface seismic coverage in the same area (figure 2).
Note the marked improvement in resolution that the VSP/CDP transform yields in this example: It is a VSP/CDP transform display made from a vertical incident VSP survey, recorded to provide better resolution over a 2-D reconstruction line from a 3-D seismic volume prior to deepening this directional well bore.
A vertical incident VSP survey requires the energy source to be positioned at the surface directly over the downhole geophone tool. Vertical incident geometry is generally preferred over the rig source option, and has proven to be a more accurate way to obtain velocity control and image a highly deviated bore hole.
Displaying the VSP/CDP Transform and the seismic section together yields a far more useful product for interpretation.
Downhole VSP Tools
Downhole tool design has improved significantly over the last 20 years.
Three-component geophone configurations are routine -- the tools have evolved from single component analog designs to digital multi-tool designs or actual downhole geophone arrays comprising up to 24 or more individual tools or satellites.
Multi-station tools greatly reduce the historic bane of bore hole seismic surveys -- rig time consumption -- and record higher quality data.
Slim (1-inch and 11/16-inch O.D.) downhole geophone tools have proven their versatility and made it possible to record high quality VSP data in producing wells.
Logging While Drilling (LWD) sonic, check shot and VSP tools are available to meet the real-time demands of directional drilling. LWD tools designed to record borehole seismic data are becoming increasingly more sophisticated as LWD logging replaces conventional wireline logging on many directional wells.
Reduced Risk and Saved Money
The two most important benefits of running a VSP survey are reduced risk and saved drilling dollars.
The VSP survey reduces risk by measuring the seismic velocities accurately in the well bore, which allows accurate time-to-depth conversion of the surface seismic data. VSP data also has been used to help reprocess older seismic data to yield more clearly interpretable results.
The accurate velocity information from the VSP helps make the velocity analysis involved in processing and stacking surface seismic data more precise.
VSP data also can be used to remove multiples from surface seismic data by providing parameters for an earth filter inversion process known as signature deconvolution. Accurate time-to-depth conversion is a must in producing reliable drilling prospect maps -- and helps avoid missed drilling targets.
Money is saved early on with VSP surveys done in the first wells drilled in a play by increasing the accuracy of the interpretation and mapping process -- and later, as more wells are drilled and the velocity field is better understood.
Most of the VSP surveys performed are of the zero or near offset type, which is primarily used for velocity determination and surface seismic correlation.
Offset VSP surveys are gaining wider acceptance, as they have proved successful in locating stepout locations to discovery and producing wells.
The sonic log and the check shot velocity survey have been the standard seismic correlation tools for many years and have proven their utility -- but today's exploration and production challenges require more precision.
The VSP survey meets that challenge, and is currently considered to be the ultimate and most effective tool available for matching the well bore to the sizable investment of surface seismic data that each exploration company made.
More VSP surveys will be needed in the future -- and may become a standard logging service -- as we strive to meet the accuracy demands of our industry.