Borehole geophysics is an aspect of geoscience that deals with the vertical analysis of the conditions of the subsurface and its fluid content. It is the technique employed in order to have a firsthand well evaluation in the exploration for hydrocarbons. One of the main results of Borehole Geophysical experiment is the Time to Depth relation curve. This curve can be used to generate an accurate calculation of interval, RMS and Average velocities. Those velocities helps us to relate our Seismic in Time to actual Geology in Depth. There are many other surveys that can be done in a Borehole other than the simple Vertical Seismic Profile (VSP), like Walk away, Walk above , 3D VSP and Walk around. Each one of those mentioned surveys gives different results and helps exploration in a different way.
Commonly used borehole surveys
Zero offset VSP
Its called zero offset because the source is very close to the wellbore , normally the source to well distance is around 200 m. This kind of survey normally provides the time to depth curve (check shots) and it can be used for multiples identification. As the source sends its waves downward, the receivers in the well bore receive those waves knowing the time of the shooting and the time of the first arrivals which connects our time to the tool Depth. The data will also show the formation reflectivity when the tool passes the formation top as a result of the reflectivity coming from the up going waves. (review types of waves below)
Deviated well VSP/walk above
In Case of Deviated wells, the best approach is to shoot the source in a line that projects the well path from the top. The output of this survey is an image of the formations below the well path which is correlated to the surface seismic data except its higher in resolution.
Far offset VSP
This kind of surveys is similar to the Zero offset, except the source is further away from the well bore. The offset increases the volume of subsurface imaged reflectors which increase the correlation with surface seismic data. Moreover, the offset provides higher chances of converting P waves into S wave. This provides a chance for amplitude variation with offset and anisotropy analysis.
Walk away VSP
In the Walk away Survey, the receivers are fixed in the borehole while the source moves in a line that passes through the well head. Such survey is useful for shear Waves and anisotropy analysis. it can be compared to a 2D surface seismic except it includes both S and P waves with higher frequency bandwidth.
Walk around VSP
The Walk around survey is mainly specialized for anisotropy analysis, since the source circles around the well bore shooting, therefore any change of the received amplitude or frequency can reflect the pressure distribution in the area, assuming flat geology.
The 3D VSP survey is comparable to the surface seismic 3D survey except for the resolution difference, in addition to the areas that VSP fills in better than surface seismic in case of infrastructure difficulties, such as shallow gas which disrupts propagation of P waves. This kind of surveys is preformed in both land an marine, it can be also acquired in parallel with surface seismic 3D by putting receivers in both surface and in the wellbore..
Another way is by monitory hydraulically induced fractures from nearby wells given an accurate velocity model to map the extent of the fracture. A different method is called passive seismic monitoring, which characterizes fractures by recording micro seismic signals generated when fluid is injected and produced modifying eh stress stated enough to cause seismic events. These micro seismic events can be plotted in space and time to identify the fractures responsible for stress changes. In some cases, the receivers are permanently installed withen the borehole due to the long periods of time needed for such surveys.
While drilling VSP
To help reduce uncertainty in time to depth correlation without having to stop drilling, geophysicist advice for seismic while drilling survey. This survey minimizes the risks that could be faced ahead of the bit, Such surveys can be done during pipe connection to avoid the noise generated by the bit while drilling.
The source used in these kind of surveys are deployed at selected depth, while the receivers are placed in the nearby well. There is a limitation of the maximum offset to few thousand feets depending on the lithology at the area. Such surveys don't cover much of the reflectors as the rays travel parallel to them, however, a lot can be learned about the velocity local changes in the inter well region.
Types of Waves
- Primary P wave
- Secondary Shear Wave
Those waves can be found in 4 forms
- down going direct arrival
- reflected up going primary
- down going multiple
- reflected up going multiple
- J. Labo (1987). Borehole Geophysics Techniques and Application. A Practical Introduction to Borehole Geophysics. SEG Digital Library
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- Blackburn, John, et al. “Borehole Seismic Surveys: Beyond the Vertical Profile.” Oilfield Review, 2007.