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THE ROLE OF VERTICAL SEISMIC PROFILES (VSPs)IN TRINIDAD AND TOBAGO

A. Aming (Trintomar)

INTRODUCTION

Vertical seismic profiles (VSPs) have been done by Schlumberger for at least 3 of the major oil companies operating in Trinidad and Tobago. These data were probably not used to their full advantage for many reasons, the chief being the lack of a good understanding of the tool. The VSP can be very useful to the explorationist and the exploitationist in many ways (e.g. to accurately correlate rock properties/log data to the seismic data). This tool has been used to predict both laterally from the well bore and below the bit. Many problems both at the acquisition and the processing stages of the VSP can produce results which may result in the condemnation of the tool. These problems arise from improper planning and coordination between the geophysicist and the geologist/engineers/logging company and from insufficient pre-survey time.
The use of VSPs in Trinidad and Tobago is relatively new and the first
VSP was run for Amoco Trinidad Oil Company in the early 1980s. Since then
VSPs have been run for Amoco (3), Trintoc (2) and Trinmar (1). Two of the 6 were acquired on land while only one had offset VSPs along with the regular zero-offset VSP. Currently 2 sets of offset VSPs are being run for Trinmar and Trintopec.
This paper will briefly discuss what is a VSP, its uses, the planning, acquisition, processing and interpretation of VSPs, and new developments in VSP techniques applicable to Trinidad and Tobago.

WHAT IS A VSP?

A VSP or borehole seismic survey is simply a seismic survey acquired in a well where, usually, the source of energy is on the surface and the recording instruments are in the borehole. The final results are similar to a conventional surface seismic survey except that the resolution is better in the VSP but the extent of coverage is limited by the survey design.
There are 3 main types of VSP techniques (Figure 1):

(1) Zero Offset or Vertical VSP, where the source is close to the well bore and the well bore is vertical.

(2) Fixed Offset (single/multiple)- the source is fixed at some offset from the well bore and the well bore is vertical, or the source is close to the well bore but the well bore is deviated.

(3) Variable Offset or Walk Away VSP - the geophone is fixed in the
well bore and the source is moved on the surface.

There are numerous methods by which VSPs can be acquired and these are only limited by the geological/ engineering objective. budgetary constraints and one's imagination.

USES OF A VSP

VSPs are very useful in both the exploration and exploitation of hydrocarbon reserves. The following is a list of uses of the VSP in Trinidad and
Tobago:

(1) It is the best means of determining the seismic travel time to the top of a formation.

(2) It is the best technique to correlate seismic reflections with borehole data. The VSP result is a seismic section that can be correlated to the surface seismic data. The travel path of the energy is direct in the case of the VSP.

(3) It is very much better than a synthetic seismogram, especially in a
well that has borehole problems.

(4) It is used to determine more accurately parameters for processing of surface seismic data, viz. amplitude recovery, deconvolutional filters, frequency decay for filter design, phase of signal for match filtering and wavelet shaping, and acoustic impedance.

(5) It allows for prediction ahead of the drill bit.

(6) It provides high resolution away from the well bore (e.g. reservoir extent or fault locations).

(7) It gives detailed information about the mechanical properties of the rock (rock moduli e.g. Poisson's Ratio) if both P and S wave energy are recorded and processed.

(8) It can be used to monitor Enhanced oil Recovery (EOR) projects.

PLANNING

Planning is the most important step in carrying out a VSP. The following are some of the key considerations that go into the planning of a VSP:

(1) Borehole conditions must allow for proper geophone/formation coupling. The deviation of the borehole is critical and determines the type of instrument to be used.

(2) If it is at all possible, the source type should be the same as that of the surface seismic data. Research should be conducted to determine the optimum trade-off between the type of source and frequency spectrum with depth, source generated noise, depth of penetration of signal, and the number of shots required for adequate penetration of the signal, consistency of the signal from shot to shot, safety and, most important , cost.

(3) Geophone tool.

a) Zero offset, P-wave work-a single geophone tool is adequate.

b) Offset, P-wave and S-wave - a three component tool is required.

c) The tool should be light with as powerful a locking mechanism as possible.

d) The geophones should be able to record when tilted at high angles.

e) The tool must be able to stabilise quickly moving from one level to the next.

(4) Modeling is probably the most important step in the planning of a VSP. The following should be deter-mined at the modeling stage:

a) Whether or not the target geo-logy can be imaged and the type and geometry for the acquisition of the VSP.

b) The distance between geo-phone levels.

c) The signal sample rate.

ACQUISITION, PROCESSING AND INTERPRETATION

The key to a successful VSP is implementation. Acquisition is the first stage

considerations must be taken into account:

(1) The tool must be coupled to the formation in the borehole. Therefore it is recommended that the VSP should be acquired in an open hole, but if the hole is cased the cementation must be good.

(2) The background noise level must be determined and the number of shots per level chosen to ensure an adequate signal to noise ratio.

Processing of the VSP data requires considerable input from the client especially at the final stages. It can be divided into 4 main stages:

(1) Data preparation - the data recorded in the field are sorted, edited, frequency filtered, stacked and any another techniques applied to improve the signal to noise ratio of the data.

(2) Velocity filtering is performed next in order to separate the upgoing from the downgoing waves.

(3) Application of deconvolution, based on the downgoing wavefield, to shape the wavelet (e.g. zero phase).

(4) The final and most important stage is the corridor stack and migration of the data. This stage requires the input of the client and takes the most time. Figure 2 is a VSP before corridor stacking showing the lines along which the data will be stacked. Figure 3 is the same VSP data after corridor stack. The corridor stack is highly dependent on the geometry of the formation and requires modeling to determine the best corridor stacking transformation. The data should then be migrated and depth converted.

All of the above steps are done for the P-wave and S-wave data separately.
Interpretation of the seismic data requires a geological model along with objective reasoning and imagination. After the VSP has been properly processed the data are similar to a conventional surface seismic line and can be interpreted likewise. The data may be manipulated to generate acoustic impedance, seismic porosity, seismic pressure or rock moduli depending on the quality and resolution of the data.

NEW TECHNIQUES

A number of ideas have emerged since the advent of the VSP technique and these include:
(1) Tomography, which is similar to a CAT scan in Neurology. It requires detailed planning and may even require that the source has to be in a well. It is not used extensively It is not used extensively because a good down hole source is still in the making. It has never been used in Trinidad and Tobago.

(2) Seismic while drilling is a technique where the energy from the drill bit is used as the source and a geo-phone, or arrays of geophones, are placed on the surface. This will produce 2D or 3D seismic data depending on the design. There are some problems e.g. zero time brake, number of channels that can be recorded, dynamic range of the instrumentation etc. The technique may be the answer to acquisition of better quality data in areas like the Central Range of Trinidad. It can be used to monitor Enhanced Oil Recovery projects. This technique is very attractive because it significantly reduces the cost of acquisition of seismic data and does not require any rig time.

CONCLUSIONS

The VSP has not been fully used and appreciated in Trinidad and Tobago because of cost considerations and improper planning. Planning (modeling) is the key to a successful VSP. The client should have personnel at the acquisition and processing sites to quality control and assist in decision making. Interpretation of the results is similar to conventional seismic data. New adaptations of the technique are on the horizon and could have significant applications in Trinidad and Tobago.

ACKNOWLEDGEMENTS

The information for this paper was taken partly from the course notes of a school GEOPHYSICS FOR NON-GEO-PHYSICISTS sponsored by the National Institute of Higher Education Research Science and Technology and created by the author and Mr. Eric Williams.

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