OIL EXPLORATION BY THE WELL SEISMIC METHOD

In the surface seismic, of which I spoke in my last posts (Geophysics - Exploration method (Elastic Impedance) and Geophysics - Exploration Methods (Anisotropic Media)), the data acquisition is done in such a way that both the seismic source and the receivers are located on the surface.

In well seismic data acquisition techniques can be classified into three types:

• Source on surface with a receiver in the well
• Source in the well with a receiver on the surface and
• Source in the well with receiver in another well

Figure 1 shows the different configurations of data acquisition in the well system.

There are many advantages of the well seismic, we can mention some of them:

• They link the images of the strata with their exact depths, which are extrapolated to other wells.
• Provide high resolution images.
• They allow to determine the elastic properties of the rocks.
• They allow converting seismic sections dependent on time to depth-dependent sections.
• Data records with the best ratio (signal / noise)
• Wave attenuations, caused by low-speed shallow strata, are minimized.

In this publication we refer to two of the well seismic configurations:

• Sonic records
• Seismic Shots (Cheskshot)
  

  SONIC RECORDS

The acoustic instrument contains one transmitter and two receivers regularly spaced 1 foot. When the transmitter is activated, the acoustic wave is transmitted through the mud column, enters the formation, returns to the mud column and registers in the receivers. The speed in the formation (propagation time or Δt) is determined by the difference in the arrival time in the two receivers (Figure 2).

Figure 2 - Sonic Tool Source

What is recorded is the travel time, which is the reciprocal of the speed. The time of sound travel in the formation is mainly a function of lithology and porosity. In general, the higher the density or consolidation, the shorter the travel time (Δt); On the other hand, an increase in travel time indicates an increase in porosity.

Figure 3 - Sonic Logs (Velocity) Source

Sonic records are the main factor in the elaboration of synthetic seismograms (Figure 4) which are essential in the seismic-well calibration.

Figure 4 - Processing of sonic records for the obtaining of synthetic seismograms Source

The velocity curve is obtained from the sonic record, which is expressed in units of microseconds per foot, so its inverse multiplied by conversion constants is transformed into velocity expressed in meters per second.

SEISMIC SHOTS (CHECKSHOT)

They consist of introducing a receiver into a well and measuring the arrival times of the seismic wave produced by the explosion of a surface energy source, to the different receiver positions.

Figure 5 - Configuration for obtaining Checkshot and VerticalSeismic Profile (VSP) Source

These seismic shots are made in order to measure the travel times of the seismic pulse from the surface to various depths, generally spaced 250 m or when significant lithological changes occur in the subsoil.

The result of the seismic shots is a table of time-depth values which once verticalized and through the use of the sonic record, will allow us to calculate the continuous conversion and exact time-depth.

At the moment the shot is taken, a receiver placed next to the source receives the pulse instantaneously to set the zero reference time. Then the seismic pulse travels through the subsoil until it reaches the geophones station, placed at a certain depth in the well (Figure 6).

Figure 6 - Seismic Shots (Cheskshot) (Source: Grade Work: "IMPACT OF THE DELTA ANISOTROPIA FACTOR IN THE SEISMIC SPEED MODEL" Engineering graduate. UNIVERSITY OF ZULIA. LORENZO RIVERO)

Figure 7 shows a graphic representation of a record obtained in the lake of Maracaibo.

Figure 7 - Graphic representation of a record obtained in the lake of maracaibo
(Source: Grade Work. "IMPACT OF THE DELTA ANISOTROPIA FACTOR IN THE SEISMIC SPEED MODEL" Engineering graduate. UNIVERSITY OF ZULIA. LORENZO RIVERO)

REFERENCE

• http://seismo.geology.upatras.gr/TOMOGRAPHY.htm
• Dobrin, M.B. “Introducción a la prospección geofísica.”2da Edición; Editorial Ω.
• José Cantos Figuerola. Tratado de Geofísica Aplicada. Librería Ciencia-Industria, S.L. Pl. San Luis de la Cruz, Madrid. 520 pag.
• Rivero L. “Uso de velocidades sísmicas en la programación de registros sónicos de pozos”, Julio 1997.Universidad del Zulia. Tesis Pregrado
• Rivero L. “Impacto del factor de anisotropia delta en el modelo de velocidades sismicas”, Junio 2005. Universidad del Zulia. Tesis Postgrado.
• Schlumberger. Sísmica de pozo. Documento Schlumberger. 43 pag. Venezuela.
• Schlumberger. Seismic Applications (WST and Geogram). Documento M-088008, Noviembre 1979.
• Telford W.M., Geldart L.P., R.E. Sheriff y D.A. Keys.”Applied Geophysics”. Cambridge University Press London (1976).
• Udías A, Mézcua J. “Fundamentos de Geofísica”. Primera Edición. Editorial Alambra (1986).

I hope that this publication has given you knowledge about the subject. If you have any questions, please leave your comment and then I will gladly try to clarify them.