SU1223177A1 - Method of seismic analyzing in wells - Google Patents

Description

FIG. one

one

The invention relates to geophysical surveys in wells, in particular to seismic surveys in order to determine the physical characteristics of the rocks in the near-wellbore space. .

The purpose of the invention is to improve the accuracy and efficiency of research.

FIG. 1 shows the impulse response of a well as a filter forming a wave field 5 in FIG. well frequency response.

The method is carried out as follows.

A source and a receiver of elastic waves, the distance between which along the vertical axis of the well is Z, is placed into a well of radius R filled with fluid with the speed of acoustic waves a in it, and the density is J. The density of the surrounding rocks is J ,. , and the velocities of longitudinal and transverse waves in them are, respectively, a and b. If you place the source on the vertical axis and combine the origin with it, then the pressure inside the well at the point with coordinates z, g, (r) is determined by the ratio

PTur.22).% IlJlj $ PoUc, (mo).

-"about

-Lao (t, d) eAp (} kg) a1c, (-1}

where cJ is the circular frequency; PO is a source function; k is the integration variable that has the meaning of the spatial frequency; Sho q - k, q. b) / a, lo and HO are the functions of Bessel and Hankel;

jxiRi

7G 3.14159 ...;

A is a function defined by

boundary conditions. If we consider a well as some filter that forms a wave field, then P (J, G :, z) is the frequency characteristic of this filter, which determines the pressure inside the well. The inverse Fourier transform of P (o), g, z) is the filter characteristic of the filter, those. seismic trace pressure in the well when excited in it is a pulse. When a source of pulses of arbitrary shape is excited, the seismic trace can be obtained as their convolution with impulse response.

o

five

0

five

55

thirty

five

0

five

50

77 2;

FIG. Figures 1 and 2 show the pulse and frequency characteristics calculated by formula (1) for the case when the velocity of the transverse waves C in the surrounding rocks is less than the velocity a of the waves in the fluid filling the well. Such a model of the medium is fundamental to engineering-geological and engineering-geophysical surveys, since it is characteristic of the upper part of the section of sedimentary and loose sediments. In the calculation model, the following parameters are accepted: a, 1500 m / s; P 1 and 1700. M / s; B 650 m / s; ft 2 g / cm, R O., 05 m; d. 0.01 m; z Yum. These parameters correspond to the section of clay deposits.

Three types of waves appear on the seismic trace (Fig. 1). Wave 1 is longitudinal, i.e. wave, propagating in rocks with the velocity of longitudinal waves, and inside the well recorded as the head wave. This wave is high frequency: its visible period at R 0.05 m corresponds to a frequency of 2 kHz and decreases as 1 / R.

Wave 2 is a hydraulic wave, i.e. surface wave, whose velocity at U) is equal to the velocity of the Lzmba tube

v.-tafi, / P.C (, / a.), i | (| t) T.

This wave is substantially low frequency. The central frequency of its spectrum does not exceed for all types of the section and all practically used radii of wells 500 Hz.

Wave 3 has a propagation velocity equal to 19 transverse wave velocities. Since the head shear wave cannot form at a given velocity ratio (bj a), it is a surface non-uniform wave. It is formed due to inhomogeneous waves entering the decomposition of a spherical wave. When passing through the boundary (the borehole wall), these waves are transformed into ordinary uniform waves propagating in the environment at a speed of transverse waves.

When observed on small bases, i.e. when the distance between the receiver and the source is unimportant, both types of surface waves (2 and 3) interfere, thus it is not possible to determine the entrance of surface wave 3 and, therefore, the velocity of the transverse waves. The use of large bases or observation at several bases is difficult: when using large bases, the detail of studies decreases, the observation at several bases reduces not only detail, but also the productivity of research.

These difficulties can be avoided by taking advantage of the difference in the spectral composition of surface waves 2 and 3. A specific feature of these waves is their resolution in the frequency domain. FIG. 2 shows the amplitude spectrum (frequency response module) of the waves recorded in the well; region 4 of the amplitude spectrum corresponds to hydro wave 2 in FIG. one; the amplitude spectrum region 5 corresponds to the longitudinal wave 1 and the surface wave 3 in FIG. 1. Thus, by recording in two different frequency ranges, interference effects can be eliminated and all three types of waves occurring during a seismic well logging can be recorded in pure form. The separation of the longitudinal 1 and surface 3 waves in this case occurs on the seismic track due to the difference in the velocities of the longitudinal and transverse waves. ,

Calculations carried out for a well model that satisfies the ratio bj а showed that for any practically used borehole diameter and for any transverse wave velocities in the near-wellbore space, the lower and upper bounds of the frequency range of the hydraulic wave do not exceed 20 and 500 Hz, respectively, and The range of the surface transverse wave does not exceed: the lower limit of 1 kHz, the upper 4 kHz. In this regard, for section 231774

For laziness of waves during seismic logging, two frequency ranges of recording can be used: the first from 20 to 500 Hz, the second from 1 to 4 kHz. In this case, the width of the spectrum of the excited vibrations should be at least 4 kHz. In this case, the width of the spectrum is understood to be the frequency band in which at least 90% of the energy of the excited oscillations is contained.

Upon registration, in the range of 1–4 kHz, the surface of a transverse and longitudinal wave is selected, the arrival times of which determine, from the velocity bj and aa, the rocks of the near-well space. When registering in the second frequency range, the hydrofiber is selected and determines e10

15

with its velocity v,. By knowing V, b, and aj, one can determine the density of the rocks, p I, since the velocity a and the density of fluid filling the well are usually well known.

Thus, the proposed method allows one to determine not only the velocities of transverse and longitudinal waves in rocks, but also their density. Thus, the problem of determining the elastic moduli of rocks during seismic surveys in wells is completely solved.

The use of the proposed method in comparison with the known one provides an increase in the accuracy and efficiency of research, which is achieved due to the fact that the velocity of the transverse waves is determined not by calculation using values that are known a priori directly from measurements of the arrival time of the surface wave propagating with shear wave speed. In addition, the method makes it possible to determine the density of rocks, and thus makes it possible, according to seismic data, to determine the elastic constants of these rocks.

a.

Compiled by H. Zhuravleva Editor S, Saenko Techred N. Bonkapi Corrector.

Order 1709/49 Circulation 728 Subscription VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., 4/5

Branch ShSh Patent, Uzhgorod, st. Project, 4

Claims (1)

METHOD OF SEISMIC RESEARCHES IN WELLS, consisting of measuring the travel time of surface borehole waves from a source to a receiver located at a fixed distance from one another, characterized in that, in order to increase the accuracy and efficiency of studies, waves with a frequency composition in the range from 20 Hz are excited up to 4 kHz, and registration is carried out in two frequency ranges, while the boundaries of the first frequency range are chosen equal to 20 and 500 Hz, and the second - 1 and 4 kHz. FIG. 1