SU1239319A1 - Method of determining the strained state of rock mass - Google Patents

Description

The invention relates to mining and is intended to determine the stress state of a rock mass at the exploration stage.

The aim of the invention is to improve the accuracy and informativeness of measurements.

The method is carried out as follows.

In the array along the well, propagation velocities of longitudinal and transverse waves are measured. Then, the selected core is loaded in the axial direction and, in parallel, the propagation velocities along its axis of longitudinal and transverse waves are measured. At the same time, the axial load is first adjusted to the values at which the speed of the longitudinal waves in the core becomes equal to the speed of these waves in the array. Under this condition, the magnitude of the axial load is taken as the magnitude of the vertical stresses in the array. Then the axial load is adjusted to the values at which the speed of the transverse waves in the core becomes equal to the speed of the said waves in the array. Considering that the speed of the transverse waves depends equally on the two main stresses (in the array — vertical and horizontal, in the core — axial and transverse), the magnitude of the horizontal stresses is determined by algebraically subtracting the previously measured vertical stresses from the specified load.

If the rocks are anisotropic, the core in addition to the axial loads also load transverse loads. The magnitude of the horizontal stresses in the array is determined directly from the magnitude of the transverse load at which the velocity of the transverse waves in the core becomes equal to the velocity of the transverse waves in the array.

The essence of the proposed method is to use, along with speed

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t longitudinal and transverse waves, which is based on the experimentally established difference in the velocities of the two types of waves with different stress components. The velocities of the longitudinal waves depend mainly on the loads acting on the rock along the direction of wave propagation, and are practically insensitive to transverse loads. The velocities of the transverse waves depend to the same extent on the two main stresses whose directions of action correspond to the direction of the waves and the oscillations of the particles in it (polarization), i.e. they are a function of the form

Vs ,,: У5 „+ A (a, + a,) + B ((1,),

where Vs.t, is the velocity of the transverse wave along the X axis and the polarization y;

Vso is the velocity of propagation of the transverse wave in the unloaded rock;

Oh, Oy, Og are the principal stresses acting along the corresponding x, y, z axes;

A (cr -f-ffj,) is an arbitrary function describing the change in the velocity of the shear wave under the application of loads along its direction of propagation (st) and polarization (o); The B (ar) function describes the change in the velocity of this wave under the action of stresses acting orthogonally to the plane of polarization (for rocks, it was experimentally established that, at equal voltages, the DN B (ar) is much less than (a) or 4U A (o ,,). The table shows the test results.

Vs

V /

where Vp, V / are the velocities of the longitudinal wave during the propagation parallel and perpendicular to the axis of the compressive load;

-speed wave velocity during propagation parallel to the axis of the compressive load;

-speed of the transverse wave during the propagation perpendicular to the axis of the compressive load and the position of the polarization plane along the axis of the compressive load.

The data presented in the table fully confirm the peculiarities of the coupling of the velocities of longitudinal and especially transverse waves with stress in rocks.

Thus, by measuring the velocities of the longitudinal wave in the massif and in the core under pressure and comparing their values, it is possible to estimate the magnitude of the vertical stresses. By determining the values of the propagation velocity of the transverse wave in the core under various loads and comparing them with the velocity values of the specified wave in the array, it becomes possible to estimate the combined effect of vertical and horizontal stresses. Taking into account that when only gravitational forces act in the array, the horizontal stresses are equal in magnitude, knowledge of such two parameters as the speed of the longitudinal and transverse waves is sufficient to characterize the stress state of the array.

Core loading, necessary to establish which stress state of the rocks corresponds to a given set of values of the elastic wave velocities, may be carried out by axial or joint axial and lateral loads. The simplest and most productive is the axial loading of the core. Its use in the proposed method of determining stresses is most correct if the rocks studied meet the requirement of uniformity and isotropy. By homogeneity of properties is here meant the absence of differences in properties in different parts of the sample. From the condition of homogeneity, it follows that the dependence of the velocities of elastic waves on the load for different directions, in this case, the axial and transverse directions (the propagation of the wave coincides with the axis of the load). For such rocks, as shown by the results of theoretical studies, the velocity of transverse waves depends on the stresses acting in the directions of propagation and polarization of the waves, exactly the same. Thus, the magnitude of the horizontal stresses can be obtained from the velocity of the transverse waves by subtracting the contribution of the vertical stresses from the total relative change in the velocities of the transverse waves in the core from the pressure 0

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laziness. In order to increase the accuracy of the method in determining the stress state of rocks, which are characterized by pronounced anisotropy and heterogeneity of properties, it is advisable to use axial and transverse core loads together.

Example 1. A vertical well was drilled on the Kola Peninsula. A core was taken from which a sampling was made every 10 m. Then, using the AKK-I equipment, a broadband acoustic logging of the borehole walls was carried out. Using the phase correlation method of the wave pattern, we determined the values of the interval propagation times of the longitudinal and transverse waves along the well and calculated the values of their velocities. In the melanocratic metagabbro located at a depth of 350 m, the velocity of the longitudinal wave V was 6950 m / s, and the velocity of the transverse wave Vs was 3910 m / s.

Melanocratic metagabbro suited well enough the conditions of homogeneity and isotropy; therefore, they were loaded on the press only in the axial direction. As a result of measuring the velocities of longitudinal waves along the core axis during its loading, the dependence of the velocities of these waves on the stresses in the rock was obtained. Compared the results of measurements of V and UD obtained that V Vp 6950 m / s under a load of 30 MPa. It was taken as the value of vertical stresses in the array (ae -30 MPa).

Then the load was increased in absolute value and the velocity of the transverse waves in the core was adjusted to the values of Vs 3910 m / s, which were measured in the array. The load required for this was equal to 41 MPa. Taking into account that the velocity of the transverse waves changed under the influence of not only horizontal but also vertical stresses, the previously measured value of the vertical components (about 30 MPa) was subtracted from -41 MPa and found that the horizontal stresses at a depth of 1000 m are equal ar -41- a, -41 - (-30) -11 MPa.

Example 2. A well was drilled in the Khibiny massif (Kola Peninsula), crossing a complex of nepheline syenites. A core was taken from which a sampling was made every 10 meters. Then, using the AKK-1 equipment, a wide-band acoustic logging of the borehole walls was carried out. Using the method of phase correlation of waves, determined the values of the interval propagation times along the well of the longitudinal and transverse waves and calculated the values of their velocities. At a depth of 1150 m in trachytoid ijolites, the velocities were V 6,660 m / s, V 3,450 m / s.

A feature of trachytoid ijolites is the presence of natural anisotropy.

elastic properties. Therefore, a stabilometer capable of creating axial and transverse loads (KP-3 design of the ARRIM) was used to load the core. During the loading process, the velocity of axial and transverse wave propagation in the axial direction of the core was measured. Axial and transverse loads were loaded until the velocities of the longitudinal V and transverse Vs waves in the core reached the velocities of these waves in the array (V m / s, Vs m / s). The axial load required for this was -34 MPa. It was taken as the magnitude of the vertical stresses in the ot array, -34 MPa. The magnitude of the horizontal stresses in the massif was determined directly from the magnitude of the transverse loads, which were equal to -80 MPa (ag -80 MPa).

Claims (2)

1. METHOD FOR DETERMINING THE STRESSED STATE OF ROCK MASS, including measuring the propagation velocities of longitudinal and transverse waves in an array along a well, measuring the propagation velocity of a longitudinal wave in the axial direction of a core when it is loaded, and determining vertical stresses in an array by comparing the values of the longitudinal wave propagation velocities in the array and in the core, characterized in that, in order to increase the accuracy and information content of measurements, when loading the core, additionally measure soon the propagation of transverse waves in its axial direction and determine the magnitude of horizontal stresses by comparing the obtained values of the velocities of transverse waves in the array and in the core, taking into account changes in the velocity of transverse waves from the action of vertical stresses. 2. The method according to π. 1, characterized in that, in order to increase the accuracy of measurements in anisotropic rocks, the cores are additionally loaded with transverse loads, and the horizontal stresses in the array are taken equal to the transverse loads at which the shear wave velocity in the core coincides with the shear wave velocity in the array. SU, „, 1239319