Evaluation of the stressed-state of a bed based on emission memory effects of rocks in near-well space

Abstract

Capacity for memory of past experiences is one of the properties of rocks.This property manifests itself in various memory effects, which are being increasingly studied to understand the underlying physical mechanism and to find possible practical applications [i]. Socalled memory emission effects (MEE) are useful for analysis of the stressed-strained state of a bed. These effects are expressed in the nonreproducibility of acoustic emissions (AE) and electromagnetic emissions (EME) during a second loading of rock up to the maximum level of the preceding cycle.One evaluation technique, which involves extraction of rock specimens from the bed, loading, and AE and EME measurement, has been described in [2]. In the present paper we discuss another variant of stress analysis with MEE, which involves measurements in situ without the extraction of a core.For this method to be practical two main conditions must be met: First, some volume of the rock must first be relieved of the stresses that act upon it; secondly, the volume has to be loaded again up to the initial stressed state while recording AE or EME pulses. A local unloading of rocks in the bed can be accomplished by drilling a well.For reproducing the initial stressed state in the near-well space, a hydraulic sensor is used into which working fluid is injected under pressure.During the course of stress restoration, AE and EME parameters are measured.A jumplike change of these parameters is indicative of the bed being acted upon by stresses.As an implementation of this method, the Moscow Institute of Mining has developed, by modifying GD-6M/42-200 sensors designed by the Institute of Mining, Siberian Department, USSR Academy of Sciences, special hydraulic sensors with built-in AE and EME converters [3, 4]. The hydraulic sensor with an AE converter (Fig. i) comprises an electric cylindrical shell 1 installed on a central tube 2 with steel end-face cups 3 and 4; pipe 2 has radial openings 5 and an annular protrusion 6. Inside the lower cup 3 a piezoelectric element 7 is installed.It is shaped as a washer pressed against the annular protrusion 6 by cup 3'with a nut 8. The piezoelement 7, together with the protrusion 6 and the cup 3, is a piezoelectric "longitudinal vibrator" converter.The main lobe of its directivity pattern is similar in shape to the cavity of the shell I. As a result, the converter is highly sensitive to signals received from the loaded portion of the bed.The hydraulic sensor with EME converter (Fig. ib) uses as the antenna the central pipe. Through a dielectric bushing ii it is divided into two equal parts 9 and i0, which form a symmetric antenna to detect electric fields.The antenna arms 9 and ii are connected with a preamplifier 12 mounted in the lower cup of the hydraulic sensor. For matching the frequency parameters of the antenna with the spectrum of EME pulses, additional tubular conductors can be installed at the end of the pipe.Inside the dielectric bushing Ii, a ferrite antenna 13 is mounted, allowing measurements in the range of 10a-107 ...