In this paper, the principle of in-situ stress determination at great depth by using acoustic emission (AE) technique is presented. A new method is proposed to accurately measure the three principal in-situ stresses by means of re" peated loading on the core sample, based on the Kaiser effect and incomplete erasion phenomena of AE activity. The preparation of rock sample, experimental procedures and data processing method are also introduced. In addition, the effetcs of rock types, experimental parameters and the core preservation time on test results are also investigated. The results agree well with the in-situ stresses measured by hydrofracturing method at three oil fields in China, which proves the applicability of AE method for in-situ stress measurement.
In-sire stress which objeetively ex/sts in formation results from the geotectonic force and overburden we/ght of rock mass. It has great influence on wellbore stability, well trace controlling and hydraulic fracturing design, therefore, its estimation is considerably important. Several methods for stress measurement have been developed, such as hydrofracturing method, overcoring method and AE method. Hydrofracturing method and overcoring method have been used successfully in recent years, but they are expensive and labor-consuming for implementation, and rock property has great effect on the result. AE method, utiliz/ng the Kaiser effect of acoustic emission for stress measurement of the rock mass, are proposed by Kanagawa et al(1977). If stress is monotonically increased, the AE activity begins to increase markedly when the compressire stress level exceeds the previous maximum stress.This is the Kaiser effect, which is observed in rocks as well as in other materials (Kaiser, 1953). By using this phenomenon, the previous maximum stress of rock sample can be estimated by observing the AE activity during the application of monotonically increasing stress. Usually, the stress measured is an effective stress of the rock sample.
Great of efforts has been made for the application of the AE method to the stress measurement on natural rock (T. Kanagawa et al, 1977, Sumio Yoshikwa 1989), and many valuable achievements have been reached. However, a perfect program of measuring in-situ stress by AE method has not been founded yet, Because the experimental procedure of AE method is very complicated, and the test results are affected by many factors, such as rock type, the core preservation, time (it refers to the time intervals between the core drilled out from the bottom and tested in the Lab.) et al. Until now, little study has been made on measuring the three principal in-situ stresses of rock mass at great depth by utilizing AE method. In this paper, the principle, method, experimental procedure and influencing factors of AE method are investigated based on the data of AE test, and the in-situ stresses in three oilfields in China have been measured by means of the method proposed by the authors.
