Experimental Study Of Anelastic Strain Recovery In-situ Stress Measurement Methods And Its Application

ASR method is a three-dimensional stress measurement method developed in recent years. After the5.12Wenchuan earthquakes, ASR method was introduced firstly for in-situ stress measurement in Wenchuan Fault Scientific Drilling (WFSD) project in China. In the ASR method, how to determine the ASR compliances, the influence of thermal expansion coefficient and temperature, and the reliability of the test results are important issues that need further experimental studies. All these issues will influence the in-depth development and wide application of ASR method. In this paper, basing on the development existing circumstances of different in-situ stress measurement methods, the theoretical background, development process and application examples of ASR in-situ stress measurement method are expatiated in detail. The anelastic strain recovery compliances and thermal expansion coefficient experimental study of rocks are focused. According to the actual in-situ stress measurement results of WFSD holes by ASR method, the reliability and influencing factors of ASR in-situ stress measurements are analyzed. The main progress and achievements are as follows:1. In reviewing the research status of different in-situ stress measurement methods, the principles, equipment and processes of ASR in-situ stress measurement method are summarized. The data processing method and related functions expression are given in detailed. It is pointed that the accuracy and long-term stability of materials and equipment are key factors of the quality of the test results under the premise of homogeneous and isotropic visco-elastic rock material.2. Anelastic strain recovery compliance is a key parameter to determine the magnitude of in-situ stress. ASR compliance experimental results show that the results of ASR compliance for different rock are different under the Uniaxial Compression Stress (UCS) conditions. And the ASR compliance of shear mode is greater than the volume mode. The ratio of shear mode ASR compliances to volume mode increases gradually with time and the change scale gradually become smaller. The relationships between conventional rock mechanics parameters such as elastic modulus, UCS and ASR compliance obey the exponential functions. So under certain conditions, the ASR compliances of rocks could be estimated from conventional rock mechanics parameters.3. Temperature changes and thermal expansion coefficient of rocks are the main factors of anelastic recovery strain measurement results. In this paper, a high precision measuring device of thermal expansion coefficient of rock was established. The experimental study of thermal expansion coefficients for different rocks were carried out under0～60℃, and it is determined that the thermal expansion coefficients of8kinds of different rocks are between5.3～8.7*10-6/℃. The results provide a basis for the analysis of effect of temperature on the ASR in-situ stress measurement method. The results show that the effect of temperature on the ASR in-situ stress measurement results can not be ignored. The measurement error of about5～9micro-strain could be caused with1℃temperature change. In order to ensure the measurement errors due to temperature fluctuations less than1micro-strain, the range of temperature changes should not exceed±0.1℃. At the same time, the influence of sample temperature equilibration process on the ASR in-situ stress test results was analyzed.4. The ASR method was successfully applied to the WFSD project. And the magnitude and direction of in-situ stress variation with depth are given. The maximum, medium and minimum principal stress are in the scale of10.8-45.7MPa,10.4-29.4MPa and6-26MPa in depth between424-1173m, respectively. The dominance direction of maximum principle stress is NW51°. The in-situ stress measurement results by ASR method are almost consistent with other in-situ stress measurement methods. At the same time, the actual ASR compliances test was carried out using the same sample for the ASR measurement from the WFSD-1hole. The effect of ASR compliances was discussed. The results show that the ratio of ASR compliances is2.9(Jas/Jav=2.9) other than the conventional2.0, and the corresponding measurement error by this difference of maximum, medium and minimum principle stress were6.3%,5.0%and4.0%. The relationship between the ratio (Jas/Jav) of anelastic strain recovery compliances and principle stress magnitude was discussed.5. The effects of time, temperature, core orientation and other factors for ASR in-situ measurement results are summarized. And the accuracy and reliability of ASR in-situ stress measurement method are evaluated. The results indicate the measurement error of principal stress magnitude and direction can be controlled within±10%in the premise that homogeneous and isotropic rock core, constant temperature control and core scanning image and image logging are adopted.In conclusion, the ASR method has low cost, high efficiency, adaptability and without measurement depth limit compared with other in-situ stress measurement methods. As long as the oriented rock from deep underground were obtained, the ASR method in-situ stress measurement can be conducted and obtain the reliable data could be achieved. ASR method will have broad application prospects in the deep resource development, underground space utilization and geodynamic studies.