An Experimental Study Of In-Situ Stress Comparison Measurement

In-situ stress state is one of the most important physical parameters of the solid earth. Carrying out stress measurement and studying stress state in the crust are of considerable interest for the issues of earth dynamics and engineering application. ISRM has recommended overcoring and hydraulic fracturing as the major methods for in-situ stress measurement. The two methods are both belong to plane stress measurement, however, their accuracy and usability has always been a concerned problem in the field.This paper based on the "an experimental study of in-situ stress measurement techniques" project and main tasks were done in experimental field in Zijingguan, Yixian of Hebei Province. The author has joined all the stress comparison measurements in the field. Based on the jobs, what this paper has completed are as following:(1) A good many of data and literature were studied to summarize the theory and data process of piezomagnetic overcoring. Analysis for the old piezomagnetic overcoring system were done and summaries for improvements and invocations of the new version were shown;(2) Data process methods of hydraulic fracturing were summarized. Proper methods in this study were chosen, combined with drilled cores and measurement curves, to determine the final stress values;(3) Piezomagnetic overcoring stress measurements at4different depths in3boreholes and hydraulic fracturing at different depths in4boreholes were finished. The deepest piezomagnetic overcoring measurement was done in the world;(4) Comparison measurement analysis for piezomagnetic overcoring and hydraulic fracturing was conducted and reliability, usability and influence factors of the two methods were discussed.By what have done above, some conclusions can be drawn as following:(1) The new piezomagnetic overcoring systems has improved a lot than the old one. In-situ stress measurement based on the system was more effective and could work in deeper rock mass than before. The deepest depth it completed in the world was213.6m.(2) Based on comparison study of piezomagnetic overcoring and hydraulic fracturing stress measurements in the same or similarly depth in4boreholes, some conclusions can be obtained:①It was proven that, above200m, take piezomagnetic overcoring as reference, the difference between the maximum horizontal principal stress obtained from piezomagnetic overcoring and that from hydraulic fracturing, was less than10%, and as for the minimum horizontal principal stress was less than36%. Also, the principal stress direction agreed with that of regional stress field, stating that results of two methods measurement agree well with each other.②Hydraulic fracturing got the concordant stress change trend in4boreholes. However, different depths in different boreholes showed different stress values, not only for intact rock mass, but also for some place with pre-existing fractures developed, which indicated the complexity of stress field. The directions of maximum horizontal principal stress between NW40°～50°, which agreed with that of regional stress field.③Comparisons between the maximum and minimum horizontal principal stress at similar depths in different boreholes stated that if there existed fractures near the measurement sites, the measurement results were to be influenced and fracture zone affected more. Intact rock mass can store in-situ stress well and stress values obtained from similar depth were similar to each other, as well.