Anelastic Strain Recovery Method to Determine In‐situ Stresses and an Application Example

Abstract

AbstractThe core‐based anelastic strain recovery method (ASR) developed in recent years is a more economical and more effective method for measurement of three‐dimensional in‐situ stresses at depth. After the Wenchuan earthquake on May 12, 2008, this method is applied to measure in‐situ stresses for the scientific drilling borehole of the Wenchuan earthquake fault zone for the first time in mainland of China. This paper describes the principle and calculation procedures of the method and the typical stress measurement results of the hole‐1 of Wenchuan Earthquake Fault zone Scientific Drilling Project (WFSD‐1). Magnitudes and directions of principal stresses of the hole‐1 at the test depth were determined. The maximum principal stress σ1 and middle principal stress σ2 are nearly horizontal, and the minimum principal stress σ3 is nearly vertical. The azimuth of the maximum principal stress is NW. At the vertical depth of 746 meters, the principal stress magnitude is 25.2 MPa for σ1, 21.5 MPa for σ2, and 18.5 MPa for σ3, respectively. This stress state can be interpreted as the same stress regime to make the Longmenshan fault to generate thrust and dextral strike‐slip movement, which is consistent with fault movement of the Wenchuan earthquake on May 12, 2008. The measurement results obtained by ASR can be compared with that by focal mechanism solutions and other stress measurement methods. The application example shows that the ASR method is suitable for determining the orientations of the principal stresses and for estimating their magnitudes in deep drilling wells and complex geology conditions where other methods such as the over‐coring technique and hydraulic fracturing tests are difficult or impossible to be conducted.