The acoustic emission technique is an effective method to determine in-situ stress, and is mainly applied to vertical wells. A new method has been developed to determine in-situ stress for non-directional cores from the inclined well. Using coordinate system transformations and solving nonlinear equations, the method was applied successfully in the Bohai Oil Field.
In-situ stress is internal stress existing in the underground rocks. During the development of oil and gas fields, the reservoir consolidation and expansion resulting from the in-situ stress will lead to the changing of the porosity and percolation. In-situ stress is an important factor to the design of the flooding networks. At the same time, study of in-situ stress play important roles in many aspects, such as the hydraulic fracturing design, the extension laws of hydraulic fractures, the sand production, and the formation slide creep resulting from earthquakes caused by water injections. During drillings, the measurement of in-situ stresses also play important roles in some problems, such as well bore stability, formation pressure gradient, the control for directional drilling trace, and casing distortion in the rheological formation. As a method to analyze the rock stress states, the acoustic emission measurement is based on Kaiser effect, which is an abnormal phenomenon appearing when the material is repeatedly loaded. The Kaiser effect shows that the rock memorizes its stress history. Firstly, when cores from the fields are compressed, acoustic emission signals will be received at the same time. When the loads on the samples are small, the acoustic emission frequencies will be low. However, once the loads attain some value, the acoustic emission will increase abruptly. The load value at that moment is equal to the maximal normal stress of the samples. Usually a vertical core and three horizontal cores are required for rests since rocks are in a three dimensional stress state. Three principal in-situ stresses will be given by experimental results. However, this is not the case for the inclined cores, since the coordinate system of the well axis does not coincide with the coordinate system of the in-situ stress principle direction any longer. As a result, a new method is needed to solve the problem. The method proposed in this paper has been used to determine the in-situ stresses at great depth by acoustic emission technique on the cores of an inclined well. The method is realized by using coordinate system transformations and solving nonlinear equations.
The in-situ stress tensor is expressed as follows in the coordinate of in-situ principal stress directions.
Six of the equations in group (6) arc independent because of the symmetry of the stress tensor.
Using the method supported by this paper, the in-situ stresses were obtained from the deflecting well cores at Bohai Oil Field BZ-34 region. The horizontal principal stress direction was given by the logging data of the formation dip angle.
