The classical model for in-situ stress estimation from hydraulic fracturing tests doesn't consider the presence of natural cracks and assumes a continuous medium in the test interval. Since one faces to natural fractures in reality, a solution was required in order to consider fracture orientations. Considering pre-existing fractures, a code which is named SAMIR has been programmed to do stress computations based on the results from hydraulic fracturing in-situ tests using Matlab© software. In this code an inversion technique is used to determine the unknown parameters which could define the in-situ stress tensor in three dimensional space. The valid database has been involved as input to verify the code. The outputs have a well coincidence. The interpretation of the results obtained from hydraulic fracturing tests in Iran Aghajary Formation is represented.
Nowadays, since there is an increasing need in construction of underground excavations such as transportation tunnels, power house caverns and etc., knowledge of the magnitude and orientation of the in situ principal stresses is essential to determine crucial design parameters and satisfy the stability of underground openings within complex geological media. There are different methods for determining the in situ stresses. Among them, due to a well theoretical base and the feature of determining stresses in deep boreholes, Hydraulic Fracturing method becomes more promising. This method, in general, can be classified in two categories. One is massive operations which are used in oil and gas well stimulations or geothermal projects. The second could be mini/micro hydraulic fracturing [1]. The latter is available for creating initial fracture in well stimulations, fracture resistance determination and measuring in-situ stresses. With this technique a portion of a borehole as an interval is sealed off with a straddle packer (Fig1.a) and then submitted to a constant flow rate injection of a fracturing fluid. The interval pressure comes up until the borehole wall fractures and a so called breakdown pressure, P b (Fig1.b) appears. Waiting for a while (for being stabilized), the injection will be stopped [2]. After the completion of the HF test, the results will be some plots of pressure vs. time from which the stress components should be extracted. In this paper first the theory of analyzing the insitu stresses from the HF/HTPFa test results will be discussed. Then go through to describe the inversion method used in the algorithm of solving the nonlinear equation of determining stress tensor components. A code using this algorithm has been programmed in Matlab© software. Finally the results of the code are compared with prescribed values in Forsmark site investigation. Then the obtained values of insitu stress state for Gotvand Dam site in Aghajary formation of Iran are represented.
