Hydraulic fracturing had been a widely applied method of measuring stresses in a drilled borehole. During the test procedure, induced forces due to the interaction between fluid flow and rock blocks deformation overcome the in-situ least stress. Thus the coupled hydromechanical behavior of rocks having pre-defined joints should be considered. In order to model the process, an interactive computer program was developed. Through a numerical analysis, at first, fluid flow forces were calculated by Extended Pressure (EP) method. Then the coupled influence of fluid on mechanical behavior of rock blocks is included using Discontinuous Deformation Analysis (DDA). The procedure was used to simulate hydraulic tests on pre-existing fractures (HTPF). Finally having pressure-time records, the inverse calculation of in-situ stresses are compared with conventional method.

1.1 Fluids and rock fractures

The behavior of fractures plays a leading role on rock treatments. This fact could clearly be recognized through design analysis applications of Rock Mechanics such as design of underground caverns to prevent collapse of structures as such, to promote fracture in quarry industry in mineral mining and so on. In addition, fluid flow and its pressure distribution in jointed rock masses are important so as even in the case of rock masses with widely spaced and very tight joints, for most rock types, the hydraulic conductivity due to the joints is much larger than that of the rock matrix i.e. the fluid flow is controlled by the network of intersecting joints.

1.2 Hydraulic fracturing stress measurement

The artificial hydraulically induced fracturing has several usages, among the rest, the exploitation of geothermal energy, nuclear waste disposal, the stimulation of reservoirs and in situ stress determination. This process called "hydrofrac" in the late 40s (Clark, 1949) as a stimulation technique in order to enhance oil and gas well production.

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