Analysis of hydraulic fracturing tests in Uma Oya Hydropower project-Sri Lanka using Rummel’s fracture mechanics approach

A total of 8 hydrofrac stress measurements to about 1km depth were conducted in borehole PH-01during the geotechnical site investigation program of Uma-Oya multipurpose project in Sri Lanka in 2011. The in situ tests were carried out by Gamaneh kav wire line system, where the straddle packer tools is moved within the 76mm (HQ size) or 96mm (NQ size) diameter boreholes on a seven conductor borehole logging cable and a winch system. In this paper we present the results of a new development of equipment for determination of the hydraulic tensile strength of rock in laboratory conditions for the first time in Iran. In laboratory mini frac tests, the rock samples were subjected to axial stress and confining pressure. Internal pressurization was achieved by injection of water in to the 4mm diameter holes at a rate of approximately 1ml/s. Fracture initiation and dynamic fracture growth is characterized by a hydraulic short circuit between axial stress, confining pressure and injection pressure, which yields the breakdown pressure Pc. The fracturing pressure Pc then is plotted versus the confining pressure pm which in general leads to a linear relationship (Rummel, 1987). The pc-value for pm = 0 is the hydraulic tensile strength pco, the slope yields the Fracture coefficient (k).According to Rummel’s fracture mechanics approach the laboratory tensile strength values pco suggest in-situ tensile strength values Pco of approximately 2.55 MPa, compared to observed strength values of approximately6.16 MPa. The result is an expression of the size effect with an intrinsic crack size ao of the laboratory samples in the mm-range, while due to the larger rock surface exposed at the pressurization during the in-situ tests, the intrinsic crack-size Ao of the rock is more than 10 times larger.