Evaluation of Hydraulic Fracturing and Overcoring Methods to Determine and Compare the In Situ Stress Parameters in Porous Rock Mass
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There are various methods to determine in situ stress parameters, but each method is having its own advantages and limitations. Among the methods available, the hydraulic fracturing method is the most commonly adopted procedure for in situ stress measurements because of its simplicity, reliability and economic viability. But, the limitations with this technique are that some assumptions implicit for this method such as that the rock mass is continuous and elastic. Moreover, the reliability and validity of this method becomes questionable in porous rocks that may encounter in underground tunnels. These limitations are experienced ever since the introduction of this method, especially in porous rocks. In general, the recognition of fracture initiation in the hydraulic fracturing test has not proved difficult. The relatively slow rates of pressurisation have ensured that when fracture initiation occurs, the sudden increase in volume has led to a marked drop in pressure in the porous section, which is easily recognised from the pressure record (Sjoberg et al. in Int J Rock Mech 40:999–1010, 2003). But the drop-in pressure in a porous rock is far more difficult to recognise. This is since pressure cannot be developed if the rate of leakage in the formation is equal to or higher than the flow rate applied for fracture initiation. This problem of non-generation of water pressure can be tackled by use of a high viscosity fluid. Stress measurement were conducted by hydraulic fracturing method by using high viscous liquid in porous rocks. The stresses evaluated by this technique were correlated with stress measured by overcoring method. The stress measured by overcoring method was used as bench-mark as this method does not suffer from the presence of porosity of the rock. This new technique will be helpful in conducting the stress measurements in porous rocks, which will be highly beneficial to both mining and hydropower related excavation.
KeywordsHydraulic fracturing Porous rocks High viscous liquid Normal stress Fracture initiation Overcoring
Author is extremely thankful to Secretary and other staff of Ministry of Mines, Govt. of India and 40th Standing Scientific Advisory Group for sanctioning the project. All the fieldwork under this project was done at Singareni Collieries Company Ltd. This involved lot of planning, co-ordination and concentrated efforts of the management and ground staff at the test sites. Author is also thankful to Mr. C. Nagaraj and Mr. D. Joseph, Mr. G. Shyam and Mr. K. Vamshidhar, Scientists of NIRM for associating in the field work and their cooperation for successful completion of the report. Author is also thankful to our Director, National Institute of Rock Mechanics for providing encouragement and valuable suggestions as the work is progressed, and for extending the facilities for carrying out the work.
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