In Situ Stress Estimation Using ISDM with Data from Hydraulic Tests for Røldal HPP, Norway

Abstract

Hydraulic test is one of the most common methods to determine the in situ stress conditions in the rock mass, especially at great depth, where the test locations are accessible by drillholes only. This paper discusses the hydraulic fracturing of intact rock (HF) and hydraulic test on pre-existing fractures (HTPF). The paper critically reviews the current understanding of HF and HTPF test methods and suggests corrections and improvements. The associated calculation methods using HF and HTPF data are also re-introduced with detailed discussions. To demonstrate the calculation process, SINTEF uses ISDM for the calculation of in situ stress with Monte Carlo simulation to get a priori estimates, which is also done by other researchers. SINTEF runs the stress calculations for different a priori estimates based on repeated Monte Carlo simulations to reduce the sensitivity of the solution to the a priori estimates. This calculation process is applied to data obtained from hydraulic tests carried out in a group of short drillholes (30 m) at Røldal hydropower project in Norway. Two calculation alternatives were performed, which are (a) Calculation using ISDM for HF tests, (b) Calculation using ISDM for HF and HTPF tests. A comparison is also made for the obtained results with the result from classical HF tests. The paper discusses the applicability of the ISDM method, impact factors on the accuracy of the ISDM, practical challenges and uncertainties during measurement and calculation processes, and possible improvements.