Probabilistic estimation of fracture transmissivity from Wellbore hydraulic data accounting for depth-dependent anisotropic rock stress

• Published in: International journal of rock mechanics and mining sciences (Oxford, England : 1997) Vol. 42; no. 5; pp. 793 - 804
• Main Authors: Öhman, J., Niemi, A., Tsang, C.-F.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2005
• Subjects: Anisotropy; Atmosfärs- och hydrosfärsvetenskap; Atmosphere and hydrosphere sciences; Borehole analysis; Earth sciences; Fracture transmissivity; Geovetenskap; Hydrologi; Hydrology; NATURAL SCIENCES; NATURVETENSKAP; Stress-field; British Isles, England; British Isles, England, Cumbria, Sellafield; Borehole analysis; Fracture transmissivity; Stress-field; Anisotropy
• ISSN: 1365-1609; 1873-4545; 1873-4545
• DOI: 10.1016/j.ijrmms.2005.03.016

A new method is introduced that incorporates the use of hydrological and rock mechanical data in assigning transmissivities for fracture-network models. The hydrological data comes from fixed-interval packer tests carried out in a borehole and the rock-mechanical data are the prevailing in situ depth-dependent stress-field and the stress-closure relationship of fractures. In the model, the fracture transmissivity distribution is considered to be constituted of two components, one deterministic stress-induced component and the other a stochastic component that describes the intrinsic variability of fractures in a network. The outcome is a tensorial description of fracture transmissivities in an anisotropic stress-regime, where the transmissivity for an arbitrarily oriented fracture in the network is determined by its orientation in relation to the ambient stress-field. These transmissivities are conditioned such that the overall results satisfy the hydraulic packer test data. The suggested procedure is applied to an example data set from a site at Sellafield, England. The results show that the probabilistic approach, relying on hydraulic data alone, may underestimate the true variability in fracture transmissivities, since the typically vertical boreholes entail a sampling bias towards horizontal fractures that are predominantly subject to vertical stress. The suggested method helps to account for the true underlying three-dimensional variability that is incompletely resolved by using the hydraulic borehole data alone. This method is likely to have the largest impact at low stress-levels, in strongly anisotropic stress-fields, for borehole directions parallel to one principal stress, and for fracture network geometries characterized by sets orthogonal to the three principal stresses.