The Relationships between Effective Porosity, Uniaxial Compressive Strength and Sonic Velocity of Intact Borrowdale Volcanic Group Core Samples from Sellafield

Abstract

The effective porosity, saturated sonic velocity and saturated uniaxial compressive strength were determined on a large number of Borrowdale Volcanic Group volcaniclastic core samples from three boreholes at Sellafield, Cumbria. The worked formed part of the UK Nirex Limited site investigation into whether the Sellafield area could be suitable as a repository for intermediate and low level radioactive waste. Most of the intact samples were of low to very low effective porosity, had a high sonic velocity and were very strong to extremely strong. However, a proportion of values deviated significantly from this. Bivariate analysis showed a negative relationship exists between sonic velocity and effective porosity.The cross plots of these two parameters with uniaxial compressive strength showed a wide range of strength values for samples of low effective porosity and high sonic velocity.Six failure types were identified during the uniaxial compressive strength tests. The strongest samples tended to fail through the matrix and the weakest rock samples tended to fail through haematized material or along haematized veins. Effective porosity and sonic velocity measurements could not distinguish between those samples that failed through the matrix and those that failed along discrete narrow veins. The presence of narrow haematized veins has a major effect on the intact rock strength.

Correlating physico-mechanical properties of intact rocks with P-wave velocity

“…As can be seen in this table, volcanic rocks give higher uniaxial compressive strength (UCS) values (172-202 MPa) and point load index Is(50) compared to sedimentary rocks. The previous literatures show that our results are in acceptable ranges (Cobanglu and Celik 2008;Entwisle et al 2005;Kahraman 2001a;Khandelwal and Singh 2009;Kurtulus et al 2010;Kurtulus et al 2011a;Kurtulus et al 2011b;Moradian and Behnia 2009;Sharma and Singh 2008).…”

Prospects for Assessing Enhanced Geothermal System (EGS) Basement Rock Flow Stimulation by Wellbore Temperature Data

“…As discussed below, the empirical poroperm relation κ(x,y,z) ∝ exp(αφ(x,y,z)) valid for the measured value of α ~ 500 for OTN1 is similar to values found for porosity-permeability data from the KTB well and the Borrowdale volcanics at the UK Sellafield nuclear facility. This discussion places OTN1 well-log spatial fluctuation and well-core poroperm data in a wider basement rock context provided by KTB, Fennoscandia and UK Nirex metamorphic basement rock data [24][25][26][27][28].…”

“…From upper left, plots (a-h) show 3D model radial temperature distributions fit to 2D steady-state radial temperature distributions (5) The fracture connectivity parameter associated with the fracture-connectivity flow structures generating observed temperature events is then κm ~ κ exp(αmφ) for φ ~ 0.01 and κ ~ 1 µDarcy is then αm ~ 1000. If we associate an effective ambient value for poro-connectivity α ~ 500 (Figures 25-29 [24][25][26][27]), then an OTN1 temperature event model flow structure poro-connectivity αm ~ 1000 is of order twice the ambient value is nominally characteristic of Peclet number Pe ~ 10 flow structures.…”

“…If we associate an effective ambient value for poro-connectivity α~500 (Figures 25-29 [24][25][26][27]), then an OTN1 temperature event model flow structure poro-connectivity α m~1 000 is of order twice the ambient value is nominally characteristic of Peclet number P e~1 0 flow structures.…”

“…Further, KTB well core from 4 km to 5.5 km depth (Figures 24 and 25; [24]) and well-flow data from highly metamorphosed low-porosity/low-permeability Borrowdale Volcanics Group (BVG) in Cumbria UK (Figures 26-28; [25][26][27]) preserve the spatial-correlation poroperm relation κ(x,y,z) ∝ exp(αφ(x,y,z)) introduced as crustal fluid-rock interaction empirical relation (III). KTB well-core data and BVG field data yield empirical values for the poro-connectivity parameter α in the poroperm relation κ ∝ exp(αφ) that is valid for low porosity basement rock.…”