Petrophysical Properties of IDDP-2 Core Samples from Depths of 3650 to 4650m
Benoit GIBERT, Didier LOGGIA, Fleurice PARAT, David ESCOBEDO, Léa LEVY, Gudmundur Omar FRIDLEIFSSON, Robert A. ZIERENBERG, Philippe A. PEZARD, Nicolas MARINO
[Université de Montpellier, France]
As part of the exploration of supercritical geothermal reservoirs at the Reykjanes Peninsula, SW Iceland, the IDDP consortium has accomplished a series of exploratory drillings and coring in order to investigate the physical conditions at depths of 4-5 km in high temperature-high pressure hydrothermal environments. A set of 21 cylindrical mini-cores samples extracted from IDDP-2 core between 3638m to 4654m depth were studied to identify their alteration mineralogy and petrophysical properties. Optical and SEM observations from thin sections indicate that samples are medium to fine grained diabase intrusions showing granular holocrystalline textures. Mineral compositions from XRD on powders and from SEM observations on thin sections indicate that the rocks are composed of plagioclase and amphibole (80-90%), replacing primary augite phenocrysts, along with magnetite and ilmenite and minor amounts of quartz. Enstatite and biotite are present in the deepest cores, below 4500m. Petrophysical properties such as porosity, acoustic velocities (Vp-Vs) and electrical conductivity were first measured on one inch diameter mini-core samples at room conditions in dry and saturated conditions. Porosity is generally low, ranging from 0.4 to 2.9 %. Electrical conductivity, measured at 103 Hz as a function of pore fluid conductivity, indicates an unusual and low electrical tortuosity and cementation index for dolerites (2.3-10.0 and 1.2-1.6, respectively). P-wave velocity is higher by 30 % on average when the sample is saturated with water, compared to dry conditions. Both acoustic and electrical properties indicate that interconnected microcracks and fissures dominate porosity. Electrical conductivity measurements at 100 MPa of confining pressure – i.e. relevant for in-situ conditions and up to 600°C, under both dry and fluid-saturated conditions, indicate that the total conductivity of the sample increases by more than one decade from dry to fluid-saturated conditions. Acoustic properties measurements at room temperature and up to 300 MPa of confining pressure, indicate that P- and S-wave velocities increase in a non-linear and reversible manner when pressure increases. This behaviour is attributed to microcrack closure with increasing pressure. Crack density calculation shows that relative high crack density ( more than 0.3) is preserved at effective pressure corresponding to in-situ conditions. Our results indicate that the density of microcracks controls the physical properties of rock samples over the whole investigated section. Microcracks may result from recent hydrothermal circulations, as suggested by Violay et al. (2010) on samples collected in the sheeted dike complex cored by IODP hole 1256D. Microcracks may also arise from drilling operations, where intense in-situ cooling and later decompression generates mechanical stresses linked to mineral contraction. Finally, the comparison of high-pressure core measurements to data from down hole geophysical tools, large scale MT, and seismic investigations in the Reykjanes peninsula is compatible with the presence of supercritical fluids at depth.
|        Topic: Geophysics||Paper Number: 13071|