Petrophysical fracture identification for rock physics ... - Net Brains

etween the fracture porosity and fracture aperture predicted by the FCP method (d and e) andthe measured FMI logs (D and E).Carbonate rocks exhibit complicated pore textures. However, in practise, the Gassmannequations (1951) are still valid for porous media of any lithology if the observation frequencyis low and the background matrix is tight (Han, 2007). Figure 5 shows a comparison betweensynthetic gathers generated assuming the conventional porosity log (PHIE) and a fractureporosity log from FCP calculation. The tracks refer to the following: (a) Volume set; (b)Saturation set; (c) Vp (m/s); (d) Vs (m/s); (e) Density (g/cc); (f) Conventional PHIE (in red)and fracture porosity log from FCP (black); (g) Initial gather calculated from Vp, Vs andDensity; (h) Oil 80% gather using FCP; (i) Oil 80% gather using PHIE; (j) difference between(h) and (i); (k) Gas 90% gather using FCP; (l) Gas 90% gather using PHIE; (m) differencebetween (k) and (l); The difference plots in tracks (j) and (m) suggest that the fractureporosity needs to be taken into account in AVO studies, as a stronger dimming response isencountered at the top of the lower reservoir level (indicated by the horizontal red line) if onetakes into account the FCP calculated fracture porosity log.(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m)Fig. 5 Fluid substitution effect on AVO with and without considering fracture porosity, asderived from the FCP method, for 80% oil (tracks h and i) and 90% gas (tracks k and l).ConclusionThe fracture comprehensive probability (FCP) is presented as a method to interpret essentialfracture properties from conventional petrophysical log data. The FCP approach generates asynthetic image log that can be calibrated to FMI image logs. A case study from a gassaturated carbonate reservoir is also shown for the case study. A significant difference isobserved in the AVO response of the reservoir when the fracture parameters are incorporatedin the modelling. It is concluded that the FCP methodology is an important precursor to rockphysics studies when fracturing is expected to be significant but image log data areunavailable.ReferencesGassmann, F., [1951]. Elastic waves through a packing of spheres: Geophysics 16, 673-685.Han, D., H., [2007]. Velocity of Carbonate Rocks, CSRPB, University of Houston, Vol. No.P1-8.Sibbit, A.M, and Faivre, O., [1985]. The Dual laterolog response in fracture rock. SPWLA26 th Symposium.Yan. J., Lubbe, R., and Pillar, N., [2008]. Modified velocity model for seismic study via AVO,Journal of Seismic Exploration, Vol. 17, p371-390.71 st EAGE Conference & Exhibition — Amsterdam, The Netherlands, 8 - 11 June 2009