Anisotropic Rock-Physics Model and its Applications to Laminated Reservoirs Analysis

Abstract

Abstract The proposed anisotropy model allows to produce five independent dynamic elastic moduli for the veetrtical transverse isotropy (VTI) case. The VTI elastic moduli matrix can be rotated to match wellbore inclination or observed lamina dips to produce tilted transverse isotropy (TTI) anisotropy matrix with thirteen elastic moduli. The model is designed for the laminated shaly sands but is applicable for other cases of thin laminated reservoirs as well. The model is based on the rock-physics model, which assumes that the anisotropy is caused by the lamination of clay and sands with a dispersed clay incorporated into sands’ matrix. Dispersed clay modeling is done using a combination of the Upper and Lower Hashin-Shtrikman and Reuss bounds. Laminated clay is incorporated using Backus averaging technique. VTI matrix can be rotated to the borehole frame using Euler rotation. Six plots and comparison of the independent laminated and dispersed clay volumes estimation technique has been developed for the model calibration. The model has been tested on a number of wells from different regions. The model has applications in geomechanics for the elastic moduli anisotropy analysis as well as for the quantitative seismic interpretation (QI seismic). Thomsen weak anisotropy parameters can be computed from the VTI elastic moduli. The model provides new tool for the analysis of lamination and for petrophysical analysis of low resistivity low contrast (LRLC) sands. The model allows to calibrate clay volume in cases when X-ray diffraction (XRD) data is not available. It allows to produce two independent estimates of the dispersed and laminated clay volumes using Thomas-Stieber and shear moduli-based methods. The latter has been put forward in the process of the model development. These dispersed and laminated clay volumes’ estimates enhance LRLC saturation evaluation. This paper provides novel dispersed and laminated clay rock-physics model. In addition, it proposes the model calibration technique based on six cross-plots and measured to modeled logs comparison. Additional validation of the model calibration correctness comes from two independent laminated and dispersed clay volumes estimates. The model provides new method of the total clay volume calibration. It opens new applications for the QI seismic inversion analysis in LRLC environments. All examples in this paper are based on the analysis of a gas bearing well located in offshore Australia. Thylacine-1 is the taken from public dataset exploration well (available in the Australian National Offshore Petroleum Information Management System https://nopims.dmp.wa.gov.au/Nopims/).