Low Resistivity Low Contrast Saturation Evaluation Applying Anisotropic Rock Physics Model to Sonic Logs

Abstract

Abstract Low resistivity low contrast (LRLC) pays can contain significant additional reserves for the producing wells in decline. The most common tools for the LRLC formations analysis are triaxial resistivity and Micro-Imaging logs. However, such logs may not be available, in particular in the legacy wells. The paper describes a new method to analyze laminated reservoirs and to estimate saturation using Gamma Ray, Neutron, Density and Sonic logging suite. The method utilizes anisotropic rock physics model to analyze sand-clay lamination. The anisotropic rock physics model allows to subdivide total clay volume estimated from Gamma Ray and Neutron-Density into the laminated and the dispersed clay fractions using shear modulus. The dispersed clay behavior is described using Reuss, Upper and Lower Hashin-Shtrikman bounds. The laminated clay is accounted for through Backus averaging. When the model is calibrated, the fluid bulk modulus is extracted, and the saturation evaluation is done using Brie equation. The Brie exponent calibration method utilizing fluid bulk modulus versus total clay volume plot has been developed. The method has been tested on a number of wells including cases, where laminated reservoir presence was confirmed by alternative methods, for example by Image logs. It allows to evaluate saturation and to identify pay zones in majority of the test cases. The method limitations are heavy hydrocarbons presence, when oil bulk modulus is close to that of formation water, or the cases of heavy mud invasion on depth greater than one wavelength. The reason for the latter limitation is that the sonic logs’ depth of investigation is close to one wavelength. Thus, if invasion depth exceeds that, a sonic tool measures only residual hydrocarbons presence. With these two notorious exceptions, the method produces excellent results. The comparison of the dispersed and laminated clay volumes estimated by different methods allows to calibrate total clay content in the cases when X-ray diffraction (XRD) data is not available. The paper presents novel method for the saturation analysis in LRLC reservoirs, where just limited logging suite is available. The method utilizes anisotropic rock physics model and requires Gamma Ray, Neutron, Density, Compressional and Shear sonic logging suite.