Improved estimation of mineral and fluid volumetric concentrations from well logs in thinly bedded and invaded formations

Abstract

Calculation of mineral and fluid volumetric concentrations from well logs is one of the most important outcomes of formation evaluation. Conventional estimation methods assume linear or quasi-linear relationships between volumetric concentrations of solid/fluid constituents and well logs. Experience shows, however, that the relationship between neutron porosity logs and mineral concentrations is generally nonlinear. More importantly, linear estimation methods do not explicitly account for shoulder-bed and/or invasion effects on well logs, nor do they account for differences in the volume of investigation of the measurements involved in the estimation. The latter deficiencies of linear estimation methods can cause appreciable errors in the calculation of porosity and hydrocarbon pore volume. We investigated three nonlinear inversion methods for assessment of volumetric concentrations of mineral and fluid constituents of rocks from multiple well logs. All three of these methods accounted for the general nonlinear relationship between well logs, mineral concentrations, and fluid saturations. The first method accounted for the combined effects of invasion and shoulder beds on well logs. The second method also accounted for shoulder-bed effects but was intended for cases where mudfiltrate invasion is negligible or radially deep. Finally, the third method was designed specifically for analysis of thick beds where mud-filtrate invasion is either negligible or radially deep. Numerical synthetic examples of application indicated that nonlinear inversion of multiple well logs is a reliable method to quantify complex mineral and fluid compositions in the presence of thin beds and invasion. Comparison of results against those obtained with conventional multimineral estimation methods confirmed the advantage of nonlinear inversion of multiple well logs in quantifying thinly bedded invaded formations with variable and complex lithology, such as those often encountered in carbonate formations.

Estimation of reservoir properties from well logs and core plugs to reduce uncertainty in formation evaluation: a case study from the Kohat-Potwar Geologic Province

“…The porosity, volume of shale, water saturation and hydrocarbon saturation are the main results obtained from the quantitative interpretation of geophysical well logs such as resistivity, gamma ray (GR), acoustic, density and neutron. Although several improved approaches along with optimized sets of equations exist to estimate these measurements (Fylling, 2002;Heidari et al, 2010), however, some uncertainty in the inferred parameters is always present because small error in any measurement may produce uncertainty in the final output (Fylling, 2002;Viberti, 2010). Theoretical models used to determine petrophysical properties may also produce the uncertainty in the final values.…”

Geophysics Dissertation Abstracts

“…The first step after pre-processing of the data is to apply a depth-by-depth nonlinear inversion of well logs to estimate porosity, water saturation, volumetric concentration of shale, and volumetric concentrations of mineral constituents (Heidari et al, 2012). We then use Leverett's RQI method to classify hydraulic rock types based on core measurements.…”

Comparative Study for the Interpretation of Mineral Concentrations, Total Porosity, and TOC in Hydrocarbon-Bearing Shale from Conventional Well Logs

“…In our proposed workflow, we first examine mineral compositions based on core XRD analysis and carefully select a mineral model to effectively reduce the number of unknowns. Secondly, we describe the approaches taken to optimize the use of the joint nonlinear inversion developed by the University of Texas at Austin (Heidari et al, 2012).…”