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Improved Detection of Bed Boundaries for Petrophysical Evaluation with Well Logs: Applications to Carbonate and Organic-Shale Formations

Petrophysical interpretation of well logs acquired in organic shales and carbonates is challenging because of the presence of thin beds and spatially complex lithology; conventional interpretation techniques often fail in such cases. Recently introduced methods for thin-bed interpretation enable corrections for shoulder-bed effects on well logs but remain sensitive to incorrectly picked bed boundaries. We introduce a new inversion-based method to detect bed boundaries and to estimate petrophysical and compositional properties of multi-layer formations from conventional well logs in the presence of thin beds, complex lithology/fluids, and kerogen. Bed boundaries and bed properties are updated in two serial inversion loops. Numerical simulation of well logs within both inversion loops explicitly takes into account differences in the volume of investigation of all well logs involved in the estimation, thereby enabling corrections for shoulder-bed effects. The successful application of the new interpretation method is documented with synthetic cases and field data acquired in thinly bedded carbonates and in the Haynesville shale-gas formation. Estimates of petrophysical/compositional properties obtained with the new interpretation method are compared to those obtained with (a) nonlinear inversion of well logs with inaccurate bed boundaries, (b) depth-by-depth inversion of well logs, and (c) core/X-Ray Diffraction (XRD) measurements. Results indicate that the new method improves the estimation of porosity of thin beds by more than 200% in the carbonate field example and by more than 40% in the shale-gas example, compared to depth-by-depth interpretation results obtained with commercial software. This improvement in the assessment of petrophysical/compositional properties reduces uncertainty in hydrocarbon reserves and aids in the selection of hydraulic fracture locations in organic shale. Introduction Petrophysical and compositional evaluation of organic-shale and carbonate formations remains an outstanding challenge in the petroleum industry. Common well-log interpretation problems arising in organic-shale and carbonate formations include presence of thin beds, extreme vertical and radial heterogeneity, and uncertainty in physical and pore-structure models. The interpretation method introduced in this paper improves conventional well-log analysis in organic-shale and carbonate formations by simultaneously correcting shoulder-bed effects and quantifying the nonlinear impact of complex lithology on well logs. Shoulder beds can significantly affect estimates of petrophysical and compositional properties in thinly bedded formations. These effects depend on factors such as bed thickness, contrast in physical properties of adjacent beds, vertical resolution of well logs included in the interpretation, and specific petrophysical and compositional properties. Experience shows that shoulder-bed effects can cause significant errors in estimates of porosity, mineral/fluid concentrations, and permeability in beds thinner than 2 ft with conventional depth-by-depth well-log interpretation. This error increases with decreasing bed thickness and increasing rock variability. Conventional techniques do not effectively correct shoulder-bed effects on well-log interpretation because of the lack of fast procedures for the numerical simulation of nuclear logs. New interpretation techniques were recently developed that