Workflow for Determining Layer Properties from Density and Neutron Logs in High-Angle and Horizontal Wells

Abstract

Abstract A multi-step, inversion-based workflow has been developed for analyzing logging-while-drilling density and neutron measurements in high-angle (HA) and horizontal (HZ) wells. The workflow produces accurate layer properties (i.e., bulk density, photoelectric factor (PEF) and neutron porosity) by taking account of bed thickness, borehole effects, and tool response to boundary crossings and adjacent bed effects. The workflow has been validated using both synthetic and field data. A layered earth model is determined as the final result of this workflow, and it can be used as input for subsequent petrophysical interpretations. The inversion relies on fast forward models for each of the nuclear measurements. These forward models are based on flux derived sensitivity function maps obtained from Monte Carlo modeling. An initial parametric model, including borehole geometry, mud properties, geometric structure, and layer properties, defines a layered earth model along the wellbore trajectory. The initial geometrical model consists of the bed boundary locations and their dip which are determined automatically from the measured compensated density image. Initial layer density, PEF, and apparent limestone porosity are automatically estimated from the respective measured logs. After the initial model setup, a flexible three-step iterative inversion determines 1) mud density and mud PEF, 2) sensor standoff from the borehole wall, 3) layer density, layer PEF, boundary location, and dip. The geometrical model determined from the density inversion is then fixed for the subsequent neutron inversion, which determines apparent limestone porosity for each layer. Gauss-Newton optimization with line search, adaptive regularization scheme, and parameter constraints is used to minimize the weighted L2-norm error between the measured and forward modeled logs. The workflow is validated using synthetic data sets that include both thick and thin-bedded formations and eccentric tool position in the borehole. The true geometrical structure, layer formation density, PEF, and neutron porosity can be recovered within the accuracy of the forward modeling even in the beds where the layer thickness is thinner that the measurements ability to fully respond to the layer property. The workflow has also been applied to field data sets. The inversion results show that it is possible to determine a common geometrical model for the density and neutron measurements, even though they have significantly different responses to the layering. Otherwise, it would not possible to manually derive accurate layer properties due to the asymmetric nature of the neutron measurements and the common practice of attempting to compare the non-azimuthal neutron measurement with the azimuthal density measurements. The workflow provides an accurate method for quantitative petrophysical interpretations. In addition, the results lead to a better understanding of density and neutron measurements in HA and HZ wells.