A Novel Cased Hole Density-Neutron Log - Characteristics and Interpretation

Abstract

Abstract We have characterized the response of a compact 2¼-inch (57mm) diameter open-hole formation density logging tool for cased hole environments. Data are processed with an established open hole transform in which the casing effect appears as a simple attenuation term in the count rate domain, and variations in cement thickness are compensated using a classical dual-detector spine-and-ribs approach applied in the density domain. The combination of through-casing density and casing-corrected neutron porosity has been applied to the evaluation of by-passed pay and shallow gas, and to the evaluation of wells where open hole acquisition has not been feasible for operational, hole quality, or economic reasons. The tool-specific neutron porosity excavation effect has been characterized for gas-bearing sands. Case history and model results suggest accurate formation densities are achievable for casing thicknesses up to about 0.35 inches (9mm) if cement is less than about 1 inch (25mm) thick, albeit with loss of precision relative to open hole. Formation sensitivity declines with increased cement thickness until practically all is lost for casing standoffs in excess of 1.5 inches (38mm). For modest thicknesses, however, cased hole density-neutron gas evaluation has advantages relative to the neutron-dipole sonic method; in particular it does not rely on good cement bond, it has generally superior spatial resolution, and (optionally) data can be acquired in memory mode on slickline in operating environments that do not favour conventional wireline units. Introduction The present work was stimulated by the search for shallow gas, and has since found broader application in the evaluation of by-passed pay (including light oil), in the monitoring of fluid contacts and saturation changes over time, and in the general evaluation of intervals that, for a variety of reasons, may not have been logged open hole. Gas behind casing can be inferred from pulsed neutron tool count rate logs, and the 111/16 inch (43mm) diameter variants may be the sole evaluation option in wells with small diameter tubing restrictions. A common alternative in other cases (and where pulsed neutron tools are precluded by other operational or cost issues) is a dual porosity approach with neutron and sonic curves scaled empirically to overlay in clean waterbearing zones. Sonic porosity measurements have the advantage of being insensitive to hole enlargement behind casing, but quality control of waveform processing is timeconsuming, and the velocity-porosity transform may be uncertain. In poorly bonded casing the formation arrival may be lost altogether. A more technically challenging third option is the density-neutron combination. This is feasible where casing thickness and standoff are modest, but is demanding because of the need to deal with variable casing standoff. Density tools respond to casing as though it were heavy mudcake, except measured density values are typically high and beyond the range for which tools are normally characterized. When this is compounded with unknown casing standoff, standard processing yields inappropriate values of delta-rho and inaccurate compensated densities. One approach to cased hole density correction is to normalize the compensated density log in zones of known porosity, or align to neutron porosity logs in clean waterbearing intervals (Cigni and Magrassi, 1987). This can be adequate for delineating zones of interest, but is unlikely to furnish good accuracy over a broad range of density values, and provides little or no information to allow variations in casing standoff to be flagged or accounted for. A better approach is to correct individual apparent density measurements (Near and Far curves in the case of a dual detector device), but it is important to recognise that measured and actual densities may not be linearly related outside the range of densities for which the tool is normally calibrated.