CASED HOLE FORMATION DENSITY LOGGING WITH A COMPACT OPEN HOLE TOOL, AND IMPLICATIONS FOR GAS ZONE EVALUATION

摘要

We describe the derivation of formation density from a compact (2?-inch diameter) open-hole density logging tool operated in cased hole environments. It uses an established open hole transform equation in which the casing effect appears in the count rate domain as a modified formation transmission term, 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 hole quality, logistical 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 if cement is less than about 1 inch 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. For modest thicknesses, however, cased hole densityneutron 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.