IMPACT OF CEMENT QUALITY ON CARBON/OXYGEN AND ELEMENTAL ANALYSIS FROM CASED-HOLE PULSED-NEUTRON LOGGING AND POTENTIAL IMPROVEMENT USING AZIMUTHAL CEMENT BOND LOGS

摘要

New-generation pulsed-neutron logging (PNL) tools can now measure thermal neutron capture cross-section (Sigma), carbon-oxygen ratio (C/O), and mineral composition of the formation through casing and cement. Environmental corrections, including casing and cement corrections, are required for proper interpretation. Such corrections are currently done through empirical manual shifts, without considering the actual cement bond quality or possible voids and channels in the cement, and the proximity of those voids to the position of the logging tool. Information from azimuthal cement bond logs can be potentially used as quantitative inputs for C/O saturation calculation and mineral composition analysis, such that voids in the cement are accounted for, and with assumptions on what fluids might be filling the cement voids. Here we use Monte Carlo nuclear modeling technique to quantitatively assess the impact of cement quality on C/O and elemental analysis. A generic two-detector PNL tool model is used in the simulation, along with realistic formation and borehole properties to reflect a horizontal cased-well. Several scenarios are simulated with a variation of casing centralization, location of a channel in cement, and type of fluid filling the channel. Neutron-induced gamma rays are tagged by the location and element of their generation and counted at two PNL detectors as a convenient measure of C/O and calcium yields. We demonstrate that a channel filled with oil-based mud could have considerable effect on C/O saturation calculation, whereas a channel filled with water-based mud has negligible effect. Calcium in cement can have considerable contribution to the total calcium signal. Any channel or void in cement will affect the calcium correction for elemental analysis. These significant impacts of the cased hole environment can be more efficiently accounted for by integrating azimuthal cement bond logs as a quantitative input to the C/O and elemental analysis of PNL, but advances in workflows and environmental corrections will be required to make this practical.