Accurate Identification of Gas-Bearing Formation in a Mature Field Using Pulsed Neutron Logs Prevented Well Abandonment

Abstract

Operators typically update formation fluid saturation from producing wells as production impacts changes in formation fluid type and volume. An operator in India deployed a multi-detector pulsed neutron well logging tool on one of the old wells in a mature field to evaluate saturation profiles across multiple clastic formations. Production from the subject well had ceased due to water loading. The objective of the logging was to identify possible bypassed hydrocarbon zones before the operator decided on the well abandonment. A multi-detector pulsed neutron tool acquired a salinity-independent gas-sensitive time-based measurement from short-spaced and extra-long-spaced gamma-ray detectors. In addition, inelastic energy spectra-based carbon/oxygen (C/O) ratios were recorded to quantify formation oil saturation in a low water salinity environment. Another critical component in the saturation analysis workflow was the forward modeling of tool responses. We used the Monte Carlo N-particle (MCNP) stochastic method to predict gas-sensitive and C/O ratio responses in logging conditions. We had limited information on well conditions, such as cement bond condition and formation fluid properties, as no recent well logging was carried out to evaluate these. Thus, we performed saturation analyses in various conditions to reduce uncertainties in the results, including well-cemented, partially-cemented, and uncemented annulus conditions and different oil and gas densities. The analysis results identified one shallow sand unit containing gas. The sand was initially considered a water-dominant zone because the same zone produced water from adjacent wells. We evaluated the uncertainty in the gas saturation calculation attributed to cement bond quality and formation gas density. This helped to remove uncertainties in cement bond conditions and in-situ gas density on gas saturation. The identified sand unit was perforated and produced a large amount of gas. The accurate result of the gas saturation analysis saved the well from abandonment and increased reserves and production capacity. Additionally, the analysis revealed that water-filled formations were predominant in other sands. The C/O log analysis showed no bypassed oil in the lower sands. This paper further discusses case studies on candidate selection for pulsed neutron well logging, uncertainties in formation parameters, and the implications for saturation results.