Integrated Pulsed Neutron Logging and Nuclear Modelling for Formation Saturation and Gravel Pack Integrity Evaluation in Complex Sand Control Completions

Abstract

Managing both sand and water production remains a fundamental challenge in brownfield reservoir management. Wire Wrapped Screen (WWS) completions are widely adopted as part of the sand control strategies. However, the complex near-wellbore environment in such completions often masks true formation signals in behind-casing evaluation. This study aims to demonstrate an integrated approach using the Multi Detector Pulsed Neutron (MDPN) tool, pre-job Monte-Carlo N-Particle (MCNP) nuclear modelling and advanced interpretation workflow for both formation saturation and gravel pack integrity evaluation.

Over 10 wells across Asia were accessed, with two detailed case studies presented. MDPN Gas and Carbon/Oxygen (C/O) modes were deployed in cased well with double or single tubing across WWS Gravel Pack (GP) section. The MCNP nuclear modelling simulated tool responses for expected saturation envelope, incorporating effects from changes in fluid type from all tubing-screen-casing interfaces and known reservoir properties. This workflow leveraged the deeper depth of investigation (DOI) of Far and Long detectors against the shallow DOI from Proximal and Near detectors to separate the near-wellbore sand screen response from the true formation signals. In Gas mode, time-spectrum inelastic and capture ratio measurements, combined with detailed GP-specific MCNP models, identified whether the gas responses were originated from trapped annular gas below the unperforated blank pipe sections, from the formation, or from both. MCNP modeling attributes were adjusted to account for trapped gas effects on saturation envelopes.

Results from one case study indicated significant formation gas in the unperforated interest zone and later post-logging perforation confirmed gas flow. In another, C/O dual-saturation results using different detector ratio mixtures revealed probable water breakthroughs in existing perforation zones, aligned with observation of high water cut. Gas cap expansion could have occurred due to reservoir depletion. GP integrity evaluation was completed by adopting silicon activation analysis using additional bottom Gamma Ray (GR) sensor measurements, allowing determination of gravel top, and packing percentage. This enabled identification of localized GP deterioration within the WWS interval, providing critical input for operators on completion remediation decisions when required.

The integrated MDPN-MCNP approach proved effective in complex sand control completions, delivering accurate behind-casing saturation answers and reliable GP condition assessment without costly intervention. Detector ratio mixture flexibility from MDPN tool has gained user's confidence in distinguishing true reservoir signals even in challenging GP completions. This capability supports improved production optimization and long-term sustainability in mature assets.