Pulsed Neutron Interpretational Challenges for Heavy Oil Steam Flood Evaluation: Case Study from Indonesia

Abstract

Heavy oil, characterized by high viscosity and low API gravity, presents significant challenges for oil production from reservoirs. Injecting hot steam into the reservoir reduces the oil's viscosity, making it more producible. The performance of steam flood injection and the remaining oil saturation are closely monitored through surveillance wells using Five Detectors Pulsed Neutron Logging (FDPNL). Annually calculating the remaining oil saturation and cased hole porosity presents a challenge, as it is crucial to ensure that the interpretation results are neither overly optimistic nor excessively pessimistic.

The observation wells across the X region of Indonesia, which has an original oil in place of approximately 6 billion barrels (BBO), are in a shallow reservoir with a depth of around 300 – 700 feet. The main lithology of this reservoir is sandstone, offering good porosity and permeability. Steam flood surveillance wells are dedicated to monitoring various parameters on a yearly basis, including oil saturation, temperature and pressure changes, and steam distribution in the reservoir, using FDPNL tools with Carbon/Oxygen (C/O) and Sigma logging modes. The formation sigma curves from main and repeat passes are used qualitatively to identify steam zones in the interval. Quantitative analysis is performed using Bin data from selected detectors to calculate ratios, which are then used to compute cased hole porosity. The C/O ratio, determined by windowing method from three passes of logging using near and far detectors, is employed to estimate oil saturation. The integration of petrophysical interpretation from both open holes and cased hole data enables precise allocation of production intervals in this challenging environment.

Over 100 wells were analyzed to track changes in steam zones and current oil in place. The FDPNL results show an increase in reservoir temperature and sigma values, with low sigma indicating the presence of the steam zone, which is then used to calculate cased hole porosity. Steam typically develops in zones of good porosity, initially appearing at the top of the reservoir and progressively moving downward with active injection. The hot temperature of the steam generally reflects injectivity, which can persist for several years. However, the injection rate is often nonuniform and discontinuous, which can impact the results. If additional steam zones are suspected, based on sigma results and cased hole porosity calculations, it is recommended to perform C/O logging to accurately assess remaining oil saturation because more direct indication of hydrocarbon presence.

Pulse neutron logging has proven to be one of the most effective methods for monitoring steam flooding. To ensure consistent and more accurate results, it is recommended to use the same tools and methodology for each survey, avoiding inconsistency & unreliable result due to inconsistent baseline interval being used. Conducting annual observations allows for reliable data collection, providing valuable insights for reservoir management, which can ultimately lead to improved production and enhanced recovery.