Time-Lapse Pulsed Neutron Well Logging in Oil Sands for Monitoring Steam Chamber Development

Day 2 Tue, November 01, 2022

Aziz

2022

Abu Dhabi's onshore oil fields are generally mature and contain carbonate reservoirs with light oil and gas. Various well types have been employed, such as vertical, highly deviated, and horizontal wells with cased hole or openhole completions. Surveillance of wellbore and formation fluid attributes is essential for reservoir management decisions and hydrocarbon production enhancement. This paper discusses surveillance techniques to understand multi-phase flow characteristics and reservoir saturation. Formation saturation monitoring and production profiling have been performed actively using pulsed neutron logging (PNL) and array production logging (APL) techniques. Challenges in employing the APL method in Abu Dhabi onshore fields include (1) many wells are horizontal with barefoot completions, (2) horizontal sections extend typically more than 2000-ft long with irregular and undulating trajectories, and (3) wells contain asphaltene, debris, and other materials preventing optimal spinner flowmeter functionality. After reviewing each well condition, well environment-specific combinations of APL, nuclear production logging applications for holdups and water velocity calculations, and an advanced three-phase formation saturation analysis technique were determined. This approach overcame several challenges to deliver surveillance objectives. We demonstrate four case examples, delineating well-based production and formation saturation profiles in various conditions. Two nuclear measurements exhibiting different sensitivities to oil and gas were combined to compute three-phase formation saturation. When a horizontal openhole wellbore was severely under-gauged, the pulsed neutron-based holdup application was used to avoid an APL tool deployment that might result in tool damage and unsatisfactory data acquisition. Additionally, for wells with good wellbore conditions, pulsed neutron-based and APL-based holdup data sets were acquired, and analysis results were compared. A stationary water velocity calculation method when water cut was high was also adopted to identify downhole water sources, and in-situ water production profiles from APL and nuclear applications were compared. An effort to evaluate production profile and in-situ saturation effectively from highly deviated- and horizontal wellbores to improve hydrocarbon production is described. The delineation of production profiles and formation fluid distribution allowed operators to determine reservoir and production management strategies.

Delineation of Multi-Phase Wellbore and Formation Fluid Distribution in Abu Dhabi's Onshore Horizontal Wells

Day 2 Tue, November 01, 2022

Aziz

2022

Efficient Through-Casing Surveillance of Steam Chamber and Reservoir Oil Using a New Pulsed Neutron Technology and an Advanced Interpretation Algorithm

Middle East Oil, Gas and Geosciences Show

Kotov

Chace

et al. 2023

Steam flooding is an essential recovery process in developing heavy oil reservoirs. Operators typically drill and case observation wells to monitor the movement of the injected steam and changes in heavy oil and water saturations. This in-well surveillance is performed using pulsed neutron well logging techniques. Pulsed neutron well logging technology has been used for more than 60 years to determine formation fluid saturation behind casing. We introduce a next-generation slim multi-detector pulsed neutron well logging tool. The new pulsed neutron tool integrates an upgraded pulsed neutron generator, lanthanum bromide scintillation detectors, and an improved electronics system. A robust data analysis technique is another vital component of through-casing multiphase formation fluid quantification. A conventional method for analyzing three-phase saturation uses two pulsed neutron logs in sequence. We have adopted a simultaneous analysis approach that combines two pulsed neutron measurements simultaneously to evaluate the volumes of multiphase fluid components. We present a case study of oil sands produced by the steam-assisted gravity drainage (SAGD) method. We also show comparisons of data acquisition with the previous-generation and new pulsed neutron tools, operating time, and data quality. We acquired time- and energy-based gamma-ray spectra from multiple detectors to extract key pulsed neutron measurements such as ratios of inelastic and capture gamma rays and carbon/oxygen ratios. Time- and energy-spectra-based salinity-independent nuclear measurements were combined to compute three-phase formation fluid saturation. The new tool acquired data of the same quality at least three times faster than the legacy tool. The new tool that offers three improved features (higher pulsed neutron outputs, denser scintillation detectors, and high-speed digital electronics) combined with a new acquisition technology that records time- and energy-spectra-based pulsed neutron data sets simultaneously enables faster reservoir surveillance. Operators using thermal methods for heavy oil recovery must understand the current underground steam distribution. This affects steam injection optimization and determines subsequent reservoir management activities. A technique for delineating steam, heavy oil, and water through cased monitoring wells was improved by incorporating a new well logging tool, an innovative acquisition mode, and an advanced nuclear data analysis workflow.

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

Sahu¹,

Tyagi²,

Day 2 Tue, March 14, 2023

et al. 2023

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.