gas-holdup log, Oil & Gas Glossary

What Is a Gas Holdup Log?

A gas holdup log is a production logging measurement that quantifies the volume fraction of gas in a multiphase flow stream within a producing or injecting wellbore as a function of depth. Holdup, in production logging terminology, is the in-situ volumetric fraction of a particular phase at any given cross-section of the flow conduit, which differs from surface-measured producing water-cut or gas-oil ratio because the slower-moving, buoyant gas phase accumulates at the top of inclined or horizontal flow sections, creating a local gas concentration higher than the flowing mixture would suggest at the surface. Gas holdup is most commonly measured using nuclear density tools, capacitance-based sensors, or optical probes, with each technique suited to different flow regimes, mixture densities, and wellbore orientations. The log is an essential component of a full production logging suite used to allocate production among contributing zones, identify water or gas entry, and diagnose artificial lift problems.

Key Takeaways

Gas holdup (Hg) is the in-situ fractional cross-sectional area or volume occupied by gas at a given depth level, bounded between 0 (no gas) and 1.0 (all gas), and is distinct from the surface gas fraction or producing GOR.
Nuclear gradiomanometer tools measure the pressure gradient of the fluid column, which reflects average mixture density, allowing holdup calculation when gas and liquid densities are known, but are limited in accuracy in three-phase oil-water-gas mixtures.
Capacitance or dielectric holdup tools exploit the large permittivity contrast between gas and liquid phases to directly sense the gas fraction at a point in the flow stream, offering faster response and better resolution than density-gradient methods.
In deviated or horizontal wells, gas segregates to the high side of the borehole and gas holdup is not uniformly distributed across the flow cross-section, requiring multi-probe tools or spinner velocity corrections to accurately quantify flow rates.
Three-phase holdup interpretation requires at least two independent sensors, typically density and capacitance, to simultaneously solve for oil, water, and gas fractions, since two equations are needed for three unknowns given that the three fractions must sum to unity.

How Gas Holdup Logging Works

Gas holdup measurement in a wellbore exploits the large density contrast between gas (typically 0.05 to 0.3 g/cm3 at reservoir conditions) and liquid phases (0.7 to 1.1 g/cm3 for oil and brine). The nuclear gradiomanometer, the most widely used holdup measurement device in production logging, contains a Cs-137 or Am-241 gamma ray source and two vertically separated detectors. The gamma ray attenuation between the source and each detector depends on the density of the fluid between them, and the count rate ratio between the two detectors is a function of the vertical pressure gradient of the fluid column at that depth. Because pressure gradient equals mixture density times the gravitational constant (with a correction for flowrate effects), and because mixture density equals the sum of phase densities weighted by their holdups, the gradiomanometer signal can be converted to gas holdup if oil and water holdups are separately estimated or if single-phase liquid flow is confirmed by a flow regime identification tool.

Capacitance holdup sensors operate on a fundamentally different principle. The dielectric permittivity of gas is approximately 1.0, water is approximately 80, and crude oil is approximately 2 to 5. A capacitance probe measures the permittivity of the fluid mixture surrounding a sensing electrode, and the measured permittivity is related to the holdups of the three phases through a mixing law model. In two-phase gas-liquid flow, a single capacitance sensor can directly solve for gas holdup using the gas and liquid permittivities. In three-phase oil-water-gas flow, the capacitance reading is sensitive to all three phases, and a simultaneous measurement of mixture density from the gradiomanometer provides the second independent equation needed to solve the three-unknown holdup problem. Modern full production logging toolstrings combine a gradiomanometer, a capacitance sensor, and often an optical holdup probe that physically identifies phase contacts from refracted light intensity, providing redundant holdup measurements that improve accuracy and allow quality-checking of individual sensor responses against the others.

Gas Holdup Log Applications Across International Jurisdictions

In the Western Canada Sedimentary Basin, gas holdup logs are routinely run in solution-gas-drive oil wells to identify intervals producing disproportionate amounts of free gas, which indicates gas coning from an upper gas cap or local depletion below bubble point in that zone. Cardium and Viking waterflood producers commonly develop gas breakthrough through high-permeability streaks after pressure falls below bubble point in the swept zone, and production logs with gas holdup measurement identify which perforations are producing this gas so that the completions can be selectively plugged or the injection pattern redesigned. AER production monitoring requirements for commingled completions in Alberta specify minimum testing frequencies, and production logging with holdup measurement is accepted as a method of allocation testing for commingled producers where wellhead flow measurement cannot separate individual zone contributions.

In the Gulf of Mexico, subsea wells with complex completion architectures and multiple pay zones require production logging to verify that each perforation interval is contributing to production and that gas or water from one zone is not cross-flowing into another. BSEE regulations require operators to demonstrate zonal isolation and to allocate production for royalty payment purposes in commingled completions, and production logs with gas holdup data fulfill this regulatory need. Norwegian North Sea operators, including Equinor, run production logging suites in Brent Group and Statfjord Formation horizontal wells to quantify the distribution of gas cap gas along the wellbore and to optimize the timing of artificial gas lift injection in producers where reservoir pressure has declined. Saudi Aramco uses gas holdup logs in Arab Formation horizontal producers to monitor the advance of the gas-oil contact in Ghawar field, where managing gas cap expansion is critical to long-term oil recovery in the world's largest oil field.

Fast Facts

Schlumberger's gradiomanometer tool, the GR-series, has been the industry standard for fluid density logging since the 1960s. The tool's vertical resolution, approximately 25 to 30 cm between detector pairs, is the limiting factor for bed identification in commingled completions with thin producing intervals. Gas holdup as low as 3 to 5 percent can be detected with well-calibrated nuclear density tools in stable flow conditions. Capacitance tools, such as Halliburton's CPC (Capacitance Probe Combination) sensor, provide holdup resolution of approximately 1 to 2 percent gas fraction. In horizontal wells at reservoir conditions with typical gas-oil ratios of 100 to 500 scf/bbl, gas holdup at the wellbore may range from 0.1 to 0.5 even when surface GOR appears moderate, due to gas slippage and segregation. Full three-phase holdup interpretation accuracy is typically plus or minus 5 percent for each phase fraction in moderate to high flow regimes.

Three-Phase Holdup Interpretation in Complex Multiphase Flow

Three-phase holdup interpretation is the most challenging aspect of production logging in oil wells because three unknowns, gas holdup (Hg), oil holdup (Ho), and water holdup (Hw), must be determined with only the constraint that Hg + Ho + Hw = 1.0 plus whatever independent measurements the toolstring provides. With only one sensor, such as a density measurement, there is insufficient information to uniquely determine three-phase holdup, and an assumption about the oil-water holdup ratio must be made, typically that oil and water are fully mixed (emulsified) in a fixed ratio equal to the surface water-cut. This assumption is often wrong in production wells with stratified flow regimes where oil and water segregate to different portions of the wellbore cross-section. Combining a density (gradiomanometer) measurement with a capacitance measurement provides two independent equations that, with the sum-to-unity constraint, allow independent determination of two unknowns if one phase density is known. In practice, gas holdup is usually the primary target because it is most distinct in density and permittivity from the liquid phases, and the combined density-capacitance system can solve for gas holdup accurately even when oil and water proportions are uncertain.

Advanced production logging toolstrings for three-phase wells include optical probes in addition to density and capacitance sensors. An optical probe uses a light-emitting diode and photodetector at the probe tip: when the tip is in liquid, total internal reflection occurs and detected light intensity is high; when a gas bubble strikes the tip, the light escapes and intensity drops. This provides a direct, phase-specific signal for gas bubble detection that is independent of the electrical and nuclear measurements, enabling cross-validation and improving accuracy in slug or churn flow regimes where the gradiomanometer signal fluctuates rapidly and time-averaging introduces error. The combination of nuclear density, capacitance, and optical probe measurements represents the current industry standard for three-phase holdup measurement in complex producing wells, particularly in inclined wells where gravitational segregation creates severe radial holdup variation that multi-probe arrays across the pipe diameter are needed to adequately sample.

Tip: When designing a gas holdup production logging program in a well with a high gas-oil ratio or known gas breakthrough, always specify the logging speed carefully relative to the expected flow regime. In slug or churn flow, where large gas slugs and liquid slugs alternate at frequencies of 0.5 to 5 Hz, a gradiomanometer tool logged at 6 meters per minute will average the density signal over a depth interval of several centimeters per reading, which blurs the distinction between gas-producing and liquid-producing intervals. Log at the slowest feasible speed, typically 3 to 4 meters per minute in zones of interest, and request that the logging contractor output high-frequency raw data rather than depth-averaged recordings if the tool has that capability. In horizontal or highly deviated wells, a single axially positioned holdup probe will systematically underestimate gas holdup if gas is segregated to the top of the pipe; request a multi-arm or articulated probe array that samples both high-side and low-side positions simultaneously. Always run a static pass before the flowing survey to calibrate the density and capacitance sensors in the wellbore environment, and ensure that the tool is temperature- and pressure-equilibrated before beginning the flowing pass to avoid artifact signals from sensor warm-up drift.

Gas holdup log is also referenced as:

Holdup measurement — generic term used in multiphase flow engineering to describe the in-situ phase fraction measurement for any phase, not exclusively gas; in production logging, holdup measurement usually refers to the full suite of density and capacitance sensors.
Fluid density log — older field terminology referring specifically to the nuclear gradiomanometer component of the production logging suite, from which holdup is derived rather than directly measured.
Phase fraction log — engineering term used in production allocation reports and regulatory submissions to describe the depth profile of individual phase holdups, emphasizing the quantitative phase-fraction output rather than the measurement methodology.

Gas Holdup Log: Definition, Production Logging Gas Fraction Measurement, and Two-Phase and Three-Phase Holdup