Hydrocarbon analysis during mud logging

This article discusses several techniques used for hydrocarbon analysis during mud logging. These tools characterize the reservoir fluids that have become entrained in the drilling fluid as it is returned to the surface.

Total gas analysis

The total gas analyzer (TGA), also referred to as the total hydrocarbon analyzer (THA), measures the total amount of gas, typically the total amount of combustible gas. The usual unit of TGA measurement is total methane equivalents (TME), which is essentially the BTU content of the gas extracted from the drilling fluid, expressed as that which would be obtained from an equivalent concentration of pure methane in air. The TGA, while giving an undifferentiated indication of gas entrained in the drilling fluid, has the advantage of operating in a continuous mode. Today, most TGAs use either a thermal conductivity detector (TCD) or a flame ionization detector (FID).

Component analysis

There are several analytical techniques used at the rig site to measure the molecular composition and component concentrations of the reservoir fluids entrained to the surface in the drilling fluid. These techniques are the gas chromatograph (GC), the mass spectrometer (MS), and the infrared spectrometer.

Gas chromatograph

The most widely used technique, and among the more accurate, is the GC (Fig. 1). This instrument separates components from a mixture by selectively adsorbing and desorbing each compound at different rates, either on the surface of a granular packing contained in a column made of small-diameter tubing or on the internal surface of the tubing itself. A small, usually submilliliter, volume of the mixture (i.e., the gas extracted from the drilling fluid) is injected into a carrier gas stream, which carries the sample into the GC column. Components start to separate, depending on their affinity for the active surface of the packing or the column tubing. At the end of the column, components elute, each with a unique retention time, and pass into a detector. The different detectors used to analyze the compounds eluting from the GC column include the FID, the TCD, the catalytic combustion detector (CCD), the MS detector, and the infrared (IR) absorption spectrometer.

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  Fig. 1 – Schematic of a single-column gas chromatograph with generic detector.

Flame ionization detector

The FID uses a flame of burning hydrogen to combust the eluting compound. As the compounds undergo combustion, ionized single-carbon intermediates are captured by the collector electrode. Very sensitive circuitry (the electrometer) measures the extremely small, microamp-level current generated by this flow of ions, which is proportional to the total number of carbon atoms in the combusting gas mixture. Fig. 2 shows the FID device schematically. The hydrocarbon species present are known because the operator, by design, knows which hydrocarbon component is eluting; therefore, concentration of that component may be calculated.

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  Fig. 2 – Flame ionization gas detector.

Thermal conductivity detector

The TCD (or hot wire detector) measures the thermal conductivity of the gas mixture passing over a filament. The typical TCD uses a Wheatstone Bridge circuit to measure the resistance of a platinum filament. The filament is heated to a constant temperature. As a gas passes over the filament, the temperature is reduced by an amount that is related to the thermal conductivity of the gas and the gas-flow rate. The resistance of the filament changes with temperature. At a constant flow rate through the sensing cell, gases of different composition and, hence, different thermal conductivity cool the filament by different amounts. In most TCD devices, there are two cells: a reference cell through which the pure carrier gas (in these cases, usually air) flows and a sample cell through which the gas mixture extracted from the drilling fluid flows. The filaments from the cells form two of the four arms of the Wheatstone Bridge (Fig. 3).

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  Fig. 3 – Thermal conductivity gas detector.

Catalytic combustion detector

The CCD measures the temperature change in a platinum filament when the gas mixture is combusted. Rather than using a flame, the CCD device has the filament imbedded in a catalytic bead, which catalyzes the combustion chemical reaction, allowing it to occur at temperatures lower than normal flames. Heat is released according to the "heat of combustion," and this released heat increases the filament temperature and, hence, its resistance. The resistance of the sample filament is compared with the resistance of a reference filament, in a fashion similar to the TCD, and provides an output that is a function of the composition of the gas mixture. This device provides an output signal that is more or less related to the BTU content of the gas mixture it is measuring.

Mass spectrometer detector

The MS detector measures the quantity of a particular compound by determining the amount of material with a specific molecular weight. (Fig. 4 shows the magnetic-sector MS.) This is done by ionizing a small quantity of material, during which multiple fragments of the molecule may be produced. Each fragment has a specific mass-to-charge ratio. Ionized fragments are separated in space or time, on the basis of their respective mass-to-charge ratio, and are then quantified by an ion detector. These devices have been used only in laboratory-based analyses until recently, when compact, robust MS detectors have been introduced for rig-site use. The MS has commonly been used to quantify components eluting from a GS (the GC-MS system) and has the advantage of being able to differentiate mixes of several components that may coelute, as long as the individual molecular species differ sufficiently in molecular weight. By principle of detection, this advantage allows simultaneous analysis of saturated and aromatic hydrocarbons, as well as the acid gases CO₂ and H₂S and inert gases such as nitrogen.

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  Fig. 4 – Magnetic sector mass spectrometer.

Recent advances in MS instrumentation and methods may allow its use at the rig site to measure directly the amounts of specific paraffinic and aromatic hydrocarbon compounds, CO₂, and H₂S without the need for chromatographically separating the components. This would replace the function of the GC for HC component analysis, along with in-mud hydrogen sulfide detectors. These specific methods are in relatively early stages of development and are not described here in detail.

Infrared absorption spectrometer

On input of IR energy to a molecule, its bonds will vibrate in any of a variety of modes, such as stretching, bending, rotating, and twisting. The mode of vibration will vary depending on the specific bond and the wavelength of the exciting radiation. IR radiation passes through a cell filled with the gas mixture. Portions of the incident IR spectrum are absorbed by the molecules in the gas, according to the characteristic absorption wavelengths of the specific molecules comprising the gas. The amount of IR radiation absorbed is a function of the path length and the concentration.

For a variety of physical reasons, the absorption spectrum is not manifested as a series of discrete wavelengths, but rather as a series of broader bands that, particularly in hydrocarbon systems, overlap to the point of being difficult to explicitly deconvolve. Because IR sensors also tend to have poor resolution compared with band. When comparing differences in principles of operation, relative accuracy over a wide dynamic range of gas concentrations and operational robustness under rig site, for most applications the FID is the preferred detector system for both the GC and the THA.

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See also

Mud logging

Cuttings analysis during mud logging

Formation evaluation during mud logging

PEH:Mud_Logging