Introduction

Natural gas analysis for hydrocarbons and hydrogen sulfide (H₂S) is required to avoid natural gas and oil escaping from the hole and is part of the ship's standard drilling safety plan.
The absolute quantity of hydrocarbons is the primary safety risk during shipboard operations. Gas monitoring via gas chromatography is a means of quantifying the hydrocarbon risk. H₂S is another significant risk factor for individuals working in the area. Emergency monitors on the drill floor provide early detection of H₂S, while later quantification is performed on the natural gas analyzer (NGA). A primary method of monitoring safety conditions is the concentration ratio of methane to ethane versus temperature (Figure 1).

Figure 1. Risk Assessment for Drilling Safety (IODP).

Hydrocarbon Generation

Hydrocarbon generation in sediments is a result of thermal decomposition (maturation) of biogenic organic matter. C₁–C₄ hydrocarbons may be generated in significant quantities in sediment via two processes:

Biogenic: biogenic hydrocarbons, typically characterized by methane, are produced in a sulfate-free environment via the reduction of dissolved bicarbonate.
Thermogenic: thermogenic hydrocarbons are produced in sediments in direct proportion to temperature. C₅ and other heavier hydrocarbons are almost always the result of thermal generation of hydrogen-rich organic matter at temperatures typically ~100°C or greater.

The evolution of sedimentary biogenic organic matter under increasing burial depth and consequent temperature rise is divided into three stages:

Diagenesis
- biological, physical, and chemical alteration of sedimentary organic matter that occurs at low temperature (<50°C) in relatively recently deposited sediments (Peters et al., 2005).
Catagenesis
- principal zone of oil formation, refers to a temperature range of 50°C~150°C. Liquid and gaseous hydrocarbons together with organic compounds with heteroatoms (oxygen, sulfur, and nitrogen) are released from the kerogen (Figure 2), so the catagenesis stage is called the "oil window."
Metagenesis
- Dry gases (mainly methane) are derived from liquid hydrocarbon accumulation in the crust (Figure 3). C₁–C₄ hydrocarbons may be generated in significant quantities in sediment via biogenic and thermogenic processes.

Figure 2. Hydrocarbon Formation Pathways in Geological Situations (Rullkotter, 1993).

Figure 3. Hydrocarbon Generation Resulting from Burial of Organic Matter during Geologic Time.

Hydrogen Sulfide

Sulfate-reducing bacteria produce hydrogen sulfide in euxinic sediments. This may occur in a relatively shallow part of the sediment. Thermochemical sulfate reduction of sulfate by hydrocarbons in reservoirs occurs under high temperature (>127°C ~ 140°C).

Instruments

The NGA systems are both based on an Agilent 7890 GCs. These systems were further customized with specialized gas injection inlets and various column, detector, and valving systems for gas monitoring

Gases

The GC requires that hydrogen and air are connected to the marked fittings on the back of the instrument. The type of makeup gas must be identified in the method file.

Air, compressed (Zero-Air +): >50 psi
Helium, compressed (99.9995% +): >50 psi
Hydrogen, compressed (99.9995% +): >50 psi

Method

Theory of method

The NGA gas chromatograph is equipped with 2 detectors:

Flame ionization detector (FID)
Thermal conductivity detector (TCD)

The TCD flow path travels through a 6 ft x 2.0 mm ID stainless steel (SS) column packed with Poropak T (50/80 mesh), a 3 ft x 2.0 mm ID SS column packed with molecular sieve 13x (60/80 mesh), and 6 ft x 2.0 mm ID SS column packed with 80/100 mesh HayeSep R (acid washed).
The FID flow path traverses a 60 m x 0.25 mm ID capillary column with 0.25 µm DB-1 film.

This instrument measures C₁–C₇ hydrocarbons as well as some additional compounds:

Methane (CH₄)
Ethene (C₂H₄)
Ethane (C₂H₆)
Propene (C₃H₆)
Popane (C₃H₈)
n-Butane (C₄H₁₀)
iso-Butane (CH₃-C₃H₇)
n-Pentane (C₅H₁₂)
iso-Pentane (CH₃-C₄H₉)
n-Hexane (C₆H₁₄)
iso-Hexane (CH₃-C₅H₁₁)
n-Heptane (C₇H₁₆)
iso-Heptane (CH₃-C₆H₁₃)
Nitrogen (N₂)
Oxygen (O₂)
Carbon dioxide (CO₂)

NGA Sample Flow Schematics

Standby Mode

He gas flow for standby mode (green lines).

Line 1: Aux-3—V1-4—V2-5—V2-3—capillary column—V2-4—V2-1—FID
Line 2: Aux-4—sample inlet—V1-2—V1-3—V1-6—V1-1—V3-3—V3-4—V3-1—V4-3—V4-2—V4-5—V4-4—Vent
Line 3: Front inlet—V3-5—V3-6—HaySep R column—V3-8—V3-7—V4-9—V4-8—TCD
Line 4: Back inlet—V4-6—V4-7—MolSieve column—V4-1—V4-10—Vent

Figure 9. NGA in Standby Mode.

Injection mode

He carrier gas (green line) and sample gas (red line) flows in the NGA in injection mode. Sample gas fills the sample loops connected to V1 (25 µL), V3 (1 cm³), and V4 (0.5 cm³). He flushes the separation columns.
He gas flow (green):

Line 1: Aux-3—V1-4—V1-5—V2-3—V2-2—capillary column—V2-4—V2-1—FID
Line 3: Front inlet—V3-5—V3-6—HaySep R column—V3-8—V3-7—V4-9—V4-8—TCD
Line 4: Back inlet—V4-6—V4-7—MolSieve column—V4-1—V4-10—Vent

Sample gas flow (purge; red):

Sample inlet—V1-2—V1-3—V1-6—V1-1—V3-3—V3-4—V3-1—V3-2—V4-3—V4-2—V4-5—V4-4—Vent

Figure 10. NGA in Injection Mode.

Run Mode at 0.01 min (open Valve V4)

He (green) and sample gas (red) flows in the NGA 0.01 min after start of run. Sample gas remains in the sample loop connected to V1 (25 µL) and V3 (1 cm³). After V4 opens, He returning from the back inlet pushes the sample gas out of the sample loop and into the molecular sieve column. Separated elements are detected by TCD.
He gas flow:

Line 1: Aux-3—V1-4—V1-5—V2-3—V2-2—capillary column—V2-4—V2-1—FID
Line 2: Aux-4—V1-2
Line 3: Front inlet—V3-5—V3-6—HayeSep R column—V3-8—V3-7—V4-9—V4-10—Vent
Line 4: Back inlet—V4-6—V4-5

Sample gas flow (purge):

V1-2—V1-3—V1-6—V1-1—V3-3—V3-4—V3-1—V3-2—V4-3—V4-4—out

Sample gas flow with He:

V4-5—V4-2—V4-1—MolSieve column—V4-7—V4-8—TCD

Figure 11. NGA in Run Mode: 0.01 min after starting run.

Run Mode at 0.07 min (open Valves V1 and V2)

He (green) and sample gas (red) flows in the NGA 0.07–1.79 min after start of run. Sample gas remains in the sample loop connected to V3 (1 cm³). After V1 and V2 open, He from Aux-3 pushes the sample gas out of the sample loop connected to V1 (25 µL) and into the capillary column (60 m) through V2, where it passes into the FID.
He gas flow:

Line 1: Aux-3—V1-4
Line 2: Aux-4—V1-2
Line 3: Front inlet—V3-5—V3-6—HaySep R column—V3-8—V3-7—V4-9—V4-10—vent
Line 4: Back inlet—V4-6—V4-5—V4-2—V4-1—MolSieve column—V4-7—V4-8

Sample gas flow (purge):

V3-4—V3-1—V3-2—V4-3—V4-4—out

Sample gas flow with He:

V4-8—TCD
V1-3—V1-6—V1-5—V2-3—V2-4—capillary column—V2-2—V2-1—FID
V1-1—V3-3

Figure 12. NGA in Run Mode: 0.07–1.79 min after starting run.

Run Mode at 1.80 min (open Valve V3)

He (green) and sample gas (red) flows in the NGA 1.80–1.82 min after start of run. After V3 opens, He from the front inlet pushes the sample gas out of the 1 cm³ sample loop into the HaySep column.
He gas flow:

Line 1: Aux-3—V1-4—V1-3—V1-6—V1-5—V2-3—V2-4
Line 2: Aux-4—V1-2—V1-1—V3-3—V3-2—V4-3—V4-4—out
Line 3: Front inlet—V3-5—V3-4
Line 4: Back inlet—V4-6—V4-5—V4-2—V4-1—MolSieve column—V4-7—V4-8—TCD

Sample gas flow with He:

Capillary column—V2-2—V2-1—FID
B3-4—V3-1—V3-8—HaySep R column—V3-6—V3-7

Figure 13. NGA in Run Mode: 1.80–1.82 min after starting run.

Run Mode at 1.83 min (close Valve V4)

He (green) and sample gas (red) flows in the NGA 1.83–8.49 min after start of run. After V4 closes, He from the back inlet flushes the molecular sieve column (backflush). Gas samples separated by the HaySep column enter the TCD through V4.
Helium gas flow:

Line 1: Aux-3—V1-4—V1-3—V1-6—V1-5—V2-3—V2-4—capillary column—V2-2—V2-1—FID
Line 2: Aux-4—V1-2—V1-1—V3-3—V3-2—V4-3—V4-2—V4-5—V4-4—out
Line 3: Front inlet—V3-5—V3-4—V3-1—V3-8

Sample gas flow with He:

HaySep R column—V3-6—V3-7—V4-9—V4-8—TCD

Backflush:

Line 4: Back inlet—V4-6—V4-7—MolSieve column—V4-1—V4-10—vent

Figure 14. NGA in Run Mode: 1.83–8.49 min after starting run.

Run Mode at 8.50 min (close Valve V3)

He gas (green) and sample gas (red) flows in the NGA 8.50–9.09 min after start of run. After V3 closes, He from the front inlet flushes the HaySep column and the line leading to the TCD (backflush).
He gas flow:

Line 1: Aux-3—V1-4—V1-3—V1-6—V1-5—V2-3—V2-4—capillary column—V2-2—V2-1—FID
Line 2: Aux-4—V1-2—V1-1—V3-3—V3-4—V3-1—V3-2—V4-3—V4-2—V4-5—V4-4—out
Line 3: Back inlet—V4-6—V4-7—MolSieve column—V4-1—V4-10—vent

Backflush:

Line 3: Front inlet—V3-5—V3-6—HaySep R column—V3-8—V3-7—V4-9—V4-8—TCD

Figure 15. NGA in Run Mode: 8.50–9.09 min after starting run.

Run Mode at 10.0 min (close Valves V1 and V2)

He (green) and sample gas (red) flows in the NGA 9.09–10.0 min after start of run. After V1 and V2 close, He flow returns to standby mode.
He gas flow:

Line 1: Aux-3—V1-4—V1-5—V2-3—V2-2—capillary column—V2-4—V2-1—FID
Line 2: Aux-4—V1-2—V1-3—V1-6—V1-1—V3-3—V3-4—V3-1—V3-2—V4-3—V4-2—V4-5—V4-4—out
Line 3: Front inlet—V3-5—V3-6—HaySep R column—V3-8—V3-7—V4-9—V4-8—TCD
Line 4: Back inlet—V4-6—V4-7—MolSieve column—V4-1—V4-10—vent

Figure 16. NGA in Run Mode: 9.09–10.0 min after starting run.

NGA Startup

The chromatography application ChemStation controls GC data acquisition and processing. It can be run either online or offline. Offline mode can be run without communication with the GCs, so it is useful for reintegrating or reprocessing chromatograms. Online mode requires communication with the GC.

Turn on the GC. WARNING: Before turning on the GC, make sure the gas lines are open.
The 6890 GC performs a comprehensive self-evaluation and shows real-time diagnostics on the screen. Warning, Fault, or Bad Main Board & Fatal Error messages require troubleshooting before moving to the next step (see Maintenance & Troubleshooting (HP6890GC)).
Click the Agilent Control Panel, then select NGA1 or NGA2, then Launch to start ChemStation. The Method and Run Control window opens. At startup, ChemStation uses the method last used (shown on the main screen). In addition, the GC LCD shows the loaded settings from ChemStation. Settings changed on the GC using the GC control panel are also made to ChemStation, and parameter changes entered into ChemStation are made to the GC. ChemStation will prompt to save changes.
To load a different method in Chemstation:
- Click Method > Load Method, select the method from the list, and press OK or
- Click the Method tab on the left side of the window and select a method to load
The system automatically loads the new method selected in ChemStation to the appropriate GC. Oven and detector temperatures may increase immediately after a new method is loaded, and the FID will ignite when the detector temperature reaches 150°C. Sometimes, the GC beeps because the FID flame is out, especially after a long idle period. See Maintenance & Troubleshooting (HP6890GC).
If the GC has been turned off for longer than a week, then bake the column for 8 hr with gas flowing (manually set the oven temperature to 175°C for GC3 or 275°C for NGA).

Instrument Operation

Before unknown samples can be analyzed for headspace gases, each GC system must have a valid calibration curve and the calibration curve must have been verified using a calibration verification standard.

Creating a Calibration Curve

1	Prepare 5–7 registered standard gases.
2	Activate NGA LIMS uploader located at Start > Program Files > IODP > MegaUploadaTron. The uploader must be activated before the calibration is run.
3	In the ChemStation Main menu, click Run Control > Sample Info.
4	Fill in the specific fields on the screen as follows: Operator name: LIMS user account (your last name) Sample name: name of the standard (e.g., STD_D) and the replicate number (STD_D-1, STD_D-2, etc.) Comment: text ID of the standard (scan the label) Click OK to close screen.
5	Slowly inject 5000 µL of the first standard gas and observe the floating ball in the flow meter move upward. Keep the outflow rate on the flow meter <80 mL/min.
6	When the ball in the flow meter indicates flow has fallen to just above 0 (is about to hit 0), press the Start button on the control panel of the GC.
7	When the run has finished, open the Data Analysis screen in ChemStation and click Calibration.
8	On the Main ChemStation menu, select Calibration > Recalibrate.
9	On the Recalibration screen, select Level # and Replace (or Average) as applicable for that level.
10	Repeat Steps 5–9 for 3 replicate standards (CH₄: A 25%, B = 50%, C 75%, D = 99%).
11	Click OK to change the calibration value. For NGA calibration, the same standard can be applied to both the appropriate TCD and FID level; you do not need separate standards for TCD and FID.

Running a Calibration Verification Standard

1	Ensure the uploader is activated and the CV standard is registered in LIMS.
2	Click Run Control in the main menu of ChemStation and select Sample Info.
3	Fill in the specific section on the window as follows: Operator name: LIMS user account (your last name) Sample name: common name for standard (e.g., STD_D-1) Comment: text ID of the standard (scan the label) Click OK to close the sample info screen.
4	Prepare the CV standard at approximately the mid-point concentration of the curve.
5	Slowly inject 5000 µL of the standard gas, keeping the outflow rate <80 mL/min.
6	Press Start on the GC control panel when the flow meter is just above 0.
7	When the run is finished, the report will automatically display the values. Click Upload in the uploader to submit the data to LIMS.

Running a Blank

1	To run a blank, in the Main menu click RunControl > Sample Info.
2	Fill in the following fields: Operator name: your last name Sample name: "BLANK" Comment: text ID of the blank (scan the label) Click OK to close window and save information.
3	Prepare laboratory air (5000 µL) and inject it into the GC in the same fashion as the standards above when the ChemStation software shows Ready.
4	Press the Start button on the GC control panel to start the run.
5	Confirm the chromatogram on the screen shows no peaks. If peaks are present, the system contamination must be found (injector, detector, sample loop, etc.).

Running a Gas Sample

1	Ensure the uploader has been activated.
2	Click Run Control in the main menu of ChemStation and select Sample Info.
3	Fill in the specific section on the window as follows: Operator name: LIMS user account (your last name) Sample name: Exp/site/hole/core/coretype/section/interval (e.g., 324-U1351A 5H4 32-35) Comment: text ID of sample (scan label) Click OK to close sample info screen.
4	Prepare a headspace or void gas sample.
5	Slowly inject 5000 µL of the gas sample, keeping the maximum gas outflow <80 mL/min.
6	Press Start on the GC control panel when the ball on the flow meter is just above 0.

Sample Preparation

There are two primary sample types used for natural gas analysis.

Headspace gas, which is obtained from core samples by heating a sample to ~70°C.
Void gas collected with a vacuum vial.

Occasionally, cores that come on deck have voids with large amounts of free gas. Free gas must be sampled using a sampling device that penetrates the liner and provides a channel for the gas to be drawn into a gas-tight syringe, vacuum vial, or gas sampling bag.

Headspace Gas

Collect samples from a freshly cut core section at a position within 0.5 inch of the inner side of the core liner (where sample has not been disturbed by contact with drilling fluid or core liner). In addition, the sample must be taken prior to the use of acetone or any other organic solvent in the catwalk area.
The curator authorizes the sampling plan before coring; therefore, the chemistry specialist must know the catwalk sampling plan before taking samples.

1	Locate a freshly sectioned core (consult with the curator).
2	Gently push the sample coring tool into the core section slightly inward of the edge.
3	Gently pull out the tool. If the sample recovery (% of coring tool with sample) is >80% (~5–7 cm³), proceed; otherwise repeat Steps 1 and 2.
4	Place the open end of the sample coring tool over a clean headspace gas vial and use the plunger to push the sediment into the vial.
5	Immediately place a gasket with a crimp top over the vial and crimp shut.
6	After sealing the vial, immediately write down the sampling interval, location, and any other information for the sample that was just taken. Generate a proper label and apply it to the vial as soon as possible.
7	Place the vial with the sample in a 70°C oven for 30 min to degas the sediment (use timer).

GCR Agilent 7890 Gas Chromatograph User Guide

Introduction

Hydrocarbon Generation

Hydrogen Sulfide

Instruments

Gases

Method

Theory of method

NGA Sample Flow Schematics

NGA Startup

Instrument Operation

Creating a Calibration Curve

Running a Calibration Verification Standard

Running a Blank

Running a Gas Sample

Sample Preparation

Headspace Gas