Mud Gas While Drilling (GWD) Detection and Analysis.

Mud Gas While Drilling (GWD) is an important parameter that is monitored during the drilling process in oil and gas fields. It refers to the detection of hydrocarbon gases (methane, ethane, etc.) and other non-hydrocarbon gases (carbon dioxide, hydrogen sulfide, nitrogen, and helium) in the drilling mud while drilling a well.

The presence of hydrocarbon gas while drilling can indicate the presence of a potential reservoir of oil or gas and reflect the reservoir fluid type, location, extent of the hydrocarbon-bearing formation, and petrophysical characteristics of the drilled rocks.

The main gas show categories during drilling are:

1. Liberated gas: gas mechanically liberated by the bit into the drilling fluid as the bit penetrates the formation (Drilled Formation Gas).
2. Produced gas: gas influx from the formation into the drilling fluid when formation pressure exceeds the opposing effective mud hydrostatic pressure (Connection gas, Pump off gas, and Trip gas).
3. Recycled gas: gas that has been pumped back down the hole and not removed from drilling mud at the surface therefore will appear a second time at the surface.
4. Contamination gas: gas artificially introduced in the drilling fluid system from a different source than rock formations (e.g. mud additives, adding oil to mud).

Importance of real-time analysis and interpretation of (GWD):

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• Reservoir Identification: GWD can indicate the presence of hydrocarbon reservoirs and potential total reserves by analyzing and interpreting the gas data, which can also define gas-oil contact and oil-water contact in a reservoir.

• Formation Evaluation and Reservoir Quality: By analyzing the gas composition and concentration and using new methods like GOR (gas oil ratio), biodegradation, and cross-plotting ratio, geologists can infer:

1. The lithology, porosity, permeability, fluid composition and variability in pay (Dry gas, wet gas, oil, condensate and water), and oil gravity.
2. Identification of barriers, baffles and non productive zone of the formation being drilled.
3. Well stratigraphic correlation and formation fingerprint.
4. This information can be used to enhance the petroleum exploration and production process, reducing uncertainties for hydrocarbon-bearing reservoirs, assessing the quality of the reservoir, and estimating its productivity.

• Well Control: GWD measurement is important for well control as it can help detect unexpected pressure changes or "kicks sign" in the wellbore. potentially causing blowouts or other hazards. By monitoring GWD in real-time, the drilling team can quickly respond to changes in the wellbore pressure and take appropriate actions to prevent well control issues.

• Safety: Gas detection while drilling is critical for safety in the oil and gas industry. It is essential to monitor and detect any gas leaks or potential gas hazards in the mud during the drilling process to prevent potential accidents (like harmful poisonous H2S gas) and ensure the safety of personnel and equipment.

• Drilling Optimization: GWD data can be used to optimize drilling operations, such as adjusting drilling parameters, indicating bit damage, or changing casing and completion design to maximize production and minimize costs by understanding the characteristics of the formation being drilled and the fluid type, it can also be used to make cost-effective decisions about wireline.

• Geosteering: GWD measurement is particularly important in geosteering drilling for a successful landing well at the top of the reservoir and more precise well placement, where it can be used to help steer the wellbore towards areas of higher gas concentration (sweet spot), knowing the reservoir boundary, and maximizing the production of oil or gas.

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Advanced gas analysis and measurement techniques used in GWD:

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1-Gas chromatography (GC): GC is a common analytical technique used to separate and analyze the components of a gas sample. GC is used to separate the different components of the gas in the drilling fluid, allowing for accurate measurement of individual components such as methane, ethane, propane, and butane.

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2-Mass spectrometry (MS): MS is a technique used to identify and quantify the individual components of a gas sample based on their molecular weight. MS can be used in combination with GC to provide more detailed information about the composition of the gas in the drilling fluid.

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3-Fourier transform infrared spectroscopy (FTIR): FTIR is a technique used to analyze the chemical composition of a gas sample by measuring the absorption of infrared light by the gas molecules. FTIR can be used to identify the presence of certain gases, such as carbon dioxide, hydrogen sulfide, and other trace gases.

4-Laser absorption spectroscopy (LAS): LAS is a technique used to measure the concentration of a specific gas in a sample by measuring the absorption of laser light by the gas molecules. LAS can be used to measure the concentration of methane, which is a key indicator of the presence of hydrocarbons in the reservoir being drilled.

Different types of gases can be encountered while drilling, depending on the type and depth of the well being drilled. Some common gases encountered during drilling operations include:

(A) Hydrocarbons Gases:

1-Natural gas: This is a mixture of hydrocarbons, primarily methane, that can be found in underground reservoirs.

2-Oil and condensate gases: These are hydrocarbon gases that are often found in association with oil reservoirs. Condensate gases are those that can be condensed into liquid form under specific temperature and pressure conditions.

(B) Non-hydrocarbons Gases:

3-Carbon dioxide (CO2): Carbon dioxide is a non-hydrocarbon gas that can be present in drilling operations. Its presence can indicate the presence of a carbonate reservoir or a depleted hydrocarbon reservoir.

4-Hydrogen sulfide (H2S): Hydrogen sulfide is a toxic and corrosive gas that can be present in drilling operations. Its presence can indicate the presence of a sour reservoir containing high levels of sulfur compounds.

5-Nitrogen (N2): Nitrogen can be present in drilling operations as a result of drilling fluid additives or from the reservoir itself.

6-Helium (He): Helium is a non-toxic, non-flammable gas that is occasionally found in natural gas reservoirs.

7-Oxygen (O2): Oxygen can be present in drilling operations as a result of air drilling or as a contaminant in drilling fluids. Its presence can be dangerous as it can react with hydrocarbons to cause a fire or explosion.

8-Argon (Ar): Argon can be present in drilling operations as a result of drilling fluid additives or from the reservoir itself.

The type and concentration of gases encountered during drilling operations can provide valuable information about the geology and potential of the reservoir being drilled. By analyzing the gases in real-time using advanced gas analysis techniques, drilling operators can make informed decisions to optimize drilling operations and maximize production.