Unit
- Origin of natural gas: Natural gas forms from decomposed organic matter buried under sedimentary layers over millions of years. Thermogenic gas results from high temperature and pressure acting on kerogen, while biogenic gas forms via microbial activity at shallow depths.
- Reservoir types: Gas reservoirs are classified as dry gas (no condensate), wet gas (some condensate present), and gas condensate (significant liquid hydrocarbons formed at surface conditions). Each type requires different processing strategies.
- Reservoir pressure and temperature: Initial reservoir conditions determine gas density, phase behavior, and flow characteristics. High pressure reservoirs produce gas at higher rates but require careful pressure regulation to prevent sand production and equipment damage.
- Permeability and porosity: Porosity measures the void space within reservoir rock where gas accumulates, while permeability reflects the ability of gas to flow through the rock matrix. Tight gas reservoirs have low permeability and often require hydraulic fracturing.
- Gas accumulation mechanisms: Structural traps formed by folding and faulting, stratigraphic traps caused by changes in rock type, and combination traps capture gas in subsurface formations. Understanding trap geometry aids reserve estimation.
- Non-hydrocarbon components: Reservoirs often contain associated water, hydrogen sulfide, carbon dioxide, nitrogen, and helium. These contaminants influence processing requirements and must be quantified during field appraisal.
- Gas composition variability: Even within the same field, gas composition can vary across different wells due to reservoir heterogeneity, gravitational segregation, and production induced changes. Sampling programs must capture this variability.
- PVT behavior fundamentals: Pressure-volume-temperature relationships govern gas expansion, condensation, and phase separation during production and processing. Phase envelopes for gas mixtures predict liquid drop-out conditions.
- Gas hydrates and flow assurance: Under high pressure and low temperature conditions, water molecules form ice-like cages that trap gas molecules, creating solid hydrate deposits. These crystalline structures can rapidly accumulate and completely block pipelines, valves, and processing equipment, posing severe operational and safety risks. Inhibition strategies include injecting thermodynamic inhibitors like methanol or glycol, or applying kinetic inhibitors to manage subsea flow assurance.
- Reservoir drive mechanisms: Most gas reservoirs primarily operate under depletion drive, where the natural expansion of compressed gas provides the energy to push hydrocarbons toward the producing wellbores. Water drive from adjacent aquifers and gravity drainage at steeply dipping structures also contribute to recovery. Understanding the dominant drive mechanism is crucial for forecasting recoverable reserves and designing optimal field depletion plans.
- Gas metering principles: Accurate measurement of gas flow rates and cumulative volumes is essential for custody transfer, fiscal allocation, and production accounting. Technologies such as orifice meters, ultrasonic meters, and Coriolis mass flow meters are deployed depending on flow rate, pressure, and required accuracy. Each technology demands proper installation, flow
Quick Revision
- Review the key concepts covered in Natural Gas Origins and Reservoir Characteristics.
- Apply these principles in practical working contexts.
- Connect this material to the wider module objectives.