Gas Reservoirs
Types of Gas Reservoirs
1. Dry Gas Reservoirs
Contain primarily methane (CH₄) with little or no liquid hydrocarbons.
Produce only natural gas at surface conditions.
Typically have high recovery factors due to efficient gas expansion.
2. Wet Gas Reservoirs
Contain methane along with small amounts of heavier hydrocarbons (ethane, propane, butane).
At surface conditions, they produce gas with minor liquid condensate.
Require gas processing to separate valuable liquid components.
3. Gas Condensate Reservoirs
Contain gas with significant amounts of heavier hydrocarbons.
At high reservoir pressure, hydrocarbons remain in a gaseous state.
As pressure drops below the dew point, liquid condensate forms in the reservoir, potentially reducing gas production efficiency (retrograde condensation).
4. Associated Gas Reservoirs
Found in oil reservoirs as solution gas (dissolved in crude oil) or gas cap gas (free gas above the oil).
Produced alongside oil and requires careful management to optimize recovery.
5. Tight Gas Reservoirs
Contain natural gas in low-permeability formations (e.g., sandstones and shales).
Require hydraulic fracturing or advanced drilling techniques for economic production.
6. Shale Gas Reservoirs
Gas trapped in fine-grained shale formations with extremely low permeability.
Production relies on horizontal drilling and hydraulic fracturing.
Key Reservoir Characteristics
1. Porosity and Permeability
High porosity allows for greater gas storage capacity.
High permeability enables easy gas flow; low-permeability reservoirs require stimulation techniques.
2. Reservoir Pressure and Temperature
Governs gas expansion and production behavior.
High-pressure reservoirs have better deliverability, while low-pressure reservoirs may require compression for sustained production.
3. Fluid Behavior and Phase Changes
Gas condensate reservoirs experience retrograde condensation, leading to liquid dropout in the reservoir, which can reduce productivity.
Proper pressure management is necessary to maximize recovery.
Development and Production Strategies
1. Well Types
Vertical Wells: Used in high-permeability gas reservoirs for simple production.
Horizontal Wells: Increase contact with gas-bearing formations, improving production from tight and shale gas reservoirs.
Multilateral Wells: Optimize reservoir drainage in complex formations.
2. Production Techniques
Natural Depletion: Gas expands naturally due to pressure reduction.
Compression: Lowers wellhead pressure to sustain gas flow.
Hydraulic Fracturing: Enhances permeability in tight and shale gas formations.
Gas Cycling: Injects gas to maintain reservoir pressure and prevent condensate loss.
3. Enhanced Gas Recovery (EGR)
Techniques like CO₂ injection and N₂ injection can increase gas recovery in depleted reservoirs.