Tight sand gas refers to natural gas found within tight gas sands, which are specific types of underground reservoirs characterized by very low permeability.
Understanding Tight Gas Sands
Based on the provided reference, tight gas sands are defined as:
- Low-permeability (tight) gas-bearing reservoirs. This means they contain natural gas but the rock allows fluids (like gas) to flow through it very slowly.
- They have an in situ matrix permeability to gas of less than 0.1 md. Permeability is a measure of how easily a fluid can flow through a porous material. A permeability of less than 0.1 millidarcy (md) is considered extremely low, making it difficult to extract the gas.
- This permeability criterion is exclusive of natural fracture permeability. Natural fractures within the rock can sometimes provide pathways for gas flow, but the definition of a tight sand reservoir is based on the permeability of the rock matrix itself, ignoring the contribution of these natural fractures.
Think of permeability like the interconnectedness of pores in a sponge. In a conventional reservoir, the pores are large and well-connected, allowing water (or gas) to flow easily. In tight sands, the pores are small, or the connections between them are very narrow, making flow difficult without intervention.
Key Characteristics
Here are the core defining features of tight gas sands, from which tight sand gas is produced:
- Low Permeability: The defining characteristic is the extremely low permeability of the rock matrix (less than 0.1 md).
- Gas-Bearing: These reservoirs contain significant amounts of natural gas trapped within the pore spaces.
- Requires Advanced Techniques: Due to the low permeability, conventional methods often aren't sufficient for economic gas extraction. Specialized techniques, like hydraulic fracturing and horizontal drilling, are typically needed to create pathways for the gas to flow to the wellbore.
In essence, tight sand gas is natural gas that is tightly held within these low-permeability sandstone formations.
