How Do The Differences In Hydraulic Fracturing Fluid Composition And Injection Rates Between Vertical And Horizontal Drilling Operations In Shale Gas Plays Affect The Mobility And Persistence Of Stray Gas In Shallow Aquifers, Particularly In Regions With Low-permeability, Coal-rich Sedimentary Rocks?

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The differences in hydraulic fracturing (fracking) operations between vertical and horizontal drilling significantly impact the mobility and persistence of stray gas in shallow aquifers, particularly in regions with low-permeability, coal-rich sedimentary rocks. Here's a structured explanation:

  1. Drilling Techniques and Fluid Usage:

    • Horizontal Drilling: This method is more efficient for accessing large shale areas. It often requires higher injection rates and larger volumes of fracturing fluid compared to vertical drilling. The fluid composition may include more additives to maintain fracture conductivity over longer lateral sections.
    • Vertical Drilling: Typically uses less fluid and lower injection rates, focusing on a smaller, vertical section of the shale formation.

  2. Impact on Stray Gas Mobility:

    • Fracturing Fluid Composition: Horizontal drilling may use more proppants and chemicals, which can interact with surrounding rock. These interactions might alter local geology, potentially creating or sealing pathways for gas migration.
    • Injection Rates: Higher rates in horizontal drilling can create more fractures, increasing the risk of gas escaping into shallow layers, especially if pre-existing fractures or coal cleats are present.

  3. Geological Considerations:

    • Low-Permeability Rocks: These rocks naturally hinder gas movement, but the presence of coal cleats or fractures can provide pathways. Fracking might enhance connectivity in these pathways, facilitating gas migration.
    • Coal-Rich Areas: Coal's natural fractures (cleats) can act as conduits for stray gas. Fracking fluids and pressure might alter coal structures, either creating new pathways or sealing existing ones.

  4. Chemical and Physical Interactions:

    • Chemical Composition: Additives in fracturing fluids can cause reactions with rock, such as clay swelling, potentially sealing micro-fractures. Conversely, reactions with coal might alter its structure, affecting gas movement.
    • Rock Stress Changes: Higher injection rates and fluid composition might induce stress changes, altering fracture openness and affecting gas pathways.

  5. Persistence of Stray Gas:

    • In low-permeability environments, gas may persist longer due to limited movement. However, existing pathways like coal cleats can allow gas to migrate and persist, despite the overall low permeability.

Conclusion: Horizontal drilling's use of higher injection rates and different fluid compositions can increase the risk of stray gas in shallow aquifers, particularly in coal-rich areas. The interaction between fracturing operations and local geology enhances gas mobility and persistence, highlighting the need for careful management of fracking operations in such regions.