Infill Drilling Definition / Meaning
Infill drilling is a reservoir management technique that involves drilling additional wells (infill wells) between existing producing wells in a developed oil or gas field. The primary goal is to increase the recovery factor by targeting bypassed or unswept hydrocarbons, improving sweep efficiency, and accelerating production. Infill drilling is typically applied after primary recovery and often during secondary recovery operations such as waterflooding. It is a key tool for improving recovery and extending the economic life of mature fields.
Overview of Infill Drilling
In a field developed with a pattern of wells (e.g., 40-acre spacing), each well drains a certain volume of reservoir rock. Reservoir heterogeneity, faults, permeability variations, and uneven fluid movement can leave significant oil or gas unswept between wells. Infill drilling reduces the well spacing (e.g., to 20 acres or less) to access these bypassed zones. The technique is most effective in reservoirs with poor vertical or areal sweep efficiency, such as those with high permeability contrasts or natural fractures.
Objectives of Infill Drilling
- Increase Recovery Factor: Access oil or gas not contacted by existing wells due to reservoir complexity.
- Improve Sweep Efficiency: Enhance areal and vertical displacement of oil by injected fluids (water or gas).
- Accelerate Production: Boost short-term oil rates and reduce the time to recover reserves.
- Mitigate Production Decline: Stabilize or reverse the natural decline of a mature field.
- Optimize Field Development: Better match well placement to reservoir heterogeneity.
When is Infill Drilling Applied?
Infill drilling is considered under these conditions:
- After primary recovery has peaked and production is declining.
- During a waterflood or gas injection program when injection fluids are bypassing oil.
- When reservoir simulation studies show significant unswept oil remaining.
- When well spacing was originally wide (e.g., 80 acres) and technological advancements allow tighter spacing cost-effectively.
- In naturally fractured reservoirs where matrix oil is poorly swept.
Technical Considerations
Successful infill drilling requires detailed reservoir characterization and modeling. Key factors include:
| Factor | Impact |
|---|---|
| Well Spacing | Optimal spacing depends on reservoir permeability, fluid mobility, and economic limits. Tighter spacing may cause well interference. |
| Reservoir Heterogeneity | Layered or fractured reservoirs benefit more from infill wells placed in bypassed zones. |
| Fluid Properties | High oil viscosity may require closer spacing to achieve efficient displacement. |
| Injection Patterns | Infill wells are often converted to injectors or producers to maintain pressure support. |
| Geomechanics | Drilling near existing wells must avoid pressure depletion or induced fractures. |
Economic Factors
Infill drilling projects are judged by incremental oil recovered versus the cost of drilling and completion. Typical economic drivers include oil price, drilling costs, decline rates of existing wells, and tax incentives. Infill wells often have higher initial production rates than original wells but may decline faster. A project is viable if the net present value of incremental production exceeds investment costs.
Usage Example
Consider a 40-acre waterflood pattern in a sandstone reservoir. After five years, the water cut reaches 90% and many wells are uneconomic. A reservoir simulation reveals bypassed oil in low-permeability layers. Management decides to drill four infill wells on 20-acre spacing. The infill wells initially produce at 100-150 barrels of oil per day with lower water cut, boosting field output by 30% and increasing estimated ultimate recovery by 8-12%. This demonstrates how infill drilling transforms a mature asset.
Conclusion
Infill drilling is a proven reservoir management strategy for improving hydrocarbon recovery from existing fields. It requires rigorous technical evaluation, economic analysis, and careful execution. When applied appropriately, it can significantly extend field life and enhance profitability, making it a cornerstone of improved recovery operations in the petroleum industry.