How is gas from tight sands produced?
Natural gas from tight reservoirs is produced from wells similar to those producing from conventional reservoirs - the well is drilled, cased, and perforated, and the pressure drop between the reservoir and the wellbore causes the natural gas to migrate through the reservoir to the well and then to the surface.
The major differences between tight and conventional production arise because of the poor permeability of tight reservoirs. In a tight reservoir, the natural gas cannot flow as quickly to the well or in sufficient volumes to be economical. In addition, the area that a well drains in a tight reservoir is much smaller than the drainage area for a conventional reservoir. Consequently, different production schemes have been developed to economically produce tight gas.
- Downspacing: A spacing unit is defined as the area which one well can effectively drain. For conventional gas wells in Western Canada, this is normally one section of land, or 640 acres. Studies in the United States have shown that in tight reservoirs, wells spaced as closely as 30 per section may be needed to drain a reservoir effectively, and perform as well as more conventionally spaced wells. In Canada, up to eight wells per section have been drilled, giving a drainage area of 80 acres per well.
- Directional Drilling: Horizontal wells are used to expose more of the formation to the well bore and to intersect natural fracture systems more efficiently. In addition, directional techniques allow for the drilling of multiple wells from one drilling pad, mitigating the environmental impact of downspacing.
- Measurement while drilling: Downhole sensors near the bit, which gather and relay information to the surface, are used to help steer the bit through portions of the reservoir with the greatest potential.
- Underbalanced drilling: In underbalanced drilling, conventional drilling mud is replaced with fluids such as foams and inert gasses. The hydrostatic pressure exerted by these fluids is less than the reservoir pressure, inhibiting fluid invasion and the consequent formation damage prevalent with tight reservoirs.
- Advanced fracturing techniques: Advances in fracturing technology provide not only more job-specific frac fluids and proppants, but also allow for multiple fracture jobs and more accurate proppant placement.
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