Air Compressor

Key Takeaways

• Minimum annular velocity for adequate cuttings transport in air drilling is the primary design constraint that determines required compressor flow rate, typically 900 to 1,500 m/min (3,000 to 5,000 ft/min) for dry air systems: Cuttings are transported to surface by the drag force of the upward-flowing air stream, and the minimum transport velocity depends on cutting size, shape, and density, as well as air density and viscosity. For typical drill cuttings (SG 2.5 to 2.7, 3 to 12 mm diameter) in a 215 mm (8.5 inch) hole with 127 mm (5-inch) drillpipe, the air velocity must exceed approximately 900 m/min at the lowest-velocity point in the annulus (typically at the large-diameter section near surface where air density is lowest after expansion). This translates to an air volume at surface conditions of 1,800 to 3,500 scfm for typical WCSB Foothills well configurations, requiring 3 to 6 rotary screw compressor units (each 600 to 800 scfm) or 2 to 3 reciprocating units (each 1,000 to 1,500 scfm) operating in parallel. If any compressor unit fails during drilling, the resulting velocity drop below the minimum transport threshold can cause instantaneous cuttings bridging and stuck pipe, so redundancy of at least one compressor unit above minimum requirement is standard practice.
• Reciprocating (piston) compressors are required for high-pressure air drilling applications above 500 psi (3,450 kPa) and are the preferred technology for Foothills Alberta wells encountering formation water that creates significant hydrostatic backpressure: Reciprocating compressors use one or more pistons driven by a crankshaft to compress air in discrete strokes. Two-stage reciprocating units with an intercooler between stages can deliver discharge pressures of 300 to 1,000 psi with volumetric efficiencies of 75 to 90%. Three-stage units extend to 1,500 to 2,000 psi. In the Alberta Foothills where Devonian and Carboniferous formations are deeply buried and naturally fractured, formation water influx into the wellbore during air drilling creates a liquid head that the compressor must overcome in addition to the drillpipe friction pressure. A 300 m column of formation water (SG 1.02) in the annulus represents approximately 3,000 kPa of backpressure, bringing total compressor discharge pressure requirements to 4,500 to 7,000 kPa, well beyond the capability of rotary screw units and requiring multi-stage reciprocating compressors with rated discharge pressures of 700 to 1,000 psi.
• Rotary screw compressors offer higher flow rates at lower capital and operating costs than reciprocating units for shallow-to-intermediate air drilling in dry formations, and are the workhorse compressor for WCSB Precambrian basement and Upper Cretaceous coal bed methane (CBM) drilling: A rotary screw compressor uses two intermeshing helical rotors to compress air continuously without discrete strokes, producing smooth, pulsation-free flow. Typical oilfield units deliver 600 to 1,200 scfm at 100 to 250 psi discharge pressure. They are smaller, lighter, and mechanically simpler than equivalent reciprocating units, with lower maintenance requirements (no piston rings, connecting rods, or valve assemblies). In northeastern British Columbia and northwest Alberta, rotary screw air compressors are standard equipment for drilling shallow CBM (coal bed methane) wells through the Mist Mountain and Gates formations at depths of 500 to 1,500 m, where formation pressures are below atmospheric (underbalanced conditions prevail naturally) and dry air drilling achieves ROP of 20 to 60 m/hr compared to 5 to 15 m/hr with water-based mud. Four to eight units in parallel (2,400 to 6,000 scfm total) constitute a typical CBM air drilling package in this region.
• Aftercoolers and moisture separators are essential accessory components that must remove condensed water from compressed air before injection into the drillstring to prevent downhole corrosion, tool damage, and formation damage from water slugs: Atmospheric air contains water vapour at a partial pressure determined by ambient relative humidity. When air is compressed, the dew point rises and water condenses on cooling. Without aftercooling and moisture removal, condensed water carries into the drillstring and accumulates at low spots in deviated wellbores, creating liquid slugs that cause intermittent flooding of the annulus, sudden changes in hole-cleaning mode, and bit wash-out from water jet impact. In steel drillpipe, unconditioned compressed air at relative humidity above 60% causes corrosion rates of 2 to 8 mm/year at 150 to 250 psi, sufficient to weaken tool joints within a single well programme. Aftercooler-separator combinations installed downstream of the compressor reduce air temperature to within 10 to 15 degrees Celsius of ambient and remove greater than 95% of free water, bringing delivered air moisture content below the threshold for condensation in the drillstring at formation temperature.
• Air compressor packages for drilling must comply with ASME Boiler and Pressure Vessel Code Section VIII and API RP 92L for pressure containment and well control compatibility, and require dedicated safety systems including relief valves, high-temperature shutdowns, and blowback prevention: ASME Section VIII governs the design and certification of pressure vessels including compressor coolers, intercoolers, and surge tanks at working pressures above 103 kPa. API RP 92L (Underbalanced and Managed Pressure Drilling Equipment and Operations) specifies the well control equipment configuration required when air compressors are connected to the wellbore, including check valves to prevent wellbore gas from flowing back through the compressor discharge line, pressure relief systems, and isolation valve requirements. On WCSB air drilling programmes, AER Directive 036 (Underbalanced Drilling) requires a documented equipment inspection and pressure test of all compressor piping, manifolds, and connections to the rotating head at the start of each air drilling programme, with records retained in the AER Directive 59 well file submission.