Dry Air Drilling
Dry Air Drilling Procedures and Operating Considerations
Making Connections
Making connections in air drilling is different from conventional mud drilling. High pressure inside the drillstring and kelly hose must be bled-off before breaking off a joint. Compressors are not turned off while making connections. The air stream is bypassed to the blooie line though the primary jet.
As a common practice, the pipe is kept on the slips until circulation is established. Increased standpipe pressure indicates that the cuttings have accumulated above the bit. As the pressure increases it eventually initiates cuttings movement, leading to decreased pressure readings at the standpipe. Therefore, it is always a good practice to wait until circulation is established at the blooie line before letting the drillstring off the slips.
Unloading Casing
The water used to displace the cement while setting casing needs to be unloaded from the hole before air drilling can resume. Compressors and boosters are used to deliver air to the standpipe so that it begins to aerate the water. Mist pumps are used to introduce a foaming agent into the air stream. Once the air circulation returns to surface, the standpipe pressure will start to decrease and eventually water will be completely replaced with air. The maximum depth for water unloading can be determined by dividing the maximum pressure capacity of the air system by the formation water hydrostatic gradient (0.433 psi/ft).
Water unloading can be a dangerous operation because of the high energy released at the blooie line during the process. Precautions should be taken to ensure that the high velocity water does not wash out the bank of the reserve pit.
After unloading the water, the hole should be dried before regular air drilling operations can resume. If the surface humidity is high, the normal air circulation will not be sufficient to dry the hole. The best technique is to drill a couple of feet at a time to generate cuttings that would absorb the moisture inside the hole. Typically two or three joints should be enough for a well to start dusting.
Water Influx During Drilling
When a permeable, water-bearing formation is encountered, water enters into the wellbore and breaks into droplets. The dry cuttings usually adsorb the water particles as they travel up the hole. If the water inflow rate is high, the flow tends to be in the “slug flow” regime, which can aggravate wellbore instability problems. The moistened cuttings then have the tendency to build a mud ring.
A mud ring will cause the drill string to stick in the hole. A water inflow leads to higher standpipe pressures. Even a small water influx has the potential to stick the drill string unless it is detected in time. Accumulation of the moistened cuttings around the drill string progressively decreases the clearance in the annulus, eventually blocking the entire annular space and forming mud rings as shown in Figure 1.
Drilling must be stopped whenever an increase in standpipe pressure is observed. If the well is not dusting as expected, this might be an indication of mud ring formation.
Squeeze cementing is the oldest technique for plugging the water-bearing zones. However, if the water is coming from the matrix rather than the fractures, then resin and catalyst systems are more effective. Water-based polymer solutions are also possible solutions in low permeability environments since they have lower initial viscosity.
There are also several water influx control techniques that use gas. Aluminum sulfate solution injection followed by gaseous ammonia generates a solid aluminum hydroxide precipitation around the wellbore.
This method is very quick, and does not require any waiting time for setting. The other technique is to inject gaseous silicon tetrafluoride, which reacts with formation water to generate a sealing blockage.
In each of these techniques, water zones must be located accurately for correct packer placement above the zone. The best remedial action to water inflow is to switch from dry air to mist or foam drilling. This at least helps the water to be lifted without slugging or forming mud rings.
The liquid carrying capacities of air/gas with and without foaming agents were investigated by Guo et al. (2008) assuming constant drag-coefficient. Tabatabaei et al. (2008) expanded Guo et al.’s model to include the effects of Reynolds number and droplet sphericity on the drag coefficient.
In air drilling, the cuttings that move with high velocity up the hole also can cause erosion in the drillstring. Without the presence of the lubricating drilling mud, abrasive wear takes place more rapidly in air drilling operations. All these factors make the drillstring more vulnerable to fatigue cracks.
Downhole Fires
Downhole fires can occur if either natural gas and air, or oil and air form the critical mixture that is concentrated within combustion limits. A concentration of five to fifteen percent natural gas is combustible at atmospheric conditions. The upper limit increases with increasing pressure as shown in Figure 1.
Mud rings can cause downhole fires by blocking the air flow. Even low hydrocarbon inflows below the mud rings can quickly lead to combustible mixture conditions. The friction between the drill string and mud rings/borehole can create the sparking needed to ignite the mixture. The typical consequence is a melted drill string which requires difficult fishing operations or even sidetracking.
Attention must be paid to keep drilling fluid concentrations below the limits of combustible mixture. The most common method is the application of mist drilling rather than dry air drilling whenever natural gas is encountered during air drilling.
Reverse Circulation Drilling
This new technique has the following advantages over regular air drilling practices:
- Less permeability damage to formation.
- Larger cuttings size that would allow petrophysical analysis on particles.
- Less wellbore erosion.
- Less air volume requirement.
- As a drawback cuttings might plug the bit more frequently.
