Applications for dry-gas drilling are in hard formations where water or oil flows are not likely to be encountered and areas where drill water is scarce. Dry-gas drilling (also called "dusting") uses compressed air or natural gas to cool and to lubricate the bit, to remove the cuttings from around the bit and to carry them to the surface. Dry gas is injected down the drill pipe while drilling and the cuttings are returned to the surface as fine particles. The returns are vented away from the rig in order to minimize the noise and dust. Cuttings are caught by a specially designed screen at the end of the blooey line.
In dry-gas drilling operations, the bottomhole pressure consists of the weight of the gas column, plus the annular pressure losses, plus the blooey-line pressure losses. The sum of these pressures will usually be far less than the formation pressure. Thus, the rate of penetration can be very rapid due to the low hydrostatic pressure. Chip-hold-down is also eliminated, making cuttings release from the bottom of the hole much more efficient. Overall, dry-gas drilling offers economic advantages in high ROP and lower operational costs per foot of hole, compared to mud drilling.
Dry-gas drilling operations require special and careful planning. Gas compressibility is a significant engineering consideration during both planning and drilling phases. Other equally important considerations are annular velocity requirements and logging suite selections. Fluid annular velocity, rather than fluid rheology, is the primary factor for cuttings transport when drilling with dry gas. The annular velocity necessary to lift cuttings determines the volume of gas that must be circulated. These annular velocities are such that turbulent flow always exists. To lift 3/8 in. to 1/2 in. cuttings, as a general rule, an annular velocity of about 3000 ft/minute is required. Although most air- or gas-drilled cuttings are quite small (dust particle size) when they reach the surface, they are larger when they leave the bot-
tom of the hole. The milling action of the drill string, impact with other cuttings, and regrinding of large particles at the bit are responsible for pulverizing them.
Logging is another factor to consider when drilling with dry gas. Wellbores containing no fluid other than air or gas can be surveyed only with devices that need no liquid to establish contact with the formation. The Induction Log is the only tool which can measure formation resistivities in such holes. The Gamma Ray Log can distinguish shales from non-shales. The Gamma-Gamma Density Log shows porosity even in gas-bearing zones where the Neutron Log indicates low apparent porosity. If both Gamma Ray and Gamma-Gamma Density Logs are run, the percent gas saturation may be computed in clean formations. In flowing gas wells, the Temperature Log detects the producing zones by showing the cooling effects of the gas as it expands into the hole. Also, in multiple-zone production, the Temperature Log indicates the relative volumes of gas coming from each zone. The Noise Log may be used to record zones of liquid or gas influx as well as zones of severe loss. A relative amplitude log is recorded and the noise may be monitored at the surface.
Water-bearing formations are the greatest limiting factor to air or gas drilling. Small amounts of water can be tolerated by adding drying agents such as CMC to the dry gas to absorb the water. However, if the cuttings become too moist they will stick together to form mud rings which can block the annulus. If this occurs, loss of circulation, stuck pipe, or even a downhole fire may result.
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