Gas-based, or dusting, drilling fluid systems are generally similar to drilling mud systems with certain obvious differences (see Lorenz, 1980). In the past, dusting was often accomplished using natural gas from a conveniently located pipeline. Today, this is rarely economic and the gas most often used is compressed air. Oddly enough, the major disadvantage of this change is the risk of underground fires!
Previously, when you drilled into a gas-bearing formation, mixing its output with other hydrocarbons has little effect. On the other hand, mixing formation hydrocarbons with hot, turbulent, compressed air yields a highly combustible mixture.
Air drilling requires, powerful compressors to be available at the well site (Figure 26) taking the place of the drilling mud pits and pumps.
With minor differences, the compressed air circulation system is analogous to the mud system until the top of the annulus is reached. The circulating air stream imposes little or no hydrostatic pressure on the bore hole bottom and walls, and so the air-drilled well requires the annulus to be sealed at all times. Unfortunately, drilling cannot be carried out with conventional blowout preventers closed around the drill string. A rotating drilling head closes around the drill string much like a blowout preventer, an internal swivel and allowing a small amount of leakage for lubrication, the drill string is free both to rotate and move up and down through the drilling head.
Obviously, large objects such tool joints or a drill bit cannot pass through the drilling head. For tripping or making connections, circulation must be stopped, the drilling head opened and a jet of compressed air in blown across the top of the bore hole to carry away dangerous produced gases. Equally obviously, this type of drilling operation is most convenient when drilling without a heavy drilling fluid yields great improvements in rate of penetration, but when there is also very little risk of massive, combustible, or explosive fluid influxes. For example, when drilling impermeable, hot, dry rock geothermal wells.
While drilling, the exhaust air, the cuttings and produced formation fluids are diverted from below the drilling head through the blooie line to a safe distance from the rig. Toward the end of the blooie line there may be a de-duster and a flare. The de-duster is a water injection system that prevents air-polluting dust emissions. The flare is ignited if the air stream is carrying combustible hydrocarbons. Extra air and natural gas is injected into the blooie line to ensure continuous full combustion at the flare pit located at the safe end of the blooie line.
Figure 26: The circulation system required for a gas-based drilling fluid system: gas (usually compressed air), mist or foam.
Dusting allows the fastest possible rate of penetration due to the complete absence of a hydrostatic head in the annulus. Unfortunately, the explosive decompression of material fractured by the drill bit blasts them into minute fragment. The turbulence and high velocity of the air flow in the annulus causes many more collisions between the cuttings, the bore hole wall and the drill string. When the cuttings are sampled by the geologist at surface, he quickly understands the significance of the term dusting — the so-called cuttings are little better than dust.
A disadvantage of air-drilling and one that has prevented it being widely used in large-hole drilling is that the drilling fluid is incapable of
carrying large cuttings loads or of suspending cuttings when circulation is halted. Water influx may also be a problems resulting in the cuttings becoming a swollen pasty mass that is of little geological value, and worse it can plug up the entire circulation system.
Mist and Foams
The mixing of soap with the air stream was first introduced to solve the water influx problem (Linicome, 1984). In this process, droplets of soap solution are carried into the drill string as a mist, and then mix with formation waters in the annulus to produce foam. The process is therefore in some areas known as misting and in others as foam drilling.
True foam drilling was first used at the Nevada Nuclear Test Site to drill extremely large diameter hole into dry porous formations. These wells required recovery of a large volume of cuttings without using a dense liquid or mud slurry that would create a pressure overbalance, invade, and damage to sensitive, low pressure, dry formations.
Stiff foam, or A.E.C. foam as it was sometimes called, is a mixture of very low density clay-water drilling mud with a surfactant which is pre-foamed prior to injection into the bore hole. As a mixture of water-based mud and air, it had hybrid properties between the two. It produces only a small hydrostatic head in the bore hole but has more stable, lower velocity fluid flow rates in the return annulus. Drill cuttings
produced in foam drilling are fine grained, and have an explosive appearance but are not so extensively damaged and ill-sorted as those produced in a conventionally air-drilled well.
Stable foam is a newer drilling fluid, consisting of a mixture of: water, air and a non-ionic surfactant. It has the consistency of shaving foam (or the kind of foam laid down by fire fighters) and is extremely stable unless mixed with relatively large volumes of water, when it relaxes rapidly to a thin watery solution and releases its large sediment load. Stable foam allows low bore-hole hydrostatic pressures, and extremely high rates of penetration. Cuttings although small are well suspended by the stable foam which is inert, non- invasive and travels at
velocities only slightly higher than that of drilling muds. Stable foam may become the drilling fluid of choice in those many drilling
applications where high overbalances are not wanted and pressure control is not a problem, for example, in very hard, and subnormally pressured rocks.
Coring
Coring (see Whittaker (Ed), 1985) is, in every consideration, the superior method of obtaining rock samples from a well bore It allows the geologist to observe the true nature and orientation of rock structures and textures in reliable, unmixed and un-worked depth relationships (although not in real time). It can also provide samples of rocks, minerals, hydrocarbons, and pore waters of suitable quality for
geochemical, petro-physical and rock mechanical analyses, that are far less contaminated or disturbed than any other source. The simplest coring method uses the standard bottom-hole core barrel (see Figure 28) and core bit (see Figure 27).