Pneumatic drilling fluids are used to drill in areas where loss of circulation and low reservoir pressures restrict the use of conventional drilling fluids. Pneumatic fluids also find application to minimize formation damage caused by: (1) invasion of mud filtrate and solid particulates into reservoir pore spaces, (2) flushing of hydrocarbons, (3) hydration of clays within the reservoir, (4) emulsion blocking, or (5) formation of chemical precipitates within the reservoir. These damage problems are all caused by having a large overbalance of pressure resulting from a high hydrostatic pressure of the mud column and from chemical incompatibility between the invading filtrate and the reservoir fluid. Cause of damage is eliminated, or at least diminished, by reducing the hydrostatic pressure of the drilling fluid column and by selecting a fluid that will not hydrate clays and will not form precipitates in the pore space.
Major equipment components which are required for pneumatic drilling, but not required for conventional mud drilling, are: gas/air compressor and boosters, a rotating head, chemical injection pumps (for foaming agents and corrosion inhibitors), and foam generator units. In some cases other equipment may be needed. Of the required components, the gas/air compressor is by far the most important and costly. The entire drilling system design depends upon the capability and efficiency of the compressors.
The ratio of final pressure to initial pressure controls the number of compression stages that is required. The number of stages of compression bears directly on the total drilling cost. Type and number of compressors required to handle each specific pneumatic drilling operation must be well thought out in advance. Calculation of the compression ratio and ratio of discharge pressure to intake pressure is the first step in deciding on the number of compression stages. The compression ratio in a single stage should not exceed four because the temperature of the discharge air will be too high. Cylinder temperatures above 300-400°F will cause mechanical lubrication problems and metal fatigue. Large capacity, high-efficiency, radiator-type coolers are required in order to reduce the air temperature between each stage of compression. Elevation and maximum air temperature at the drillsite must be considered because compressor efficiency decreases with increasing temperature and elevation.
Pneumatic drilling fluids may require one or more of three basic chemicals, but simple air drilling may not need any of these:
Surfactants as Detergents and Foaming Agents - Anionic or nonionic surface active agents (surfactants) are injected into the inlet air stream when formation water is encountered. These foaming agents also help clean the hole and keep the bit and drill string free of sticky solids. Surfactants prevent the cuttings from sticking together and from forming mud rings which can plug off the annulus.
Surfactants may also be added to the air stream or they may be added along with varying amounts of water to generate foam, as required by hole conditions. When water influx is minor, surfactants may be added in slugs down the drill pipe on connections. The surfactant (foaming agent) builds a homogeneous mixture that has ample consistency to bring out the water and cuttings, thereby cleaning the hole. The amounts of injection water and foaming agent used will vary according to the hole size, formation characteristics, available air, and quality and quantity of water/oil influx.
Corrosion Inhibitors - Corrosion during pneumatic drilling can be "disastrous" unless the drill string is properly protected by corrosion inhibitors and scavengers. Oxygen, carbon dioxide and hydrogen sulfide in the presence of water are extremely corrosive in pneumatic drilling. The rate of corrosion can be minimized depending on the type of pneumatic fluid being used, by: (1) maintaining a high pH (10 or above) with NaOH or KOH if water is being injected or (2) injecting corrosion inhibitors into the gas or air. Sulfide scavengers such as zinc carbonate, zinc oxide or zinc chelate are used to react with hydrogen sulfide to form inert zinc sulfide. Water-soluble and/or coating-type corrosion inhibitors (such as filming amines or phosphonates) should also be added to the fluid in order to further protect the steel components which are exposed to the circulating fluid.
Drying Agents - A wellbore which contains only a minor amount of water may be dried simply by discontinuing drilling and circulating air for a short time. Another method, which may prove to be more economical, is to add slug treatments of drying agents such as CMC or silicate powders. These additives require the use of a dry chemical injector.
When deciding if a pneumatic drilling fluid is applicable, one must consider pore pressures, rock types, porosity and permeability, reservoir fluids, economics, and location. Types of pneumatic drilling fluids discussed in this section are:
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