The extreme under-balanced that results when drilling with air or gas allows large disc-shaped cuttings to break from the formation with the impact of the bit tooth. These cuttings are degraded to dust as the turbulent airlifts them to the surface. Solids control while air drilling (including natural gas and nitrogen) consists of controlling atmospheric pollution, collecting samples, and disposing of cuttings and liquids. Normally there is no recovery or reuse of the air or gas except for a few occasions in natural-gas drilling when the gas is recycled to the gas plant. An elaborate separator/cleaning/recompressing system is required to recycle the gas. It is usually more economical to flare the gas than to recover and clean it (see Figure 1.)
Environmental Contamination
Solids-control problems that result when drilling with air or gas are primarily environmental. Dust drilling creates a large cloud of fine solid particles unless some type of dust-control device is used. When ‘‘dusting,’’ the ultra-fine particles tend to remain airborne for great distances. To properly control the dust, it needs to be wetted and settled in a tank.
The wetted fine solids from the well can create environmental problems including naturally occurring radiation (NOR), saltwater or hydrocarbon
contamination, danger to wildlife, or just a mass of unsightly gummy sterile cuttings. Instead of in an earthen pit, solids and liquids should be collected in a steel tank—a ‘‘frac’’1 tank is a good choice.
The waste can then be disposed of in accordance with appropriate environmental regulations.
Air separator and silencer systems often consist of just a vertical separator with a tangential intake and a water spray injected into the blooie line (Figure 2). Water, cuttings, and dust are thrown outward as the air spins when it enters the tank. The wetted dust and cuttings settle to the bottom of the tank as a damp mass. The tank also acts as a muffler and directs the sound upward. The separator must be grounded to avoid static electricity in case gas is present. Water requirements average roughly 300 gal/hr for an 8¾-inch hole, but some of the water can be recycled to the blooie line spray.
Several commercial-rental dust abatement systems are available. They range from closed-tank separator systems in Canada to a slotted pipe and steel tank in Oklahoma and Arkansas.
Drilling with Natural Gas
The primary requirement when drilling with natural gas is to safely flare the gas. In this case, the simplest is best, subject to any special environmental requirements. Historically, a horizontal blooie line, with an igniter attached to the end, extending into an earthen pit was satisfactory. Environmental regulations now set requirements for gas handling, dust control, liquid disposal, noise abatement, etc. In Canada and Europe, and in an increasing number of U.S. states, there are specific regulations about heat radiation from flares that preclude the use of an unregulated ground flare. Under regulated conditions, gas and cuttings with spray water have to go through a separator tank. From the separator tank, gas goes to a designed vertical flare stack. A free-water knockout tank may have to be used, depending on the amount of water required. A small back pressure, 5 psi, may be required to force the gas to the flare stack. A purge system with natural gas or nitrogen is also needed to keep oxygen out of the tank. The tank must be grounded to prevent static electricity. The sludge that results from wetting the dust and cuttings may be removed from the tank by hand or recirculated through the tank and over the shaker with a small circulating pump (Figure 3.).
Sample Collection While Drilling with Air or Gas
Sample collecting can be done at the end of the blooie line or in a collection chamber. Catching samples at the end of the blooie line presents several problems. Cuttings are extremely fine and are hard to collect. Safely and conveniently getting to the end of the blooie line may be a problem, since the minimum safe length of a blooie line is 300 feet. When drilling with gas, the gas is usually flared. The intense heat at the end of the line prevents catching representative samples. The heat destroys some of the samples and any hydrocarbons associated with them.
A collection chamber can be installed in the blooie line. The device can be as simple as a tube or pipe welded into the bottom of the line. A tong die-welded inside the blooie line makes a satisfactory deflector. At least one valve is needed at the bottom of the tube to prevent returns from continuously escaping. Two valves are better, especially when gas drilling because cuttings can be collected in the chamber with the bottom valve closed while the top valve is open. When the sample is retrieved, the top valve is closed and the bottom valve is opened. The process is simply reversed after the sample is retrieved. The collection chamber provides samples that are much more representative of the formations being drilled than does the practice of collecting samples off the cuttings pile at the end of the blooie line. It is also readily accessible and convenient to use (Figure 4.).
Air or Gas Mist Drilling
In air mist systems, the problem with cuttings is again environmental. The cuttings are finely ground and mixed with water. The simplest solution is to blow the cuttings into a steel tank and later dispose of the damp cuttings as solid waste. The water can be separated and clarified and sent to a disposal well (Figure 5.).
Mist systems use in the range of 1000/1 to 3000/1 gas/liquid. For example, a typical 8¾-inch 8000-foot misted hole will use about 2000 standard cubic feet per minute (scfm) of air (about 3 MMscf/d) and about 4 to 5 gallons of water per minute, or a maximum of about 7000 gallons of water in a full drilling day. The cuttings will contain material from the well—oil, NOR, traces of detergent, and corrosion inhibitors. The fluid should be slightly alkaline, as part of the anticorrosion treatment, but the greater mass of damp cuttings should be pH neutral.
Mist systems will surge. A mass of damp cuttings will form in the annulus and build up a plug until enough pressure is developed below the plug to blow it out of the hole. This will cause a large surge of cuttings, water, and gas, possibly equal to a rate of 10 MMcf/d for a period of up to several minutes. The surge will have a high velocity and impact, caused by the expanding gas and the mass of the cuttings. The separator system must be able to handle this surge. The simplest solutions utilize.
- A long tank to allow the velocity to decrease.
- A spiral system in a circular tank (separator) to consume the energy of
the slug. - An open pit.
- Or, One of the commercial mist separator systems.
Cuttings from misting are larger than air drilling cuttings. The misting detergent will make the cuttings wet.
Misting with natural gas and diesel oil requires a closed separator in which the oil and gas are separated. This allows the oil-wet cuttings to be dumped to the bottom of the separator or recirculated to the shaker after the gas is flared. This is not a common practice. It is documented in SPE 62896 (Labat, Benoit, & Vining).
