Drilling fluids and chemical exposures

Drilling fluid does many jobs at once, and to do them it is loaded with chemicals: weighting agents, viscosifiers, fluid loss additives, pH control agents, lubricants, shale inhibitors, biocides and, in many wells, a base of diesel, mineral oil or synthetic oil. The derrickhand, motorhand, floorhands and mud specialist mix those chemicals, handle the fluid and breathe the air above the mud tanks and shale shakers. Returns from the well bring up hydrocarbons, hydrogen sulfide in sour zones and sometimes naturally occurring radioactive material. This lesson covers the chemical hazards of the mud system and how to control them. HSE-102 covers WHMIS and hazard communication in general, HSE-240 covers respiratory protection and HSE-244 covers hydrogen sulfide in depth.

Water-based and oil-based muds

Water-based muds use fresh water or brine as the continuous phase, with bentonite clay, polymers, barite and other additives. They are used for many vertical and surface sections. Oil-based and synthetic-based muds use diesel, mineral oil, synthetic hydrocarbons or esters as the continuous phase, with emulsified brine, emulsifiers, wetting agents, lime and organophilic clays. They are favored for long horizontal sections in shale, such as in the Montney and Bakken, because they stabilize reactive shales, reduce friction and improve hole cleaning.

From a health point of view the difference matters. Oil-based muds give off hydrocarbon vapors and oil mist, especially when hot and agitated at the shakers. Skin contact with oil-based mud can cause dermatitis, and some base oils contain aromatic compounds such as benzene, toluene, ethylbenzene and xylenes (BTEX) or polycyclic aromatic hydrocarbons, although modern low-aromatic mineral oils and synthetics contain far less than diesel. Oil-based mud is also flammable when heated and adds to the fire load on the rig. Water-based muds bring fewer vapor hazards but can be strongly alkaline, and both types carry dusty powders and chemical additives during mixing.

Where exposure happens

Exposure in the mud system concentrates in a few places and tasks.

Powders, dust and silica

Mixing mud means handling large quantities of fine powders. Barite (barium sulfate) is the main weighting agent and is used in metric tons per well. Bentonite and other clays, lime, cement for spacers, and various polymers and additives come as powders in sacks or bulk. Fine dust irritates the eyes and lungs, and some products contain respirable crystalline silica as an impurity or a component, which can cause silicosis and lung cancer with long-term exposure (HSE-242). Lesson 11 covers silica in hydraulic fracturing, where exposures are much higher.

Controls start with engineering. Bulk handling systems that pneumatically convey barite and bentonite from silos to the mixing hopper remove most manual sack handling. Where sacks are still used, a sack-cutting table with local exhaust ventilation, careful technique and a vacuum rather than a broom for cleanup reduce dust. Respiratory protection, from filtering facepiece respirators to elastomeric respirators with particulate filters, is the last line of defense and requires a proper program with fit testing (HSE-240).

Caustics and other corrosive chemicals

Caustic soda (sodium hydroxide) and lime are added to control pH and alkalinity, and some muds use potassium hydroxide. Caustic soda is strongly corrosive and can cause severe eye injuries and deep skin burns, and dissolving it in water releases heat that can make the solution spit. Acids are used during some operations, especially completions. Controls include mixing caustic only in a designated chemical barrel with a lid, adding caustic slowly to water and never water to caustic, wearing goggles and a face shield, chemical-resistant gloves and an apron or suit, and having an emergency eyewash and shower within a short distance of the mixing area, tested regularly and protected from freezing. HSE-102 explains safety data sheets and labels, which must be available for every mud chemical on the rig.

Gas in the returns

When the bit drills into gas-bearing rock, gas enters the mud and travels up the annulus with the returns. Background gas, connection gas and trip gas are normal in many wells and are monitored by the mud logger. At surface, the gas breaks out of the mud at the bell nipple, the flow line, the possum belly and the shale shakers, which is why those areas must have gas detection for lower explosive limit and, in sour wells, hydrogen sulfide. In the Pinecrest wells, with about 0.5 percent H2S in the formation gas, even a small amount of gas in the returns can produce dangerous H2S concentrations at the shakers. Fixed detectors with alarms at the shakers, bell nipple, mud tanks and rig floor, personal monitors on every worker and breathing air readily available are standard on sour wells. HSE-244 covers H2S hazards and response; Lesson 8 explains how increasing gas in the returns can be an early sign of a kick.

Mud treatment can also remove H2S chemically: zinc-based scavengers and high pH tie up dissolved sulfide in water-based muds, and lime does the same in oil-based muds. Scavengers are a useful control but do not replace detection, because their capacity can be overwhelmed by a large influx.

Long shifts and exposure limits

Occupational exposure limits are usually set as 8-hour time-weighted averages for a 40-hour week. Rig crews work 12-hour tours for 14 days in a row, which means more hours of exposure and less time to recover between shifts. Several jurisdictions require or recommend adjusting exposure limits for extended shifts, and industrial hygienists often use the Brief and Scala model to estimate an adjusted limit. The program below applies that model to an invented hydrocarbon vapor measurement at Rig 31's shakers. The limit used is an illustrative value, not a regulatory limit for any specific substance.

# Invented exposure readings at the shakers over a 12-hour tour (ppm).
readings = [  # (hours, average concentration in that period)
    (2.0, 40), (3.0, 85), (1.0, 20),   # includes a meal break at low exposure
    (4.0, 110), (2.0, 60),
]
hours = sum(h for h, _ in readings)
twa_shift = sum(h * c for h, c in readings) / hours     # average over the shift
twa_8h = sum(h * c for h, c in readings) / 8            # dose expressed as 8 h TWA

limit_8h = 100   # illustrative 8-hour limit, ppm (not a real OEL)
rf = (8 / hours) * ((24 - hours) / 16)                  # Brief and Scala
adjusted = limit_8h * rf
print(f"Shift length {hours:.0f} h, shift average {twa_shift:.1f} ppm")
print(f"Dose as 8-hour TWA: {twa_8h:.1f} ppm vs limit {limit_8h} ppm")
print(f"Brief and Scala factor {rf:.2f}: adjusted limit {adjusted:.0f} ppm for the shift average")
print("Exceeds adjusted limit" if twa_shift > adjusted else "Within adjusted limit")

Shift length 12 h, shift average 76.2 ppm
Dose as 8-hour TWA: 114.4 ppm vs limit 100 ppm
Brief and Scala factor 0.50: adjusted limit 50 ppm for the shift average
Exceeds adjusted limit

The shift average of about 76 ppm looks comfortably below the illustrative 100 ppm limit, but the 12-hour tour means the dose expressed as an 8-hour TWA is about 114 ppm, and the Brief and Scala adjusted limit for a 12-hour average is only 50 ppm. Either way of looking at it shows the crew is overexposed. Controls might include enclosing the shakers with extraction, moving the derrickhand's routine tasks away from the shakers, and using respiratory protection while engineering controls are installed. A qualified industrial hygienist should design a real sampling plan and interpret results against the applicable limits.

Skin protection and hygiene

Skin contact with mud is constant on many rigs: hands in mud when handling pipe, splashes at the shakers, soaked clothing on wet trips. Oil-based mud defats the skin and can cause irritant and allergic dermatitis, and prolonged contact with some base oils is linked to more serious skin conditions. Controls include chemical-resistant gloves suited to the base oil (nitrile is common), splash suits or aprons, changing out of soaked clothing promptly, washing skin with soap and water rather than solvents, using barrier and after-work creams, and providing laundry for work clothes so that contaminated clothing is not taken home. Workers should report rashes early; dermatitis is easier to treat before it becomes chronic.

Naturally occurring radioactive material

Some formations contain naturally occurring radioactive material (NORM), mainly radium isotopes that can concentrate in scale inside pipe and equipment or in sludges in tanks. Drilling itself rarely produces significant NORM exposure, but workers handling old tubing, cleaning tanks or dealing with scale on service rigs may be exposed. HSE-330, Lesson 6, introduced NORM and the inverse square law. Where NORM is suspected, surveys with a suitable meter, control of dust and ingestion, and handling under the applicable guidelines are required.