Image 1. The Oklahoma oilfield in which the heavy-duty slurry pumps ran reliably for more than a month, even with regular dry running Throughout the U.S., hundreds of companies work every day to discover and develop onshore oil and natural gas fields. Thousands more drill deep into the earth to bring water to the surface for commercial, industrial and residential use. Some firms have only a few employees, while others are among the nation’s largest corporations, with thousands of employees and tens of thousands of wells. However, they all need to cleanly and efficiently manage the spent drilling mud that is a byproduct of their work.
Many drillers—whether producing oil, natural gas or water—use a fluid containing bentonite clay to lubricate the drill bits as they cut boreholes through soil, sand and rock. The lubricant is pumped into the well. As drilling occurs, the used (spent) mud is circulated out of the borehole to a holding tank, cellar or open reserve pit near the well. The contents of the spent drilling mud depend on the formation, the type of drilling and the compounds used in the wellbore. However, it often consists of the following:
- Drilling water
- Mineral oil or diesel
- Accumulated storm and wash water
- Bentonite clay
- Weighting agents and other chemicals
- Well cuttings—such as pulverized stone, sand, sediment, coral and other gritty substances
All these potential components make spent drilling mud a heavy slurry that is often left in the reserve pits to be treated and reused or to settle out and dry. After evaporation, the solids are encapsulated within synthetic liners in the reserve pits or carried away for treatment, disposal or recycling.
According to the American Petroleum Institute, about 1.2 barrels of drilling waste is produced for every foot of well depth drilled. The U.S. Fish and Wildlife Service estimates that about 402 million barrels of drilling waste were produced in 2008. The size of the reserve pits ranges from a small tank to more than a half-acre containing 15,000 or more barrels of spent mud. In extremely tough conditions, the used drilling mud must be pumped into and out of the reserve pits.
Until recently, drilling rig operators typically used cast-iron waste-water submersible pumps to move the spent drilling mud in and out of the reserve pits, but these pumps experienced high failure rates. Because of the weight and high solids content of the mud, the pumps often ran dry or against closed discharges. Some failure modes included:
- Broken shafts and impeller bolts
- Loosened impeller keyways
- Failed seals
Across the country, including many of the shale oil and gas plays in Oklahoma and North Texas, operators and drilling-equipment rental companies have replaced their wastewater pumps with heavy-duty submersible slurry pumps. Because these are top-discharge pumps, they are more forgiving of low fluid-level conditions. They also have agitators to fluidize settled solids back into the slurry for more efficient pumping.
The key components of these pumps—including the impeller, wear plate and agitator—are made of high-chrome iron to withstand abrasion, and the bearings are tough and oversized. These pumps perform better and last far longer than their standard wastewater predecessors in the harsh oilfield environment. The Oklahoma site in Image 1 used a 7.5-horsepower, heavy-duty slurry pump. It has run reliably for 24 hours per day, seven days each week for more than a month at a time—even while running dry regularly. Of the approximately 80 pumps sold in Oklahoma in 2007, few have had problems. Most required only replacement power cords, which were worn from the constant movement from one location to another.