Fact Sheet - Drilling Practices That Minimize Generation of Drilling Wastes
How Are Wells Typically Drilled?
The conventional process of drilling oil and gas wells uses a rotary drill bit that is lubricated by drilling fluids or muds. As the drill bit grinds downward through the rock layers, it generates large amounts of ground-up rock known as drill cuttings. This section of the Drilling Waste Management Information System website discusses several alternative drilling practices that result in a lower volume of waste being generated.
Oil and gas wells are constructed with multiple layers of pipe known as casing. Traditional wells are not drilled from top to bottom at the same diameter but rather in a series of progressively smaller-diameter intervals. The top interval is drilled starting at the surface and has the largest diameter hole. Drill bits are available in many sizes to drill different diameter holes. The hole diameter can be 20" or larger for the uppermost sections of the well, followed by different combinations of progressively smaller diameters. Some of the common hole diameters are: 17.5", 14.75", 12.25", 8.5", 7.875", and 6.5".
After a suitable depth has been reached, the hole is lined with casing that is slightly smaller than the diameter of the hole, and cement is pumped into the space between the wall of the drilled hole and the outside of the casing. This surface casing is cemented from the surface to a depth below the lowermost drinking water zone. Next, a smaller diameter hole is drilled to a lower depth, and another casing string is installed to that depth and cemented. This process may be repeated several more times. The final number of casing strings depends on the regulatory requirements in place at that location and reflects the total depth of the well and the strength and sensitivity of the formations through which the well passes.
Historically, wells were drilled to be relatively vertical and were completed at a depth to intersect a single formation. Thus, one full well was required for each completion. Modern technology allows modifications to several aspects of this procedure, thereby allowing more oil and gas production with less drilling and less waste generation. The following sections describe how drilling can be done to intersect multiple targets from the same main well bore, how wells can be drilled using smaller diameter piping in the wells, how drilling can be done using techniques that minimize the amount of drilling fluid, and drilling fluid systems that generate less waste. The U.S. Department of Energy describes these and other environmentally friendly oil field technologies in a 1999 report, "Environmental Benefits of Advanced Oil and Gas Exploration and Production Technology" (DOE 1999).
In the mid-1970s, new technologies like steerable downhole motor assemblies and measurement-while-drilling tools became more prevalent and allowed drilling to proceed at angles off of vertical. Drillers could now more easily turn the well bore to reach targets at a horizontal offset from the location of the wellhead. This opened up many new possibilities for improving production. Three variations of directional drilling include extended-reach drilling, horizontal drilling, and multiple laterals off of a single main well bore.
Extended-Reach Drilling: In some situations, it is impractical or too expensive to drill wells from locations directly above the target formations. For example, offshore drilling is much more expensive than drilling from a shore-based facility. If the target formation is a mile from shore, it may be much more effective to directionally drill from a shore-based location. Another option involves using a single platform or drilling pad to drill multiple extended-reach wells in different directions or to different depths, thereby minimizing the number of surface well-pad facilities. An interesting example of this comes from the THUMS operations, which take place on four man-made islands in the harbor off the coast of Long Beach, California. These are disguised, camouflaged, and sound-proofed to look like a residential development. More than 1,200 wells have been directionally drilled from the islands to reach targets under the harbor. More than 60 percent of the wells drilled from the islands deviate from the vertical by 50 degrees or more. A bird's-eye view of the well bores looks like a spider's web stretching in all directions from the islands. Using extended-reach drilling allows many wells to be completed from a single location and avoids the environmental impacts of multiple surface structures.
Horizontal Drilling: Some productive formations are not thick but extend over a large lateral area. Prior to the advent of directional drilling, such formations were either uneconomical or required multiple wells to recover the resources. Modern technology allows wells to be drilled and completed in a relatively thin horizontal layer. A single horizontal well can contact more of the resource and therefore takes the place of several traditional vertical wells. Because the well bore interval from surface to producing formation is drilled only once, a horizontal well generates less waste than several vertical wells.
Multiple Laterals: Some formations contain multiple, small, oil-bearing zones or zones at several different depths. To recover these resources using traditional vertical wells would require many wells. With directional drilling technology, lateral well bores can be drilled off of a main vertical well bore to reach individual targets. The main well bore is drilled only once, followed by drilling of several smaller-diameter laterals. The total volume of drilling waste is lower than would be generated if several full wells were drilled.
Drilling Smaller-Diameter Holes
The amount of drill cuttings generated is directly related to the diameter of the hole that is drilled. Several technologies, often employed together, can drill smaller-diameter wells.
Closer Spacing of Successive Casing Strings: The sizes and ultimate volume of cuttings are a function of the type of drill bit and casing diameters used. In the past, only standard-sized bits and casings were available, such that each reduction in hole size was quite dramatic. The number of available bit- and casing-size options has increased dramatically in recent years. Now, adjacent casing strings can fit closer together, so the outer of the two strings need not be so far from the inner string. This reduces the volume of cuttings generated.
Slimhole Drilling: According to DOE (1999), slimhole wells are defined as wells in which at least 90% of the hole has been drilled with a bit six inches or less in diameter. Although slimhole technology has been available since the 1950s, it was not commonly used because the small-diameter well bore restricted stimulation, production, and other downhole manipulations. Modern technology has overcome these disadvantages. In addition to generating less drilling waste, slimhole rigs have a smaller footprint on a drilling pad.
Coiled Tubing Drilling: This type of drilling does not use individual sections of drill pipe that are screwed together. Instead, a continuous length of tubing is fed off of a reel and sent down the hole. The coiled tubing has a smaller diameter than traditional drill pipe so a smaller volume of cuttings is generated. In addition to reducing waste volumes, the surface footprint is smaller, the noise level is lower, and air emissions are reduced.
Mono-bore and Expandable Casing: Recent developments and success with expandable casing hold the promise of allowing a true mono-bore type well to be constructed.
Drilling Techniques That Use Less Drilling Fluid
Drilling fluids play an important role in traditional well drilling. However, the fluids become contaminated by their use. At the end of the drilling job, they must be disposed of or processed for recycling. For some types of wells, drilling can proceed with minimal or no drilling fluids.
Pneumatic Drilling: In selected formations, wells can be drilled using air or other gases as the fluid that circulates through the drilling system. DOE (1999) describes four different types of pneumatic drilling: air dust drilling, air mist drilling, foam drilling, and aerated mud drilling. These all rely on gas or blends of gas and mud to lift cuttings to the surface. Pneumatic drilling often does not require the large surface reserve pits common to traditional drilling. Thus, this technique can be used in environmentally sensitive areas.
Drilling Fluid Systems That Generate Less Waste
The choice of drilling fluid can affect the overall volume of used muds and cuttings that is generated. Synthetic-based muds (SBMs) drill a cleaner hole than water-based muds (WBMs), with less sloughing, and generate a lower volume of drill cuttings. SBMs are recycled to the extent possible, while used WBMs are generally discharged to the sea at offshore locations.
Other Waste Minimization Issues
Waste minimization can be looked at strictly from the perspective of solid waste volume. A more comprehensive view of "minimization" looks at the overall environmental impacts associated with a process or technology. This website primarily focuses on minimization of solid waste or wastewater streams. Readers are encouraged to consider other issues, like air emissions and energy usage, as they evaluate technology options.
There are many relatively simple processes that can be used on drilling rigs to reduce the amount of mud that is discarded or spilled. Examples include pipe wipers, mud buckets, and vacuuming of spills on the rig floor. These devices allow clean mud to be returned to the mud system and not treated as waste. Solids control equipment, like centrifuges, can be used to remove solids from the recirculating mud stream. Although such a process does generate some solid waste, it avoids the need to discard large volumes of solids-laden muds.
This website focuses on wastes arising directly from drilling or downhole processes. From a different perspective, the entire process of manufacturing, storing, and transporting muds to a drilling location generates wastes. Management of those wastes, used containers (e.g., drums, sacks), and washwater can benefit from waste minimization efforts too.
The technologies and practices described are not universally applicable. Some of them only are appropriate for use in specific niches. Others can function well in many settings but may not be cost-effective; consequently, they are not selected. The total cost of drilling a well is usually hundreds of thousands to millions of dollars. These technologies are not chosen simply for their ability to reduce drilling waste volumes, because waste management costs are only one small component of the total well cost. The technologies must provide increased performance and save money for the operators. Nevertheless, as they are employed, they contribute to a waste management benefit.
DOE, 1999, "Environmental Benefits of Advanced Oil and Gas Production Technology," DOE-FE-0385, U.S. Department of Energy, Office of Fossil Energy, Washington, DC (Available at: http://www.osti.gov/bridge/product.biblio.jsp?osti_id=771125.)