Onshore disposal options aim at incorporating drilling waste into either the surface (or rooting zone) or beneath the rooting zone. The former is
called land application. The latter is called burial.
The term land application refers to a disposal technique of incorporating drilling wastes (cuttings and associated drilling fluids) into the top few inches or feet of soil so that the resulting soil is still good for agricultural use. The same basic thing is meant by other terms, such as land farming, land application, and land treatment. Occasionally, the user might intend some fine points of difference. For instance, if the land is to be tilled, then land farming might be used, whereas in land application, no tilling is intended. No such distinction will be made here. It is assumed that some sort of mixing of waste and soil to form a waste/soilmixture will be required.
The technique of land spreading is ideally suited to the large amounts of solids (cuttings) and fluid generated from the drilling process. The relatively small amounts of contaminants are easily incorporated into waste/soil mixtures, sometimes resulting in an improved soil. This is especially true if the soil has been neglected. By land spreading, the waste
is actually becoming soil.
Land application occurs when drilling waste is mixed into the top few inches of soil. The drilling waste is spread in a thin, calculated layer and the waste is tilled into the soil, usually using farming plows. Several natural mechanisms help reduce the contaminant concentration in the resulting waste/soil mixture. Rapid biodegradation of the oil occurs when the oily cuttings are exposed to air, water, and naturally occurring microorganisms in the soil. By managing the sodicity of the clays in the soil and drilling waste, and the salinity in the drilling waste, soil structure can be maintained. The structure controls how air and water pass through the soil; for example, dispersed soils reduce hydraulic conductivity, while flocculated soils increase it. There is also a reduction in concentration of potential contaminants due to mixing with soil.
The contaminants most commonly found in drilling waste that must be addressed are salts, oil, and heavy metals. Salts in this case refers to the large family of chemicals generated by mixing an acid with a base. Salt content is determined by measuring the electrical conductivity in the extracted water of a saturated paste. All salt measurements must be related back to the saturated condition from the condition of the waste/ soil mixture at the in situ condition.
Oil and heavy metals are measured by determining the weight of oil or heavy metal in the sample after all the water is removed. This is called the dry weight basis. The oil referred to in this test is total petroleum hydrocarbon (TPH).
Occasionally, lead, zinc, or chromium may be of concern, but lead and chromium have generally been replaced in oil field use. Barium levels in waste cuttings from high-weight fluid systems (say, 17 or 18 ppg) may be as high as 300,000 mg/l. While barium from barite has very, very low solubility and is not bioavailable, many areas of the world have regulations that limit the concentration of barium that can be incorporated into the soil.
Deuel and Holliday have described acceptable limits of contaminants commonly found in E&P (exploration and production) wastes in agricultural soils. These limits were examined by the state of Louisiana and adopted, with slight modification, as the regulatory limits known as 29b. These limits are shown in Table 1.
Table 1. Waste/Soil Mixture Limits
|Parameter||Limit, La. 296|
|Oil and Grease ( % weight )||1|
|Electrical coductively, mmhos/cm||4|
|Metals (mg/kg )|
By knowing the levels of potential contaminants in the waste, one can estimate the amount of dilution with dirt needed to reduce the contaminant level to the acceptable range. Many times, dilution with dirt is all that is needed. A simple method is to divide the waste concentration by the established limit for the waste/soil mixture and subtract 1. While not mathematically rigorous, this gives a close approximation of the amount of dirt required to dilute the concentrations down to the acceptable level.
Sometimes large amounts of oil are encountered, and it is desirable to allow some oil to biodegrade before mixing with dirt to achieve the final waste/soil mixture. Oily cuttings can easily be 30% by volume (about 15% by weight) when they are returned to shore. Most oils biodegrade readily to a natural endpoint. Diesel biodegrades to about two thirds of its original content. In warm climates, where moisture conditions are kept reasonable (neither flooded nor dry), biodegradation rates are very rapid. The cuttings should be spread in a thin layer at about 35% and kept moist. If the conditions are correct, then the oil content should be reduced to 1% within 3 months.
Managing salinity is complicated. Salt is toxic to plants at fairly low levels. However, salts tend to flocculate soils containing clay, making them more permeable to water infiltration. Sodium has the opposite effect, dispersing them. Thus, sodium chloride salt may flocculate the soil initially, but as the sodium adsorbs onto the clay matrix it may cause the clay to collapse and reduce water infiltration, precipitating a condition of sodicity, which is a major problem in agricultural chemistry.
Since oil-based fluids are dominated by calcium content, sodicity is not a problem with land application of oil-based cuttings. However, waterbased
fluid is dominated by sodium content. Thus, any salt content in water-based cuttings will need some sort of amendment, which may take the form of calcium additives. Another approach is to incorporate soil humus into the soil by mixing horse manure or sawdust with the waste/ soil mixture. Soil humus adsorbs large amounts of sodium, allowing the clay in the soil to remain flocculated.
If the soil is flocculated, then rainfall or irrigation will remove salt by infiltration. For this reason, land application works better in wetter climates, but irrigation can replace rainfall. Also, groundwater should be reasonably deep to prevent salt from reaching it.
Heavy metal content cannot be reduced. It must be diluted with dirt to the acceptable level. The method to achieve this is to plow the waste mixture into the land surface. Most plows till only about 6 inches deep, but with sufficient power, tilling 2 feet deep can be accomplished.
Several best practices should be observed:
- Land application is meant to be a onetime event. A limited amount of waste material can be applied to a given area. A commonly cited value is 1000 metric tons of material per hectare, although this depends on the nature of the waste material. If the drilling-fluid system being used contains a high amount of barite, then the amount that can be supplied will be severely limited.
- Waste/soil mixtures that are to be biodegraded should be kept moist, but not saturated. Both water and air are required for biological degradation of oil. Land application in desert environments should not be used unless water can be obtained. Percolation and drainage should be considered in very wet locations.
- Make sure that the waste brought to the site is acceptable for land application. Highly saline solutions, such as zinc bromide, are not acceptable.
- Obtain equipment for spreading and tilling that can adequately do the required job.
- Get help from someone who has performed land farming or land application and understands the objectives.
Land farming ensures the proper chemical balance in the final waste/soil mixture, but further enrichment by fertilizer addition is usually needed.
The main advantage of land application is that, if done right, the waste is incorporated into the land and the land can be returned to its original status (e.g., for growing crops). While it cannot be said that liability is completely eliminated, proper land spreading can certainly minimize any long-term liability.
Not understanding that a limited amount of waste material can be managed in a given area is the biggest pitfall in land farming. Time and again, so-called land farms are set up and waste is dumped into the area in an uncontrolled manner. Land farming is meant to be a mixing of waste and soil in the top foot or two of soil. If 3 or 4 feet of waste (or the equivalent volume in barrels) is brought to a site and dumped, then this is not land farming. These types of misapplication usually end up being long-term liabilities.
Burial is a method of disposal in which cuttings are mixed with dirt to achieve physical and chemical properties in the resultant waste/soil mixture that are suitable for burial. The waste/soil mixture will be placed in a burial cell. The top of the burial cell should be below the common rooting zone of 3 feet. Since the material is out of the rooting zone, the chemical properties are less strenuous than with land application. The bottom of the burial cell should be at least 5 feet above the seasonal high water table. Burial is the most common method of disposing of cuttings collected in a reserve pit while drilling onshore. However, burial practices have developed over time with little scientific input. Thus, the method of burial varies widely among operating areas and contractors. The most common practice is to bury the solids in the existing reserve pit after the water is allowed to evaporate. However, this practice should be examined closely in view of waste/soil mixtures and placement of the burial cell.
In order to apply scientific knowledge to the practice and responsibly bury cuttings, several issues need to be addressed. These are:
- Chemical content of the buried cuttings
- Depth or placement of the burial cell
- Moisture content or condition of buried cuttings
- Leakage or leaching from the burial cell
Chemical Content of Buried Cuttings
Chemical content of the buried material is important to prevent potential contaminants from affecting surrounding soil or groundwater. The level of potential contaminants can be higher than would normally be tolerated in good soil (i.e., soil not adversely affecting crop growth), because the burial cell is out of the rooting zone. But it must not be so high that leaching to groundwater or the rooting zone can occur. Table 2. lists the chemical criteria and maximum levels suggested for each material.
Table 2. Burial Limits
|Parameter||Limit, La. 296|
|Oil and Grease ( % weight )||3|
|Electrical coductively, mmhos/cm||12|
|Moisture content ( %)||50|
|Metals (mg/kg )|
|Depth of cell ( ft )||5|
|Height about water ( ft )||5|
Most of the parameters in Table 1 cannot be met without mixing the cuttings with dirt or soil prior to burial. Thus, burial in situ will not, generally, meet the above criteria. Mixing the drilled cuttings with dirt will increase the volume of material to be buried but decrease the concentration of potential contaminants.
Depth or Placement of the Burial Cell
Burial can be considered entombment of waste/soil mixtures. The waste/ soil mixture contains oil, salt, or heavy metals in excess of the surrounding dirt and soil. This means that, over time, equilibrium is established with the surrounding soil, or there exists a transition from soil properties to waste/soil mixture. The concentration of potential contaminants is within the waste/soil mixture but gradually decreases to background levels in the surrounding soil.
If the waste/soil mixture to be buried were spread as soil, then it would adversely affect some crop growth. This is because the chemical concentrations exceed established limits for crop growth. But the material is not spread; it is buried in a cell. Thus it is important that the burial cell be placed below the rooting zone of future plants. The rooting zone for most plants is 3 feet. If the top of the burial cell is 5 feet below the surface, then 2 feet of buffer zone protects the rooting zone from contamination from the buried material.
The cell should also be placed 5 feet over any groundwater to prevent migration to the groundwater. The zone over the groundwater should be discontinuous. This means that there should be some sort of clay barrier between the waste material and the groundwater, especially in sandy soil.
If the burial cell is placed in this manner, future problems with the cell are unlikely. The waste/soil mixture should not adversely affect crops and should not contaminate groundwater.
Placement of the cell should be considered, too. The cell should not be placed near sensitive resources, like water wells or streambeds. In hilly areas, the cell should not be placed where leaching out of the hillside could occur. Other natural environments may make burial difficult or impossible. Sites in swamps, tundra, or desert that blows sand may not be possible.
Moisture Content or Condition of the Buried Cuttings
Moisture content is expressed as the weight of water contained in the cuttings divided by the mass of the dry sample. This means that the moisture content is 100% when the weight of water contained in the sample equals the weight of dry cuttings. Pit contents have more than 100% moisture content without dewatering. Moisture content is important because dissolved solids could potentially migrate to groundwater. By limiting moisture content, the carrier fluid is limited and the potential for contamination is minimized.
Moisture content is also important because burial of cuttings with high moisture content can result in so-called slumping of the burial cell: When wet cuttings are buried, moisture content will be lost to the surrounding dirt. As this water is lost, the water in the cuttings matrix is replaced with air. As more air is incorporated into the matrix, the strength of the matrix is reduced. Eventually the weight of the covering dirt/soil causes the cuttings matrix to collapse.
Excessive amounts of water associated with the cuttings can be pumped off and processed for reuse or disposed of. Smaller amounts of moisture can be incorporated into the waste/soil mixture by mixing dirt with the cuttings. Clays will contain about 60% moisture at saturation and will drain to about 38% at field capacity. Sands will contain about 40% moisture at saturation and drain to about 7% at field capacity. Cuttings containing high amounts of clay will probably not require much drying or extra work. Cuttings containing high amounts of sand, though, will probably require water to be extracted and/or require mixing with dirt/soil. In tropical or subtropical climates, collection areas may need to be covered to prevent additional moisture being absorbed into the cuttings.
Leakage and Leaching from the Burial Cell
Leaking from a burial cell refers to whole fluid escaping. Improper cell
construction or poor practices usually cause this. If the cell is properly
constructed, leaking should not occur.
Leaching refers to potential contaminants dissolving in water (usually rainwater) and escaping the cell with the solvent (water). This can occur when sufficient rain saturates the soil and water moves gravitationally down to the burial cell. Soluble chemicals may dissolve in the water and may move laterally or upward while trying to establish an equalized concentration. If the cell is constructed with discontinuous zones (such as clay barriers), movement of lightly contaminated water will not occur or will be very limited.
The disadvantage of limiting leaching is that the main mechanism for removing salt from cuttings is through leaching. This means that high concentrations of salt in cuttings should not be buried. High concentrations of salt can be minimized by leaching prior to burial (as in land application) or by mixing with dirt/soil.
Pit design has a dramatic impact on burial operations. There are several types of pit design. They are:
- Partially aboveground
- Perched (a variation of aboveground)
The in-ground pit is dug into the earth. The top of the pit is level with the surface of the adjacent drilling pad. This type of pit makes it easy to operate. Wash water can be collected in ditches surrounding the rig and gravimetrically drained into the pit. If the ditches do not drain on their own, then they can be flushed with water easily. These types of pits are also easy to close. If the pit is deep enough, then it can be dried and dirt can be backfilled over the top. The pits need to be deep enough so that the contents (after drying) can be covered with 35 feet of dirt. Groundwater in the area needs to be deep enough that the bottom of the pit is at least 5 feet above it. A hazard with this type of pit is lack of control over what goes into the pit from the ditches. Accumulated hydraulic fluid or used motor oil in the ditches might be flushed into the pit and jeopardize its exempt status.
The partially aboveground pit is a shallow hole dug into the earth using the excavated material for berm walls. This increases the holding capacity of the pit and decreases the amount of digging. It might also be used in areas with shallow groundwater. Since the ditches surrounding the rig do not drain gravimetrically into the pit, sumps are placed strategically between the pit and the drilling pad. Wash water collects in the sumps and is pumped or jetted into the pit in batches. If the sand trap is dumped, then it is usually dumped into the sumps. Whole fluid discard is usually jetted rather than dumped. One positive is that there is better control over what gets into the pit. A disadvantage is that the pit is usually not deep enough for direct burial. This means extra handling and cost during the closure operations.
The aboveground pit is constructed where it is impractical to dig a pit. This can occur in very soft, sandy conditions or in very hard surface conditions. The aboveground pit has similar operational advantages and disadvantages as the partially aboveground pit. Burial is obviously not possible using an aboveground pit. In soft, sandy conditions, a large burial cell or trench is usually dug especially for burial during closure operations. In very hard surface conditions, burial becomes a difficult problem. If the pit could not be dug due to very hard surface conditions, then a burial cell is probably not economical either. Construction of an aboveground pit is fairly cheap and simple, but closure costs are slightly higher.
The perched pit is a special case of the aboveground pit. This type of pit is used in hilly terrain. It is constructed by making a berm out of fill material on the outside of a hill. In this case, the pad is located on the cut portion of a cut-and-fill location. The inside wall of the pit is the hillside. The outer wall is the constructed berm. These types of pits are cheap and easy
to construct. If conditions are ideal, then operations will run smoothly, as well. But all too often, problems occur and the berm wall breaks. Then, the contents of the pit can run down the hill and collect in whatever is at the bottom. This is usually a stream, creek, or other water body. For this reason the perched pit is usually not recommended.