OIL-BASED DRILL CUTTINGS TREATED BY LANDFARMING

Peter K. Zimmerman, James D. Robert
Amoco Canada Petroleum Co. Ltd.
Calgary

Over the past 3 years, Amoco Canada Petroleum Co. Ltd. has been developing a technique to successfully landfarm oil-based drill cuttings.

The oil-based drill cuttings are a residue of diesel invert mud and rock cuttings.

One of the primary advantages of landfarming oilbased drill cuttings is the disposal of waste through the use of a natural process with minimal energy input. An additional benefit is the relatively low cost as compared to other common disposal methods.

Landfarming of oil-based mud cuttings does not cause air emission problems in comparison to incineration where smoke and particulate matter may be of concern, nor does it simply cover up or store waste as in a landfill.

Basically, landfarming of the cuttings handles the waste on-site, and transport of the material is not usually required.

The Amoco method disposes of the cuttings and mud residue by the use of the native soil microorganisms to degrade the oil phase of the invert fluid. Also, natural dilution and leaching action reduce the high chloride levels in the water phase.

This landfarming method is based on the following:

Modifications to the drilling mud solids control system such that the amount of oil retained in the cuttings is substantially reduced
Minimization of the oil-to-soil ratio through even spreading of the cuttings over a suitable land area
Use of the soil's natural capacity to biodegrade hydrocarbons and enhancement of this capacity through the application of chemical fertilizers and mechanical cultivation.

In the Grey Wooded soil zone of Alberta, 33 well sites drilled with invert mud have been treated by landfarming and have shown positive results. During the 3 years the program has been active, each landfarming area has been given one to two treatments per year.

Each location has had a significant reduction in the oil content of the soil, in electrical conductivity, and in salt levels. It is anticipated that many of the sites will meet or exceed government revegetation standards by the end of the 1990 growing season. To date, no detrimental effects from leaching or fluid migration have been observed, and the sites are still monitored regularly.

This kind of treatment procedure is low cost, even considering sampling, analysis, treatments, and the 2-4 years required to reclaim the site. Amoco Canada hopes to reduce this time frame as more data become available and the technique is "fine-tuned."

Although this approach has apparently been successful in this soil zone, it could be limited by the availability of biologically active soil horizons, the surface area to cutting volume ratio, and the oil-to-cuttings ratio.

Further evaluation is still required to determine the limits of this reclamation method. However, it is believed that this approach will become a primary method of handling oil-based drill cuttings.

SITE DESCRIPTION

The landfarming sites almost all occur on crown lands in the Grey Wooded soil zone (luvisol) of west central Alberta. This soil zone is typified by a shallow, weathered topsoil horizon that covers a mostly clay subsoil with occasional seams of sand and gravel.

The landform is glacial in origin, with gentle-to-severe rolling topography. The forest cover ranges from typical aspen woodlands to boreal species of trees (pine, spruce, poplar).

The mean annual precipitation is approximately 22 in., and most of this generally occurs during the spring breakup period and early summer months.

There are 33 individual landfarm sites: 21 clustered in the Ricinus field, 8 in the Brazeau field, and 4 others at scattered, deep-hole exploration wells. Cuttings and mud residue volumes are approximately 260-390 cu yd per location in the Ricinus field, 390-520 cu yd in the Brazeau field, and 590-910 cu yd in the four other locations.

Most of these wells are on production and have an average location size of approximately 4 acres. Roughly one third to one half of each well site is available for the landfarming operation.

Thus, the average ratio of cuttings-to-surface area is roughly 240 cu yd/acre. This is equivalent to a layer of cuttings 13/4-in. deep, but this varies somewhat from site to site.

LANDFARMING OPERATION

Amoco Canada's standard invert diesel mud system is based on a one-to-four, water in oil emulsion. The oil component is No. 2 diesel fuel, whereas the water phase is a CaCI2 brine with chloride ion levels in the order of 200,000 mg/I. The fluid also contains emulsifiers and wetting agents (surfactants) and may contain lime and other additives in small quantities.

The invert-mud landfarming operation consists of several steps:

Invert fluid and water that has drained from the cuttings pile is first treated and removed for off-lease disposal. The cutting pile is diked, so this fluid phase tends to be mostly rainwater. A conventional, activated carbon/flocculent treatment is generally used.
The oil-contaminated cuttings are then spread as thinly as possible over the designated landfarm area with a bulldozer. A general guideline is to keep the layer of cuttings less than 2-in. thick. However, this is not always possible because wet cuttings can be difficult to spread uniformly, and the available surface area is sometimes limited.
The previously stockpiled, stripped surface material (topsoil and humus layer) is then bulldozed to mix in with the cuttings. The area is cultivated with a set of discs or a tractor-mounted tiller (Fig. 1). At this time, highnitrogen fertilizer is broadcast and mixed into the cuttings and topsoil layer. If a soil conditioner or bacterial culture such as manure is to be added, it would also be introduced and spread at this time.

The three objectives of this operation are to maximize the surface contact between the cuttings and soil bacteria, to aerate the soil/cutting mix to promote aerobic decomposition, and to boost the soil microbe count by providing additional limiting nutrients in the form of high nitrogen fertilizer.

These are the same general principles applied to treating small oil spills or making a compost heap in one's backyard. The findings are consistent with those of Scroggins, et al.1

The cultivate/fertilize cycle is repeated as often as required, generally twice a year for 2 or 3 years. The kind of fertilizer and recommended application rate is determined by a regular soil sampling and analysis program. The two most frequently used fertilizers are 34-0-0 and 11-510. Fertilizer application rates vary a great deal but have generally been in the order of 1,000 lb/acre.

Once the analytical results demonstrate that the oil-to-soil ratio has dropped to the 1-2% range, the site is given a final treatment, and a suitable grass seed mixture is sown. The most common seed mixture consists of 40% Creeping Red Fescue, 24% Climax Timothy, 24% Canada Blue Grass, and 12% White Dutch Clover.

MUD SOLIDS CONTROL

Amoco Canada's drilling department has placed an emphasis on "closed-loop" drilling .2 3 Closed-loop drilling has resulted in a substantial reduction in the amount of invert fluid dumped with the drill cuttings.

Fig. 2 shows a typical invert cuttings and fluid pit prior to the introduction of the advanced solids control.

Initial modifications to the mud system involved:

Increasing screen mesh size in the shale shakers
Substituting two or more centrifuge units for the desander/desilter
Continuously agitating the shaker tank.

Fig. 3 is a typical surface pit containing invert-contaminated cuttings after adopting better mud cleaning systems and closed loop drilling.

Other modifications and in situ treatment systems are also under evaluation. These efforts produce real dividends during the cleanup and landfarming operations because the reduction in total invert-fluid volumes left on site is a key factor in reducing bioremediation time.

The present time frame to reclaim a site completely is 24 years. Improved solids control at the rig should cut the recovery time by as much as one half to reduce costs and to minimize any potential negative impacts of invert-fluid landfarming.

SOIL SAMPLING

Soil sampling was initiated in 1986 at several sites in the Brazeau area and is currently conducted at 33 sites in the Brazeau and Ricinus areas on an annual basis. Landfarming sites are divided into two to four sections for composite soil sampling, and a minimum of 15 soil cores are taken in each section to make a composite sample. The intent is to sample soil at similar locations each year.

The depths of soil sampling are 0-6 in. and 6-12 in., with a few sites sampled at 12-18 in. A composite soil sample is obtained for each depth and section of the landfarmed area. A control sample is also taken for comparison with soils analyzed from the landfarm area.

Composite soil sampling from each section is important because it is difficult to evenly spread the spent invert mud residues and cuttings by mechanical means. The concentration of total hydrocarbons or soluble salts will vary on the site because of the limitations of mechanical spreading. By division of a site into two to four sections, treatment can be directed specifically to each section.

Each soil sample is analyzed for the following parameters: pH, total hydrocarbons, electrical conductivity, saturation, sodium absorption ratio, calcium, magnesium, sodium, sulfates, chlorides, theoretical gypsum requirement, nitrogen, phosphorous, and potassium.

From these analyses, fertilizer recommendations are determined. At present, government guidelines and regulations do not specify the parameters for the soil analysis.

INITIAL RESULTS

The pH of the soil has typically averaged in the 6.4-7.4 range. For most of the landfarming sites, the pH of the soil has been very close to 7. At a few sites, the soil pH has ranged from 5.1 to 5.7 because of the kind of soil and the characteristics of the area.

Before modifications were initiated at the drilling rig and the appropriate spreading depth of mud residue and cuttings was determined, the total hydrocarbons in the soil in the first year was much greater than 1%. Table 1 shows the hydrocarbon percentage over time for various sections of the Ricinus and Brazeau fields.

The total hydrocarbons in the 0-6 in. depth for Amoco Brazeau 10-28-45-14-W5M has decreased from 7.37% to 1.45% in the first year and ultimately to 0.58% in 1990. Fertilizer applied twice yearly in 1987 and 1988 and extensive cultivation account for this rapid decrease in hydrocarbons.

Several landfarming sites, such as Amoco Ricinus 5-3432-7-W5M and Amoco Ricinus 10-2-34-8-W5M, had total hydrocarbons in the 0-6 in. depth in the first year of 1.97% and 3.29%, respectively. After three fertilizer treatments and cultivation, the hydrocarbon content dropped dramatically to approximately 0.14% and 0.39%, respectively.

This rapid decrease in the hydrocarbon content is primarily a result of the addition of nitrate fertilizers that assist in increasing soil bacteria.

Many landfarming sites have a total hydrocarbons content of less than 1 % in the first year. Amoco Ricinus 1434-32-7-W5M is shown in Table 1 to have a total hydrocarbon content in the 0-6 in. depth of 0.72% in 1988, 0.2% in 1989, and 0.13% in 1990.

Amoco Ricinus 10-28-348-W5M shows a similar decrease in hydrocarbons. The oil content in the shale cuttings has been greatly reduced by the use of centrifuges and modifications to the shale shaker screens. This has resulted in an initial lower concentration of oil.

The soil analyses for the 33 sites shows that hydrocarbons are not migrating or leaching. Initially, the hydrocarbon content in the 0-6 in. and 6-12 in. depths may be similar, depending upon the depth of cultivation.

However, the maximum hydrocarbon content is normally found in the shallower depth of 0-6 in. The hydrocarbon content does not appear to increase in the 6-12 in. or 12-18 in. depths over time. Other studies have also indicated that hydrocarbons are not migrating or leaching.4

There is a wide range of electrical conductivity for the sites, with some initial values reaching 66.7 millisiemens/cm. Table 2 shows the range of electrical conductivity for landfarming at well sites Brazeau 10-28-45-14-W5M, Ricinus 5-34-32-7-W5M, and Ricinus 10-28-348-W5M. Calcium chloride and other soluble salts are factors contributing to the wide range of electrical conductivity. The trend has been for the electrical conductivity to decrease rapidly in both the 0-6 in. and 6-12 in. depths. For many sites, the electrical conductivity is well below 5 millisiemens/cm in the second or third year and very similar to that of the control soil samples.

Table 3 shows a wide range of chloride levels for the first year of landfarming. Chloride levels in the first year may vary from several hundred to several thousand parts per million. As a result of leaching and an abundance of rain in 1988-90, chloride concentrations dropped significantly each year. Overall sodium concentrations have not been a problem. However, in areas where sodium levels have been excessive in comparison to the control site, gypsum has been added to alleviate the problem.

There have been elevated levels of magnesium, calcium, and sulfates in comparison to the control sites. However, no major problems with landfarming have been encountered as a result. Amoco Canada is in the process of setting up groundwater monitoring wells at four sites to determine movement of leachates.

RECLAMATION TIME

The percentage of total hydrocarbons tends to be the limiting factor for the reclamation. Sites with a higher hydrocarbon content of 3-7% may take 2-4 years for reclamation. Many sites with an initial hydrocarbon content of 0.5-2% have been successfully revegetated in the second growing season.

Overall, once the hydrocarbon content in the soil is 1% or less and chlorides are less than 1,000 ppm, the site can be revegetated successfully.

Fertilizer is added at least once per year, and some sites receive two fertilizer applications per year. The addition of fertilizer containing nitrates and phosphorous has been a successful strategy.

Manure may be added to landfarming sites, and previous field studies indicate this would be beneficial .5 The addition of straw is beneficial in areas of very little topsoil and especially where the topsoil is poor. The addition of sewage lagoon material is being assessed to determine if this would expedite the reclamation process. The objective is to increase the number of bacteria to enhance breakdown of the hydrocarbons.

COSTS

Thus far, the landfarming operation as outlined has proven to be a very cost-efficient method of handling invert cuttings. The average landfarming costs, beyond normal site cleanup costs, are approximately $7,000 per location. This includes the landfarming operation, the soil sampling program, and the lab analysis.

This compares very favorably with other disposal options currently used in Alberta. Based on recently conducted field trials with fixation and incineration, it is estimated that costs for these methods may range from $35,000 to $50,000.

LIMITATIONS

Government regulatory bodies in Alberta have given the oil industry a somewhat cautious green light for this disposal procedure. There have not yet been any serious problems or shortcomings. At this point, it appears to be a satisfactory method of handling invert cuttings.

However, there are some limitations:

The oil/soil ratio must be less than about 1 % by weight to establish a satisfactory vegetative cover. If there is a restricted area available to spread the cuttings, if there is a large volume of cuttings, or if there is a high ratio of oil to cuttings, the site may take a very long time to reclaim.
Volatile organic compounds may evaporate from the farmed cuttings. This has not been of great concern because most landfarm sites are not near any residences. Furthermore, only trace amounts of volatile organic compounds may be involved.
Salt loading of the soil from the brine phase of the diesel invert mud residue and rock cuttings does not appear to be a problem. Regulations stipulate that no fluids with a chloride concentration above 1,000 ppm may be disposed off site. Landfarming operations are conducted on the well site to comply with these regulations. However, the soil analysis indicates that the CaCI2 quickly leaches out. There has not yet been any detection of observable detrimental effects that would suggest salt damage on either the surrounding forest or the new grass crop.
Possible groundwater contamination is a concern, although there have been no obvious indications that this is having a significant impact. This aspect of the operation shall be further investigated through a groundwater monitoring program in the near future.
Lack of topsoil or humus material to mix with the drilling waste can be a limiting factor. Extra effort made during well site construction to conserve organic material is more than made up for during the landfarming operations. Trucking in and spreading a soil conditioner such as manure is an expensive option.

To date, Amoco Canada's landfarming of oil-based drill cuttings has shown positive results. A key factor to expedite the process is the reduction of invert residue on the cuttings through improved solids control during drilling operations.

There have been no noticeable negative environmental impacts, but studies continue to try to determine the implications of leaching. The revegetation of many sites shows that landfarming is an acceptable disposal option for oil-based drill cuttings.

REFERENCES

Ashworth, J., Scroggins, R.P., and McCoy, D., "Feasibility of Land Application as a Waste Management Practice for Disposal of Residual Diesel Invert-based Muds and Cuttings in the Foothills of Alberta," International Conference on Drilling Wastes, Calgary, 1988.
Braun, B., "Invert Mud Systems in Amoco Ricinus Field," November 1988.
Braun, B.. "Revised Solids Control Hookup in Amoco Canada's Ricinus Field," November 1988.
Ashworth, J., Scroggins, R.P., and McCoy, D., "Landfarming Invert Cuttings from Sour Gas Wells in the Rocky Mountain Foothills," APCA: The Air & Waste Management Association, Chemicals in the Environment Conference, Whistler, B.C., Nov. 9-11. 1988.
Ashworth, J., "Field Study to Assess the Feasibility of Disposing of Diesel Invert-Based Cuttings Residues Using Land Application," draft copy, October 1989.