The cuttings separated from the mud at the shale shakers may be coated with so much mud that they are unsuitable for the next reuse or disposal step or are difficult to handle or transport. Constituents of the cuttings or the mud coating them (e.g., oil, metals) may leach from the waste, making them unsuitable for land application or burial approaches. Various materials can be added to cuttings to solidify and stabilize them. The processes of solidification and stabilization can be defined as follows:
Solidification refers to techniques that encapsulate the waste in a monolithic solid of high structural integrity. The encapsulation may be of fine waste particles (microencapsulation) or of a large block or container of wastes (macroencapsulation). Solidification does not necessarily involve a chemical interaction between the wastes and the solidifying reagents but may mechanically bind the waste into the monolith. Contaminant migration is restricted by vastly decreasing the surface area exposed to leaching and/or by isolating the wastes within an impervious capsule.
Stabilization refers to those techniques that reduce the hazard potential of a waste by converting the contaminants into their least soluble, mobile, or toxic form. The physical nature and handling characteristics of the waste are not necessarily changed by stabilization.
Types of Additives
Historically, cement, fly ash, lime, and calcium oxide have been used most frequently as solidification/stabilization additives for treating drill cuttings and other types of wet solids. A recent study tested seven other types of additives for stabilizing cuttings and assessing the performance of stabilized cuttings as a substrate for growing wetlands plants. These included: medium-ground mica-based material, fine-ground mica, three different commercial mixtures of recycled cellulose fibers, walnut nut plug, and pecan nut plug (Hester et al. 2003). Various other commercial products with proprietary compositions have been marketed.
Not all drilling wastes are amenable to chemical fixation and stabilization treatments. Solidification/stabilization should be adapted for site-specific applications depending on the end-use of the treated material and the chemical characteristics of the waste. Conducting laboratory tests to determine the proper blend of additives to achieve the desired material properties is recommended.
Some companies have used solidification/stabilization for drilling wastes. The resulting materials have been used for road foundations, backfill for earthworks, and as building materials (Morillon et al. 2002) and may be used for other purposes (BMT Cordah Limited 2002).
In contrast to these examples, others (in particular, ChevronTexaco, one of the companies that partnered with Argonne to develop this website) have tested different additives and found that they either did not achieve the desired goals once the solidified or stabilized wastes were placed into the environment or that the cost of using the additives was prohibitive. Most of the solidification/stabilization systems produce conditions both of high pH and high total alkalinity. Much concern has been expressed about the long-term stability of the processes. Of greatest concern is the failure of the additives to keep the waste constituents from releasing into the environment over the long term or the sudden release of contaminants due to breakdown of the matrix. No long-term data are available because the technology has only been practiced for about 20 years, although ChevronTexaco has tested about 8 different commercial products and found that all failed leachate testing (Fleming 2000).
A solidified waste is an amorphous solid at least partially saturated with water. It consists of one or more solid phases, entrapped air in the form of air voids, and a liquid phase; all are in chemical equilibrium or close to it. When the solid is exposed to leaching conditions, equilibrium is disturbed. The leaching mechanisms assume that no chemical reactions occur (other than those involved in dissolution of the constituents in the solid). While this might be true in laboratory leaching tests, it is not the case in the real environment. Rain, surface water, and ground water all contain constituents that may increase or decrease the leaching rate (e.g., redox potential, pH, anions such as carbonate, sulfide and silicate, organic chelating agents, and adsorptive particulates).
There are limitations on the applicability of stabilization/solidification systems. For example, cement-based systems do not work when:
- the organics content is above 45% by weight,
- the wastes have less than 15% solids,
- excessive quantities of fine soil particles are present, or
- too many large particles are present.
As noted above, the most commonly used additive materials have a high pH, which can pose a problem if the stabilized wastes are subsequently land-applied or used as a soil supplement. In a series of studies to test the suitability of using treated cuttings to grow wetlands vegetation, researchers at Southeast Louisiana University discovered that cuttings stabilized in a silica matrix had a pH higher than 11. The stabilized cuttings did not support plant growth as well as unstabilized cuttings (Shaffer et al. 1998).
By adding other materials to the drilling waste, the overall volume of stabilized wastes is larger than the original volume of unstabilized waste. Depending on the amount of additive that is required to meet the applicable waste quality standards, the cost of additive may not be cost-effective.
Because of the equipment and space requirements for solidification/stabilization, this is not a practical process to use at offshore locations. It can be used at onshore locations for either onshore drilling wastes or offshore wastes that have been hauled back to shore.
BMT Cordah Limited, 2002, "Use of Oily Drilling Wastes in Civil Engineering Applications, " report no. COR027/Programme/2002, September.
Fleming, C., 2000, "A Discussion of: Chemical Fixation and Solidification (CFS), Solidification/Stabilization, Microencapsulation, and Macroencapsulation, " prepared for the API/NOIA Synthetic Based Fluids Research Group, June 15.
Hester, M.W., G.P. Shaffer, and J.M.Willis, 2003, "Wetland Restoration Mesocosm Studies: Examination of Hematite-based Drilling Mud Phytotoxicity and Suitability as Wetland Substrate, " prepared by the University of New Orleans and Southeastern Louisiana University, for the U.S. Department of Energy, National Petroleum Technology Office, March.
Morillon, A., J.F. Vidalie, U. Syahnudi, S. Suripno, and E.K. Hadinoto, 2002, "Drilling and Waste Management, " SPE 73931, presented at the SPE International Conference on Health, Safety, and Environment in Oil and Gas Exploration and Production, Kuala Lumpur, Malaysia, March 20-22.
Shaffer, G.P., M.W. Hester, S. Miller, D. J. DesRoches, R.F. Souther, G.W. Childers, and F.M. Campo, 1998, "Restored Drill Cuttings for Wetlands Creation: Results of a Two-Year Mesocosm Approach to Emulate Field Conditions under Varying Hydrologic Regimes, " DOE/BC/14849-3, prepared by Southeastern Louisiana University Wetlands Restoration Laboratory, Hammond, LA, for U.S. Department of Energy, National Petroleum Technology Office, November.