Fact Sheet - Using Muds and Additives with Lower Environmental Impacts
Introduction to Drilling Muds
Drilling fluids or muds are made up of a base fluid (water, diesel or mineral oil, or a synthetic compound), weighting agents (most frequently barium sulfate [barite] is used), bentonite clay to help remove cuttings from the well and to form a filter cake on the walls of the hole, lignosulfonates and lignites to keep the mud in a fluid state, and various additives that serve specific functions.
Historically, the drilling industry has used primarily water-based muds (WBMs) because they are inexpensive. The used mud and cuttings from wells drilled with WBMs can be readily disposed of onsite at most onshore locations. WBMs and cuttings can also be discharged from platforms in many U.S. offshore waters, as long as they meet current effluent limitations guidelines (ELGs), discharge standards, and other permit limits. WBMs do not present environmental problems for organisms living in the water column or widespread problems for organisms living on the sea floor. However, for difficult drilling situations, such as wells drilled in reactive shales, deep wells, and horizontal and extended-reach wells, WBMs do not offer consistently good drilling performance. For these types of drilling situations at onshore sites, the industry relies primarily on oil-based muds (OBMs). OBMs perform well, but may be subject to more complicated disposal requirements for onshore wells. OBMs contain diesel or mineral oil as the base fluid and may be harmful to the environment when discharged to the sea. Consequently, the EPA prohibited any discharge of OBMs or their cuttings from offshore platforms.
Synthetic-Based Muds (SBMs) Offer Alternative to Traditional Muds
In the 1990s, drilling fluid companies devised new types of muds that used nonaqueous fluids (other than oils) as their base. Examples of these base fluids included internal olefins, esters, linear alpha-olefins, poly alpha-olefins, and linear paraffins. SBMs share the desirable drilling properties of OBMs but are free of polynuclear aromatic hydrocarbons and have lower toxicity, faster biodegradability, and lower bioaccumulation potential. For these reasons, SBM cuttings are less likely than oil-based cuttings to cause adverse sea floor impacts. The EPA has identified this product substitution approach as an excellent example of pollution prevention that can be accomplished by the oil and gas industry. SBMs drill a cleaner hole than water-based muds, with less sloughing, and generate a lower volume of drill cuttings. SBMs are recycled to the extent possible, while WBMs are discharged to the sea.
New Drilling Fluid Systems
Drilling fluid companies are developing variations of fluid systems that are much more amenable to biotreatment of the subsequent drilling wastes (Growcock et al. 2002; Getliff et al. 2000). It is likely that companies will continue to develop fluids with suitable drilling properties that contain fewer components or additives that would inhibit subsequent break down by earthworms or microbes. In some circumstances, the constituents of the muds could actually serve as a soil supplement or horticultural aid.
Other developments in drilling fluids could lead to entirely different formulations. Drilling fluids based on formate brines have been suggested as being more environmentally friendly than traditional fluids. Formate brines are created by reacting formic acid with metal hydroxides. Common examples are cesium formate (HCOO-Cs+), potassium formate (HCOO-K+), and sodium formate (HCOO-Na+).
The primary environmental drawback to WBMs is that they require a high dilution rate to maintain desirable properties (on the order of 5 times the volume of the hole drilled) as compared to OBMs and SBMs, which require very little dilution. WBMs do not offer the same level of drilling performance and well bore stability as OBMs and SBMs. A new generation of WBMs has been developed and used effectively in the past few years. They have demonstrated improved drilling performance and significantly reduced dilution rates.
Alternate Weighting Agents
Substitution of some of the key components of drilling fluids with more environmentally friendly products could reduce mass loadings of potentially harmful substances to the environment. Barite is the most commonly used weighting agent. Other readily available weighting agents include hematite (Fe2O3) and calcium carbonate (CaCO3). Other wells have been drilled using ilmenite (FeTiO3) instead of barite as a weighting agent.
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.)
Getliff, J.M., A.J. Bradbury, C.A. Sawdon, J.E. Candler, and G. Loklingholm, 2000, "Can Advances in Drilling Fluid Design Further Reduce the Environmental Effects of Water and Organic-Phase Drilling Fluids?," SPE 61040, SPE International Conference on Health, Safety, and the Environment, Stavanger, Norway, June 26-28.
Growcock, F.B., G.W. Curtis, B. Hoxha, W.S. Brooks, and J.E. Candler, 2002, "Designing Invert Drilling Fluids to Yield Environmentally Friendly Drilled Cuttings," IADC/SPE 74474, IADC/SPE Drilling Conference, Dallas, TX, February 26-28.
Veil, J.A., 2002, "Drilling Waste Management: Past, Present, and Future," SPE 77388, SPE Annual Technical Conference and Exhibition, San Antonio, TX, September 29-October 2.