Thread Compounds + Environment = Change
Authors: McDonald Herschel B. 
Published: December 2007

Introduction

Environmental awareness, concern, and regulation have affected virtually every industry in the last several years, and the oil patch is no exception. Changes in the methods required for storage, use, and disposal of materials ranging from pipe coatings to drilling muds have spawned a flood of new products that are more environmentally acceptable. These new products not only differ in composition but also can exhibit substantially different performance properties. The change in performance properties will require changes in operational practices that have been in place for decades. Nowhere is this more true than for thread compounds.

Thread Compound Background

Historically, compounds used for both drilling and production applications have consisted of a petroleum lubrication grease containing a substantial percentage of heavy metals (lead, copper, zinc, or a combination of the three).The metals are present in a finely divided form as a powder or flake and may make up as much as 60 wt % of the thread compound. These metals have been used since the early days of the industry because their unique material properties provide two main performance requirements of a thread compound:protection against metal/metal contact and galling at high connection bearing stresses andthe sealing of potential leak paths at high internal pressures.

Lead, copper, and zinc are malleable and ductile. These properties allow them to compact and deform without fracturing, thereby forming an effective seal either in the thread form of the casing or tubing or between the shoulders of rotary connections. Their ability to prevent wear and damage of ferrous metal surfacesis evidenced by their wide use in a variety of bearing materials.

Environmental Factors

Unfortunately, restrictions on the use and disposal of materials containing heavy metals have and will continue to become increasingly severe. U.S. Environmental Protection Agency (EPA) guidelines for determining whether a waste material is hazardous include lead and several other heavy metals on the list of toxic substances. If the defined test procedure (the Toxicity Characteristic Leaching Procedure) indicates that any substance on the toxicity list is present in an extract solution above the stated limits. the material is considered a hazardous waste and must be disposed of according to the appropriate regulations. Zinc currently is not on the list but most likely will be added within the next year. Copper also is not listed and probably will not be for the near future. Both zinc and copper, however, are listed for a similar test procedure required by the State of California. Other areas of concern, besides disposal of waste material residues and contaminated containers, are not covered specifically by EPA regulations. Some examples are contaminated effluent from hydrostatic testers that may be discharged to a sanitary sewer system, rain-water run-off from pipe yards and threading facilities, and contaminated mud pits and surrounding soil in field operations. These situations are, for the most part, subject to state and local regulations that can vary considerably, depending on the location. State and federal regulations based on marine biotoxicity and other environmental-impact properties also restrict discharges from offshore operations. Clearly the composition of thread compounds must and will continue to change as environmental regulation increases and other substances are added to the toxicity lists.

Performance Considerations

What effect do the changes in thread compound composition have on performance properties and operational practices? The answer is complex. The compounds used for the past 40 or 50 years have been fairly generic with well-defined categories sharing similar performance properties. The tables of recommended torque values and operational practices that the API and pipe manufacturers publish are based on the use of these compounds. The new compounds that have appeared recently are of proprietary formulation and can vary widely in both composition and performance properties.

Drilling Compounds. Nonmetal drilling compounds rely on such materials as graphite, nonheavy metal oxides or other solid lubricants, and extreme-pressure additives to provide protection against connection galling. Most nonmetallic solids, however, fracture under applied stresses and do not compact and deform into a continuous gasket-like seal between the shoulders as do powdered metals. Seal failure can result in connection damage from washout caused by high-solids drilling muds under pressures as high as 6,000 psi. Because of the malleability of the metal powder particles, high metal compounds also are more effective than nonmetallics at uniformly distributing bearing stresses (60,000 psi in drillcollars) around the connection shoulders. This is particularly critical if the shoulders are not exactly parallel because of wear or substandard field reworks. Uneven distribution of shoulder bearing stresses can cause pin fatigue failures and reduce drillstring life.

JPT

P. 614^