Drilling fluid type affects elastomer selection

Oct. 26, 1998
One of the damaging results of mud/elastomer incompatibility is a "chunking" effect. Note that chunks of the stator's elastomer have broken loose, impairing the surface (Fig. 1[17,210 bytes]). This drilling-motor stator has failed because of hysteresis (the conversion of mechanical energy into heat) generated by cyclic pressures in the wellbore. New synthetic drilling fluids contribute to the hysteresis (Fig. 2 [8,942 bytes]). Because of drilling-fluid/elastomer incompatibility, the
Venu Bodepudi
Halliburton Energy Services Inc.
Houston

J. Michael Wilson
Halliburton Energy Services Inc.
Duncan, Okla.

Arvind Patel
M-I LLC Drilling Fluids Co.
Houston

Thorough research and field studies, coupled with effective communications among interested parties, can help operators find the best elastomer for use in drilling operations.

Because of increasingly stringent environmental standards, the oil and gas industry has developed more-environmentally friendly, synthetic-based drilling fluids as alternatives to conventional oil-based muds (OBMs). Some of these synthetic-based muds (SBMs), however, are incompatible with the conventional elastomers used in downhole equipment and drilling tools-a situation that can impair elastomer performance and result in costly, premature failures.

Currently, researchers are examining the relationships between elastomer design and SBM formulation to find the most successful correlation between formation needs, drilling fluids, and elastomers.

Ultimately, however, the real solution to the incompatibility problem may be found not only in this ongoing research, but also in cooperative efforts among the various contractors directing the onsite operations.

The compatibility dilemma

In recent years, complex and diverse environmental regulations have affected the logistics and economics of drilling operations. 1 As a result, the oil and gas industry has developed an array of synthetic drilling fluids that comply more closely with environmental standards than conventional OBMs.

Although the synthetic fluids may be biodegradable and less toxic, some are incompatible with downhole elastomers. Particular components in SBMs and their additives can often chemically and thermally degrade elastomers by altering such elastomer properties as volume (swelling or shrinking), elongation, hardness, tear strength, and tensile strength.2 Such occurrences usually result in costly downtime and parts replacement.

With the ever-growing selection of SBMs and additives available for use in these new drilling fluids, the drilling contractor has an almost unlimited number of formulation possibilities. This situation benefits drilling companies because they can further customize the mud to fit the formation's unique conditions.

However, when this broad spectrum of SBM designs is combined with the wide range of elastomers used in oil field equipment, compatibility between the fluids and elastomers becomes more complicated.

Downhole conditions such as bottom-hole circulating temperatures and intrinsic formation pressures can further promote damaging effects.

SBMs vs. OBMs

In many respects, oil-based muds have distinct advantages over water-based muds. They can furnish borehole and thermal stability, inhibit corrosion, withstand contamination, and reduce maintenance requirements. In contrast, their use results in higher initial and operational costs.

In addition, these muds contaminate byproducts, such as cuttings. Therefore, with these muds, operators must use expensive measures to comply with environmental mandates.1 During the past decade, drilling fluids have been designed that perform much like diesel and mineral OBMs without the associated disadvantages. These developments have resulted in a variety of new muds based on synthetic, organic materials.1

Specific base-fluid selection is based on several factors-toxicity, biodegradability, thermal stability, viscosity, cost, required performance, and type of application. SBMs are classified according to the chemical composition of the synthetic-based fluids (SBFs) used in their formulation.

In a synthetic drilling fluid, the aqueous phase (water or brine) is emulsified in the continuous phase (the SBF). Trends indicate that synthetic fluids are typically more biodegradable under both anaerobic and aerobic conditions, and that they are much less toxic, resulting in fewer handling hazards than conventional OBMs.1

Synthetic-based drilling muds differ in their designs, but the synthetic-fluid components generally account for 70-95% of the blend, with water comprising the remaining percentage. These components include such materials as esters, ethers, olefins,3 and other synthetic-organic fluids.

Olefins include such materials as linear alpha olefins, isomerized olefins, polyalpha olefins,3 and linear paraffins.

Compared with mineral and diesel oils, which are purified through distillation, SBFs are generated through reactions with certain purified chemicals. Each fluid features a specific molecular weight and unique structure.1

Elastomers

Each type of elastomer contains 15-25 ingredients. The compounds used in drilling and downhole tools may include nitriles, hydrogenated carboxylated nitrile blends, hydrogenated nitriles, hydrogenated carboxylated nitriles, epichlorohydrins, chlorinated elastomers, 2 and fluorocarbons. These materials exist in a variety of oil field equipment, such as blowout preventers, pulsation-dampener bladders, drill motors, O-rings, boots, and packers.

Because each tool application requires specific elastomer properties, diverse designs are necessary. For example, an annular BOP calls for moderate rubber-strength levels but extremely high-tensile elongations, while certain BOP rams require superior tensile strengths combined with relatively low-tensile elongations.2

Elastomer damage

At one time, elastomers used in oil field drilling tools were tailored exclusively for oil-based or water-based drilling fluids. With the introduction of synthetic muds, however, toolmakers and operators began to examine the impact of SBMs on equipment.

They found property changes in the elastomers that would cause the oil tools to fail within their design limits.2 To prevent premature failure in a drilling operation, all parties should know the adverse effects that new SBMs could have on elastomeric sealing elements.1

In addition to the swelling and "chunking" effects mentioned earlier (Fig. 1 [17,210 bytes]), synthetic drilling fluids can induce hysteresis, a conversion of mechanical energy into heat caused by internal friction produced during cyclic loading (Fig. 2 [8,942 bytes]).4

These fluids can also destroy the bond between the elastomer and the tool component (Fig. 3 [14,418 bytes]). They can also cause the elastomer to split (Fig. 4 [16,123 bytes]). Furthermore, elastomers are susceptible to such forces as:

  • Cyclic loading
  • Leaching of plasticizers, causing components to shrink or become brittle
  • Chemical attacks by such downhole agents as sodium silicate and other high-pH fluids.
These attacks cause the elastomer chains to break.

Whenever a "kick" occurs in a well, high-pressure elastomeric sealing elements such as those found in packer elements and blowout-preventer rams must protect against high well pressures.1 If the elastomer's sealing integrity becomes impaired, extremely hazardous situations and well failure can result.

However, the SBMs' capability to mechanically alter elastomers is not limited exclusively to downhole tools. The drilling equipment itself may also be affected. For example, the positive displacement motor (PDM) is used in a wide range of drilling applications. This increasingly popular drilling tool has a dual-section design that combines a rotor with a helical cavity or passageway composed of an elastomeric material.4

Drilling programs that incorporate equipment such as PDMs appear to be simple, but are complex and costly to implement when fluid compatibility is undetermined. For these systems to be effective and cost-efficient, the equipment and materials should be compatible with each other and the comprehensive drilling plan.4

Through advances in motor technology, most PDMs now have an increased service life; however, synthetic fluids could reduce PDM service life by degrading the stator elastomer compounds.

Although reliable service histories for PDMs are not generally available, studies have revealed the kinds of problems that result from such incompatibilities.4

One common type of failure is the "chunking" of the stator section (Fig. 1), which is indicated when chunks of rubber appear in the drilling-fluid returns from the well bore. Other problems include severe swelling and changes in mechanical and physical properties. The swelling can reduce the clearance between the stator and rotor. This tighter fit, especially when combined with a change in the elastomer's hardness, can cause the stator lobes to "chunk," resulting in PDM failure.

One solution to the problem is to use PDMs with undersized stators that can then be expanded to the normal operating tolerance. However, this solution is expensive, and can result in diminished motor performance.

No single elastomeric material exists that can withstand all types of drilling fluids. Therefore, when one drilling fluid is used with multiple PDMs and stators made from different compounds, equipment will sustain damage and one or more of the stators will eventually fail.

Laboratory research

To solve incompatibility problems, researchers are studying both drilling fluids and elastomers. The researchers' major focus is to develop newer and better elastomers and drilling fluids.

Through testing and the process of selectively formulating various polymer blends, the chemical and mechanical properties of the elastomers are modified, resulting in more extensive and effective choices to the industry. Researchers are altering the molecular structure and molecular weight of SBFs to enhance drilling-fluid properties such as viscosity, polarity, solvency, low toxicity, and biodegradability.1

This research also examines, quantifies, and monitors the effects of SBMs and other field-related drilling factors on elastomers. When operators, drilling contractors, and service companies pre-analyze and select the proper fluids and elastomers, they help save time and expense and can reduce the risk of downhole failures by a substantial margin.

In studies with simulated downhole conditions, researchers periodically conduct elastomer tests that extend up to 200 hr in small time increments. The researchers examine such dynamic and mechanical properties as chemical resistance, strength, and reaction to ordinary wear.

By quantifying the test material's degradation resulting from a particular synthetic drilling fluid, laboratory personnel can predict the outcome of the job.

In one recent series of tests, 14 muds of various types-oil-based and water-based, as well as isomerized olefins and other kinds of synthetic-based fluids-were tested. Selected examples of this testing are shown in Figs. 5-8.

Fig. 5 [119,106 bytes] shows an example of excessive swelling and incompatibility with the tested elastomers. In this example, motor failure is inevitable. The predicted failure mode would be primarily chunking and possibly debonding. This fluid indicates the need for a new elastomer.

Fig. 6 [121,061 bytes] shows the effects of a popular ester-based drilling fluid and Fig. 7 [121,741 bytes] shows a popular internal olefin (I/O)-based drilling fluid. The elastomers in the ester-based fluid are expected to degrade faster than those in the I/O-based fluid.

The ester fluid tends to solvate, swelling the tested elastomers faster than the I/O-based fluid. While either of these fluids can be used, the lifetime of PDMs in the ester-based fluid would be considerably shorter than the I/O-based fluid.

Fig. 8 [122,363 bytes] shows a clear example of a given elastomer's compatibility with the specific drilling fluid. In this case, the XNBR-HNBR40 elastomer is used in the drilling motor assembly, leading to a much improved lifetime (200%).

Comprehension, communication, cooperation

While today's synthetic-based drilling fluids successfully solve environmental concerns, they often introduce new concerns downhole because they alter the structural composition and mechanical properties of the elastomers used in oil field equipment. Impaired sealing elements can fail suddenly and unpredictably,4 which can lead to hazardous conditions and/or well shutdown.

Research conducted on both SBMs and elastomers is intended to provide compatibility without compromising performance. Although laboratory tests are valuable, the ideal solution to the incompatibility problem may be obtained through better cooperation and communication among the operator, drilling fluid company, and supplier of the elastomer. This sort of alliance would minimize the risk of failure and economically benefit the producer.

Such a cooperative effort was staged with a major oil company. The service company supplying the elastomer compounds used in the drilling-motor stators conducted a series of tests to determine the optimal rubber compound for these stators. Three stator elastomer compounds were evaluated for compatibility and property retention after being heat-aged in diesel-based drilling fluid.

In addition to testing the compounds' tensile, tear, and compressive strengths, researchers measured density and volume changes and conducted dynamic property evaluation and adhesive-peel tests.

As a result of this study, researchers were able to select the elastomer compound that could withstand the effects of the drilling fluid without failure; these measures resulted in substantially reduced drilling costs.

References

  1. Patel, A.D., "Choosing the Right Synthetic-Based Drilling Fluids: Drilling Performance Versus Environmental Impact," paper SPE 39508 presented at the First SPE India Oil & Gas Conference & Exhibition, New Delhi, Feb. 17-19, 1998.
  2. Badrak, R.P., "Effects of New Generation Drilling Fluids on Drilling Equipment Elastomers," paper SPE/IADC 27452 presented at the SPE/IADC Drilling Conference, Dallas, Feb. 15-18, 1994.
  3. Fleming, C., Wardall, T., and Caveny, B., "Effects of Environmentally Friendly Drilling Fluids on Hydrating Cement," paper presented at the 20th International Conference on Cement Microscopy, Guadalajara, Mexico, Apr. 19-23, 1998.
  4. Kubena, E. Jr., Ross, K.C., Pugh, T., and Huycke, J., "Performance Characteristics of Drilling Equipment Elastomers Evaluated in Various Drilling Fluids," paper SPE/IADC 21960 presented at the SPE/IADC Drilling Conference, Amsterdam, Mar. 11-14, 1991.

The Authors

Venu Bodepudi is group leader of the Elastomers Group of Halliburton's Houston Technology Center, where he has served as a senior polymer engineer since 1995. In this capacity, he has developed and helped implement new formulations for stator applications. Before joining Halliburton, Bodepudi was employed with a number of other firms, including Lockheed-Martin Corp., where he worked on the NASA space shuttle structural composites and cryogenic insulations, and Uniroyal. He earned a bachelor's degree in chemistry and physics at the Detroit Institute of Technology and a bachelor's degree in chemical engineering with an emphasis on polymers at the University of Detroit, where he also worked as a research assistant. Bodepudi has done graduate work in polymer science at the University of Detroit and the University of Akron. He holds a patent for the processing of composites, with another patent pending on the processing of high-temperature composites.
J. Michael Wilson is a principal scientist in the zonal isolation group of Halliburton Energy Services Inc.'s Duncan Technology Center. He is also the technical liaison to M-I LLC. During his 22 years with Halliburton, Wilson has served as a group leader in analytical research and as technical manager for various departments including zonal isolation and water and sand control. Wilson holds a doctoral degree in organic chemistry and master's and bachelor's degrees in chemistry, all from the University of Oklahoma. Wilson performed post-doctoral work at Texas A&M University synthesizing potential antitumor agents for cancer research. He holds nine patents pertaining to the petroleum industry.
Dr. Arvind Patel is a senior research scientist for the product development section of M-I LLC Drilling Fluids Co.'s research and development division. He has served as a research scientist for 25 years, the last 17 of those for M-I. Patel earned a bachelor's degree in chemistry at Baroda, India; a master's degree in organic chemistry at Stephen F. Austin State University; and a doctoral degree in organic chemistry at North Texas State University.

Patel has taught at Texas A&M University, conducted cancer studies at a research institute in Philadelphia, and performed post-doctoral research. He holds 21 U.S. patents for drilling fluids.

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