Multi-functional solid lubricant reduces friction/prevents mud loss
Resilient graphitic carbon (RGC) has the property of “resiliency”, induced by process into graphite carbon particles, combined with lubricity to minimize lost circulation, differential sticking, torque and drag and casing wear in water-, oil- or synthetic-based mud
This article discusses a multi-functional product that has been successfully used in drilling fluids and oil well cementing operations in more than 200 wells in Louisiana, Texas, Oklahoma, the Gulf of Mexico and the North Sea. RGC is composed of 50- to 80-vol.% graphite and can reduce well costs by:
1. Controlling loss of circulation in pores and microfractures
2. Preventing seepage loss and incidents of differential sticking
3. Reducing torque and drag in tight holes and in landing casing
4. Reducing casing wear by physically separating sliding metal surfaces.
RGC will not plug mud motors at high concentrations, is non-toxic, nonabrasive and does not significantly disintegrate in drilling fluid at high shear. Also, it does not contribute to surface sheen development under the protocol of the U.S. EPA; it does not affect yield point or gel strength; and it is compatible with fibers and mineral blends.
Important studies/reports representative of those addressing mud loss in recent times include: 1) Gin-Fa Fuh and associates using a properly sized “Loss Prevention Material” (LPM)(1); 2) Whitfill et al. employing calcined petroleum coke with a very specific particle size distribution(2); and 3) Kubena et al. blending various polymers and petroleum coke for borehole stabilization and prevent lost circulation.(3)
Newhouse recommended a Permeability Plugging Apparatus (PPA) to prevent differential sticking of drill pipe, casing and logging tools and to relieve tight hole conditions.(4)
White and Dawson investigated casing wear using non hard-banded tool joints.(5) Data proved that casing-metal removal is related to the amount of energy dissipated as friction in the wear process and that a good quality lubricant will likely reduce the coefficient of friction in a drilling fluid, but is unlikely to reduce wear. For example, cutting oil used in machine shops will reduce tool chatter and give a smooth cut, but do nothing to prevent metal removal. A similar effect is suspected downhole when a liquid lubricant is added to mud. This view of wear as a problem is supported by the work of Bruno Best.(6)
[TABULAR DATA FOR TABLE 1 OMITTED]
Table 2. Resiliency comparison
petroleum Calcium Cellulosic
RGC coke carbonate fibers
Resiliency, % 100-150 20 0 0
Test procedure: Superior Graphite method 17017 resiliency test
A solid lubricant physically separates surfaces; a liquid lubricant cannot. Moreover, a solid lubricant is particularly effective in reducing hard-faced tool joint wear, liner running, and aids in casing rotation and placement, Fig. 1.
ENGINEERING PROPERTIES OF GRAPHITE
Natural graphite occurs worldwide and differs from coal or diamond carbon by virtue of its laminar, hexagonal, crystalline structure, inset Fig. 1. Graphite is classified based on its formation mode: flake, lump and amorphous. Natural amorphous graphite is commonly used in drilling fluids and is about 70-w/w% pure. Graphite is the softest mineral in the world, Table 1. This property is due to carbon layer spacing of about 0.35 nm apart, these platelets slide parallel to one another. In addition, graphite hardness exhibits a unique ability to physically keep sliding metal surfaces apart.
Particle composition. RGC is further defined as:
* Carbon subjected to high thermal treatment resulting in a macroscopic particle of high purity, ordered crystalline structure, and it exhibits resilient characteristics of up to 150% at 10.000 psi. Table 2
* Being free of extractable hydrocarbons. heavy metals and sulfur. It is odorless, tasteless, non-toxic and will not react with any drilling fluid additive. In addition, it does not soften in mineral or crude oil, over time or with temperature to 425 deg C (800 deg F).
When manufactured from delayed coke, the particles, observed under a microscope, resemble black-colored “popcorn,” Fig. 2. In terms of shape, the particle has a 0.7 sphericity and 0.1 roundness.
The Desulco process graphitic carbon can be readily differentiated from similar appearing, i.e., black or dark brown color, particulates such as petroleum coke, ground polypropylene plastic, sulfonated asphalt or mined natural asphaltite because of its resiliency, Table 3. Other LCM materials show little or no resiliency. Most are permanently crushed when the pressure reaches plastic flow conditions, and/or compressive strength of the material is exceeded, Fig. 3.
Resiliency contributes to reduced drilling costs. It is postulated that when RGC is pumped downhole, particles are forced into pores or microfractures by differential pressure where the material elastically deforms. Since lab tests show that deformed RGC helps increase formation strength, it is assumed that a greater pressure would be required to initiate fracture.
Table 3. RGC physical characteristics
Source Desulco process delayed coke
Color Black with a silver sheen
Shape Irregular, angular particles with
a “popcorn” appearance
Specific gravity 1.45-1.75
Resilience Up to 150%
Ro-Tap: 100% less than 16 mesh
98% greater than 200 mesh
Coulter LS in water
with tspp: Mean, [[micro]meter] 361.4
Median, [[micro]meter] 315.5
area, [cm.sup.2]/ml 361.7
More than one-half volume of each particle is graphite, thus facilitating migration and seating of lubricious particles in a fracture. If differential pressure later increases above that of the original plugging event, increased stress may cause fracture enlargement. RGC expansion maintains a seal to block fluid loss.
LAB EVALUATION RESULTS
Four samples of one-barrel equivalent of 12-lb/gal PHPA mud was mixed. RGC was added in concentrations of 10, 20 and 30 lb/bbl and tested at 100 psi in an API filtration cell containing a 1-in. thick bed of 16/30-mesh gravel pack sand, Table 4. The cell without RGC blew dry immediately. However, samples containing RGC sealed off loss of whole mud. At 30 lb/bbl, Run 4, no mud passed through the sand bed. The filtrate was clear and free of solids. Similar results have been obtained in PPA cells at 500 and 2,500 psi across ceramic discs and a metal slot.
Resistance to attrition by high shear. The sample was stirred at high speed for 90 min. in a stainless steel container on a Hamilton Beach mixer. After stirring, a 30-ml aliquot of mud was wet screened over a 200-mesh sieve. The volume of residue on the screen, after washing away colloidal clay solids, is a measure of change in particle size due to shear.
Losses were about one volume percent, indicating that RGC is resistant to shear forces. Base mud color changed from tan to medium gray indicating size reduction occurred. However, the amount was small as attested to by the 200-mesh recovery data. Resistance to attrition may be attributed to particle resiliency.
Abrasivity index of graphitic carbon. A modified API test was used to determine abrasiveness of weighting materials. Sample C indicates a 0.19-mg/min.-weight loss of 1020 mild steel in a 200-lb/bbl sample. This compares favorably with a 0.06 mg/min. loss obtained with 20-lb/bbl bentonite clay, Sample A, Table 5. Furthermore, RGC is one-third of the weight loss obtained with [D.sub.50] calcium carbonate. Calcined petroleum coke gave the highest weight loss of 6.81 mg/min.
Effect of RGC on rheological properties. Graphitic carbon can be added at concentrations up to 120 lb/bbl without “locking up” the mud, Table 6. It is particularly important to note that initial and 10-min. gel strengths of the blank sample were 8 and 15 lb/100 [ft.sup.2] respectively. With 120-lb/bbl oilfieldgrade RGC, initial gel strength was 15lb/100 [ft.sup.2]. The 10-min. gel strength was unchanged.
Reducing lost circulation in cements. RGC has also been tested for plugging a 0.20-in. slot to prevent loss of Type H cement. Setting time may be increased or decreased by changing composition. Since RGC absorbs water, a formulation containing RGC must be pilot tested before use in cement. A number of field jobs have been completed with cement/Pozmix slurries. RGC is now routinely used by both mud and cementing companies to control loss of slurries following displacement and squeezing.
The following are field applications/advantages of RGC.
Wharton County, Texas – invert emulsion. After setting a 9 5/8-in. string of casing at 7,800 ft, a chrome lignosulfonate (CLS) mud was displaced with a 15.5-lb/gal invert emulsion mud system. An 8 1/2-in. PDC bit was used to drill ahead with no problems. Mud weight was increased to 15.8 lb/gal at 10,250 ft due to increased background gas. A fluid loss of 1 to 3 bbl/hr was observed.
It was decided to go with two sacks cellulose fiber and two sacks calcium carbonate per hour. Two linear shakers were fitted with 175-mesh screens at this point.
Drilling continued to 10,735 ft, where mud weight was increased to 16 lb/gal for background gas control. Seepage losses now ranged from 5 to 15 bbl/hr. Cellulosic fiber and calcium carbonate treatments were increased to four sacks (each) per hour. Linear shaker screens were changed to 84-mesh in an attempt to retain system LCM. The interval was drilled to 11,485 ft with continuous seepage losses. Mud weight was increased to 16.4 lb/gal to POOH and log.
While out of the hole and logging, the well began flowing. To kill the well, 150 bbl of 17.5-lb/gal invert emulsion was bullheaded down the backside. Logging tools were removed and a very slow process of reaming back to bottom began. While reaming, continued seepage losses were observed. It was obvious that cellulosic fiber and calcium carbonate were not successful in controlling mud losses.
Table 4. Seepage control tests over 16/30 gravel pack sand at 100
psi (one bbl base mud: 12.1-lb/gal seawater PHPA)
Run No. 1 2 3 4
RGC conc., lb/bbl – 10 20 30
Vol. to shut-off, ml Blowout 21 14 0
Time to shut-off, s – 12 3 0
Filtrate color after shut-off Whole mud Muddy Clear Clear
[TABULAR DATA FOR TABLE 5 OMITTED]
[TABULAR DATA FOR TABLE 6 OMITTED]
A 40-bbl pill containing 40-lb/bbl RGC was pumped downhole. Additionally, RGC was added to the mud system for an increased 5-lb/bbl concentration. Linear shaker screens were changed from 84-mesh to 20-mesh to retain RGC in the system. Mud weight was increased to 16.2 lb/gal from 16.0 lb/gal with no losses. Final mud weight was 16.4 lb/gal, again with no mud loss.
It was feared that increasing mud weight would induce additional lost returns. For well control, it was decided to increase mud weight to 16.5 lb/gal (previously unachievable), allowing reaming operations to continue and successfully run a 7 5/8-in. liner.
High Island block 140 – offshore Texas. A 50 [degrees]-angle well was drilled off into a massive, depleted sand section at High Island Block 140, offshore Texas. An Enhanced Mineral Oil (EMO) mud was used due to the small wellbore size (6-in.) and potential for becoming stuck in depleted sands.
Initial mud contained 40-lb/bbl calcium carbonate. While drilling, RGC was added at a rate of three to five sacks every three hours to prevent mud loss and provide lubricity. Total depth was reached three days under projection with only normal fluid loss to cuttings.
Southern Louisiana, horizontal well. Unable to slide while drilling a horizontal well in Southern Louisiana, it was decided to use RGC for lost circulation treatment. The first sweep of 40 lb/bbl in a 50-bbl pill showed immediate improvement of weight transfer to bit and allowed drilling ahead. An RGC pill was kept on standby in the slugging pit until interval completion. No adverse effect was noted on MWD tools or mud motor.
High Island 24-L – offshore Texas. Using a low pH/FILTERCHEK mud system, RGC was credited for stopping loss circulation experienced on a conditioning run after logging. Very little success had been achieved with a variety of LCM pills used originally, including cement. About 20-lb/bbl RGC was successfully used with 20lb/bbl cellulosic fiber and 40-lb/bbl calcium carbonate.
Southeast Oklahoma, drilling very hard quartzite rock section. Excessive torque problems were experienced while drilling at 10,300 ft in southeastern Oklahoma. Several liquid lubricants were unsuccessful in lowering torque amps. Torque dropped by 300 to 275 amps soon after a 30lb/bbl RGC slug cleared the bit. This procedure was repeated as needed until casing was set.
South Louisiana well, drill and slide. The problem was an inability to slide while drilling horizontally in South Louisiana. It was decided to use a STEELSEAL (RGC) 40-lb/bbl sweep. A 50-bbl pill allowed drilling to continue with no adverse effect on MWD tools or mud motor.
South Texas – 50/50 cement/Pozmix slurry. Applications to date have been in 50/50 cement/Pozmix slurries containing 8-w/w% cement. Resulting lost circulation mixtures have been squeezed into the formation before weighting up and drilling ahead. In those cases, a balanced plug, preceded and followed by spacers, was spotted below lost circulation zones. Several stands of drill pipe were racked back and a RGC/cement lost circulation squeeze was bullheaded into the formation. Pumping the squeeze was continued until pressure reached a desired equivalent mud weight. Drilling was then resumed and RGC was batch mixed with cement before transporting to the rig.
Increasing cement density is much faster when Diacell M is used. Slurties of 50/50 Pozmix containing RGC have exhibited good compression strengths and are economical to use. RGC/Pozmix slurries can be used in water-based and oil-based systems. Although filtration agents can be added to squeeze mixtures, successful applications have been with slurties exhibiting high filtration rates. These slurries can be mixed on location “on the fly” if necessary.
Conclusion. In summary, RGC is multi-functional, with a resiliency of up to 150%. Lab and extensive field results show that this material serves to economically seal fractures and/or permeable formations against adverse effects of increased mud weight in water-, oiland synthetic-based drilling fluids.
RGC structure provides ideal properties for reducing torque, drag and wear in all types of drilling fluids. It is particularly useful as a solid lubricant in lime-based mud where conventional liquid lubricants normally fail to perform.
The authors thank Rowan Petroleum Co. management, Baroid Drilling Fluids and Superior Graphite Co. for permission to publish this material. In particular, we thank Todd Shepard and Peter Zaleski of Superior Graphite Co. for providing background material on graphite chemistry and properties.
Desulco is a registered trademark of Superior Graphite Co., Chicago, Illinois; STEELSEAL and BAROFIBRE are trademarks of Baroid, a Division of Dresser Industries, Inc.; Soltex and Diacell M are trademarks of Drilling Specialties Co., Bartlesville, Oklahoma.
1 Fuh, G. F., N. Morita, P. A. Boyd and S. J. McGiffin, “A new approach to preventing lost circulation while drilling,” SPE 24,599 (Oct. 4-7, 1992).
2 Whitfill et al., U.S. patent 4,957,174 (Sept. 18, 1990).
3 Kubena et al., U.S patent 5,211,250 (May 11, 1993).
4 Newhouse, C.C., “Successfully drilling severely depleted sands,” SPE/IADC 21,913 (December 1992).
5 White, J. P. and R. Dawson, “Casing wear: Laboratory measurements and field predications,” SPE Drilling Engineering, March 1987.
6 Best, B., “Casing wear caused by tool joint hardfacing,” SPE Drilling Engineering, February 1986.
Jerry Alleman is operations VP for Alleman and Associates, Houston, and has 25 years drilling operations experience.
Bobby Owen is Texas Gulf Coast sales manager for Baroid, Houston, working with drilling fluids and products. He earned a BS degree in biology, University of North Texas, Denton, and has 19 years of industry experience. E-mail: email@example.com
Freddie Cornay is worldwide technical marketing manager for Baroid, Houston, working in drilling fluids operations and technical support. He received an industrial technology degree from the University of Southwestern Louisiana, Lafayette, and has 16 years of drilling fluid experience. E-mail: firstname.lastname@example.org
Donald J. Weintritt, president of Weintritt Consulting Services in Lafayette, Louisiana, and founder of Weintritt Testing Laboratories has worked for NL Industries and Baroid for the past 25 years. He has been involved in R&D projects, including field use of water and oil based drilling, completion and workover fluids. He earned an MS in geology, University of Houston, and is a registered PE in Texas and Louisiana. He has published 45 articles, holds seven patents, and co-authored Water-Formed Scale Deposits. Additionally, he is a member of SPE, NACE, AADE, ACS and CMS and serves on the API Standardization Committee on Drilling Fluids. E-mail: email@example.com
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