suspended in frac gel was pumped through the samples at 30 barrels per minute. This equates to an internal velocity of 33 ft/sec. One of the internally coated pup joints was subjected to 200,000 pounds of proppant and the other joint to 750,000 pounds. The bare pup joint was tested to 1,000,000 pounds of proppant.
The pup joints were cut in two-foot section lengths and then split longitudinally. All three pup joints exhibited ridges approximately nine inches from the pin nose on both ends. It was assumed this was inherent to the pipe’s manufacturing process. All pup joints also exhibited areas of rough metal. Additionally, the internal diameter (ID) of the test loop increased at the 8rd coupling connection (commonly known as the “J” area).
After pumping 200,000 lbs. of proppant, the internal coating exhibited no holidays when tested with a wet sponge at 67 ½ volts DC at 70,000 to 80,000 ohms. This is in accordance with NACE TM0384-94 holiday standards for film coatings (thickness less than 10 mils).3 A “holiday” is a condition of the coating which causes the coating to fail to meet specified electrical resistance values.4 Holidays can be caused by defects such as thin coating areas, foreign material on the substrate or imbedded in the coating, coating pinholes, or metallic slivers in the pipe wall. Photo 1 shows the outer surface of the coating (0.4 mils thick), which is darker due to surface oxidation from the final baking processes, was eroded along a very thin ridge nine inches from the pin end. This exposed the lighter coating material below the oxidized surface. Coating thickness for this sample ranged from 5 to 7 mils, well within the coating specifications.
Photo 1. 200,000 lbs proppant test at 30 BPM
After pumping 750,000 lbs. of proppant (Photo 2), the coating generally is lighter colored. This indicates some wear of the 0.4 mil thick oxidized outer layer of the coating. The coating also exhibited 9 small holidays within 9” from both pin ends and from the pin noses. The pin nose is exposed to the flow through the 8rd connection and coating thickness in the areas of no holidays again ranged from 5 to 7 mils.
Photo 2. 750,000 lbs proppant test at 30 BPM
After pumping 1,000,000 lbs. of proppant (Photo 3), the bare pup joint exhibits similar areas of wear near the connections. These results are consistent with the bare steel Taber Abraser tests. Like the coated joints, the ridges and areas of rough pipe exhibited the most wear.
Photo 3. 1,000,000 lbs proppant test at 30 BPM
In summary, the coating performed well enough in this test to be used for the planned completion program. Although there were small and infrequent holidays over ~3% of the surface area after pumping 750,000 lbs of proppant, the coating served its purpose of minimizing metal exposure and wear (~97% of the area was protected with coating). Numerous laboratory and field case histories document that pipe with coating holidays or wireline cuts has a lower corrosion rate than if uncoated.4,5,6,7,8,9
The coating loss for the 750,000-lb. proppant test was roughly 0.4 mils of the topcoat in some areas and 7 mils in the areas of the holidays. This equates to a maximum coating loss of 65 lbs. (<0.01% by weight of the proppant pumped) for a 15,000’ workstring of 5” drill pipe. This coating loss should not cause a measurable reduction of gravel flow capacity based on industry proppant conductivity tests with contaminating foreign particles.10
Ridge of 0.4 mil oxidized surface eroded Flow Direction Coating is intact
Holidays
Flow
Direction
Ridge
Eroded Bare Pipe
Flow
The pup joint’s ridges, rough spots, and connection areas exhibited the most coating wear. These results infer that the irregular internal surface of the second-hand workstring would be susceptible to erosion greater than experienced in the test loop or what would be expected of a newly coated workstring. Also, since the area of most wear was near the connection profile change, the planned workstring’s IF connection with 2.75” tool joints and 4.276” tubes should be likely to develop wear.
Field Results – Well Productivity
Well productivity can be impaired by contaminants which reduce the formation’s permeability, reduce proppant permeability, reduce gravel pack screen conductivity, or plug the perforation tunnels creating extra near-well pressure drop (skin effect). The main contaminants typically present are mill scale, iron sulfides, pipe dope, sand, and other fine particles which are picked up during storage and transportation.11 The Genesis wells overcame these damaging contaminants and have good productivity. The average skin factor is 2.9 and the average production rate is 14,000 BOEGPD.
Iron scale and sulfides on the pipe wall are usually treated with an acid pickle and pipe dope is usually treated with a solvent just prior to frac packing. Frequently, the pipe dope prevents the acid from fully contacting and removing the scale. With a coated workstring, all that is needed during the pickle treatment is solvent to remove pipe dope since the internal coating greatly reduces the iron scale presence. No acid pickle treatments were needed throughout the 17 completions saving $170,000. These savings more than offset initial coating costs and any re-coating costs.
Dissolved iron could be a critical factor in formation damage since precipitation of iron complexes can occur in some brines if dilution with formation waters causes an elevation of pH.12 Iron scale solubilized from downhole tubulars introduces dissolved and suspended solids to the completion fluid which can create an endless cycle of circulation and filtration to reach fluid specifications. Often this remedy is temporary or not effective. The CaCl2/CaBr2 completion fluids used did not have any unusual dissolved or suspended iron problems. This is partially attributed to the internal workstring coating, coated pits, and coated troughs.
Field Results – Pipe Debris
Managing pipe debris is critical to completion processes. As an example, pipe debris can indirectly be a safety hazard if live TCP guns have to be pulled due to a stuck firing bar. Also fishing stuck tools (wireline, packers, gravel pack washpipe, etc) can get quite costly.
At Genesis, 8 wireline trips, 8 TCP bar drop operations, and 24 ball drop packer setting jobs were conducted through the subject workstring without any trouble time caused by pipe debris (scale). The gravel pack tools were thoroughly checked for debris following each gravel pack operation and, on only
one occasion, small flakes of coating remnants were recovered from the gravel pack crossover tool. Throughout the completion program, the major debris components observed from these checks was steel perforating debris and/or cement.
Field Results – Proppants
High rate Frac Packs are known to erode surface and downhole equipment. Proppant mass, concentration, and pump rate must be reviewed and compared with equipment’s ratings when designing completions. Equipment should be pressure tested to the maximum anticipated surface treating pressure. The second-hand 5” drill pipe workstring was used during the Frac Packs of all seventeen completions. Stimulation vessels pumped 1.8 MM lbs. of proppant at 10 to 25 BPM and a total of 0.5-MM lbs. of excess proppant was reversed from the wells during the completion program. All completions used ceramic proppant, except one, which used bauxite. Obviously the proppant schedules pumped during the completion program were not a constant 12-ppg concentration like the abrasion loop tests. Additionally, the workstring had to endure annular screen outs with peak pressures over 10,000 psi and the excess proppant had to be reversed from the wells. Both of these events are sometimes forceful and agonizing. Due to significant internal restrictions, the tool joint flow velocity is 2.4 times higher than the tube velocity. The tube velocity ranged from 9.4 to 23.8 fps; whereas, the tool joint velocity ranged from 22.7 to 57.6 fps and exceeded the flow loop test velocity of 33 fps.
Field Results – Mechanical Abrasion and Impact
In addition to the abrasion from proppants, the workstring withstood the mechanical effects of 8 wireline trips to correlate completion tools, 8 TCP bar drop operations, and typical (rough) drill pipe handling.
Both braided electric line and single core slickline operations occurred during the completion program. As a result, roughly 10% of the tool joints have wire line cuts through the coating. Photos 4 and 5 show a representative wireline scared coated tool joint and also previously wireline scared pipe which has been re-coated. A total of 8 wireline trips occurred and equate to a maximum possible wireline exposure of 136,400 ft. Industry and NACE RP0291-91 recommended wireline procedures to minimize coating damage were not used at Genesis.6,7,13,14,15 Some of these recommendations include maximum wireline speed of 100 feet per minute, no free falling tools, no sharp edge tools, single strand line, and well lubricated slickline. Previous industry studies indicate that coatings succumbed to wireline abrasion after 18’-15,000’ of wire was passed across the samples.7,16,17 Abrasion rates of each coating were related to the type and condition of the wire, the wire velocity, and the applied wire force against the coating. Although each joint of the workstring was subjected to varying wireline lengths and forces, it is estimated that the footage of wire exposed to the coating far exceeded any of these industry tests.
The TCP bar drop process occurred 8 times and involved dropping a 1¼” x 10’ steel detonating bar weighing 35 lbs. inside the workstring from the surface. Roughly 650 ft of the workstring was dry (air cushion) and the remainder of the workstring was liquid filled during this operation. This bar must certainly make quite an impact with each restricted tool joint since every well was directionally drilled. During previous industry tests, numerous coatings were checked for impact resistance using ASTM D 2794-82 and most had an impact resistance of 30-300 in-lbs. before cracking or chipping.16,18
Photo 4. Coating is wireline scared
Photo 5. Boroscope image of well bonded coating over